Chestnut and Grapevine By-Products: Bioactivity, Biotransformation, and Nutraceutical Applications

Abstract

The growing interest in sustainable food systems has spurred research into the valorisation of agro-industrial by-products as sources of bioactive compounds. This review provides a comprehensive overview of the phytochemical composition, bioactivity, biotransformation, and potential nutraceutical applications of by-products from chestnut (Castanea sativa Mill.) and grape (Vitis vinifera L.). Recent studies identify matrices such as chestnut leaves, shells, and burs, as well as grape pomace, skins, seeds, stems, and vine shoots, as rich in phenolic compounds, dietary fibres, vitamins, and minor bioactives, with antioxidant, anti-inflammatory, and antimicrobial properties. Emerging evidence highlights the importance of gastrointestinal digestion and microbial biotransformation in modulating the bioavailability and biological efficacy of phenolic compounds, particularly fibre-bound phenolics. The review further discusses state-of-the-art analytical approaches for chemical characterisation, including chromatographic and spectrophotometric methods, as well as emerging strategies for extraction, encapsulation, and delivery to enhance stability and bioavailability. Finally, the integration of chestnut and grapevine by-products into nutraceuticals, functional foods, and natural preservatives is critically examined from technological, safety, regulatory, and sustainability perspectives. Overall, this synthesis underscores the potential of these underutilised biomass streams as multifunctional raw materials that support waste valorisation, resource efficiency, and the development of next-generation health-promoting ingredients aligned with circular bioeconomy principles.

1. Introduction

The growing demand for sustainable and functional food solutions has driven scientific interest in the valorisation of agro-industrial by-products as alternative sources of bioactive compounds [1,2,3]. In this context, the valorisation of agro-industrial waste is a central strategy for promoting the sustainability of food systems and consolidating the transition to a circular economy [4,5,6,7]. The growing pressure to increase food production, coupled with the urgent need to reduce environmental impacts, has intensified interest in economic models that optimise resource use and minimise losses throughout the production chain. In this context, the circular economy represents an alternative socio-economic paradigm that departs from traditional linear models. Instead of a logic of extraction–production–disposal, it proposes a system in which material flows are cyclical, favouring reduction, reuse, recycling, and resource recovery to minimise waste generation and maximise overall efficiency [8,9].

In this review, grapevine (Vitis vinifera L.) and chestnut (Castanea sativa Mill.) by-products were selected because these species represent two of the most economically, socially, and culturally relevant crops in northern Portugal and in the world, generating large volumes of agro-industrial residues whose valorisation offers significant environmental, technological, and socio-economic benefits. Traditionally considered waste, the by-products of these two species emerged as raw materials with high biotechnological and nutritional potential [10,11,12]. These secondary materials, resulting from agro-industrial and oenological processing stages, have a particularly rich polyphenol, prebiotic fibre, vitamin, and other phytochemical composition, with proven antioxidant, anti-inflammatory, and metabolic activities [13,14]. The growing interest in these natural matrices therefore stems from their ability to provide functional ingredients for the formulation of nutraceuticals, supplements, and fortified foods while simultaneously contributing to human health and reducing food waste [15,16,17,18].

From a compositional point of view, chestnut by-products, including the inner and outer shells, burs, flowers, and leaves, stand out for their high concentration of phenolic compounds, such as ellagic acid, gallic acid, hydrolysable tannins, and flavonoids, as well as for their significant levels of vitamin E and structural fibres [17,19,20]. These constituents confer antioxidant, anti-inflammatory, and antimicrobial properties, as widely described in the scientific literature. Furthermore, their stability and chemical diversity make these materials particularly promising for functional and technological applications [13,21,22,23,24]. Similarly, grapevine by-products, such as stems, skins, and pomace resulting from fermentation or pressing, have complex phenolic profiles that are rich in resveratrol, catechins, proanthocyanidins, and anthocyanins [12,16,25,26]. In addition, they contain significant amounts of dietary fibre with prebiotic potential, which can modulate the gut microbiota and promote beneficial metabolic and immunological effects [27,28]. This biochemical diversity has sustained the expansion of its applications, which range from antioxidant and antimicrobial extracts to natural colourings and functional ingredients.

Despite significant advances in characterising the chemical composition and potential applications of chestnut and grapevine by-products, there remains an essential scientific gap regarding the bioaccessibility, biotransformation, and physiological relevance of their bioactive compounds. Although numerous studies have documented the presence of polyphenols, vitamins, and prebiotic fibres in these matrices, few have explored in depth how these constituents are transformed throughout the gastrointestinal tract, their absorption efficiency, or the role of the resulting metabolites in promoting measurable beneficial effects. Understanding these processes is essential, given that the mere presence of bioactive compounds in raw materials does not, in itself, guarantee effective biological action in vivo. The chemical transformations that occur during digestion—including hydrolysis, methylation, glucuronidation, and microbial bioconversion—can substantially modulate the potency, bioavailability, and functional properties of these compounds [29,30,31]. Therefore, in-depth evaluation of these dynamics is essential to validate the real health-promoting potential of chestnut and grapevine by-products and to support their safe and effective integration into functional foods and nutraceuticals. This review summarizes current knowledge on the composition, bioaccessibility, and biotransformation of bioactive compounds from chestnut and grapevine by-products, highlighting how digestive and microbial transformations modulate their biological activity and identifying key research gaps that must be addressed to support their effective use in functional foods and nutraceuticals.

Literature Search Strategy, Scope, and Limitations

This work was conducted as a narrative review to provide a conceptual and mechanistic overview of the bioactivity, biotransformation, and nutraceutical potential of chestnut and vine by-products. The literature survey was performed using PubMed, Scopus, and Web of Science, employing combinations of general keywords such as “chestnut by-products”, “vine by-products”, “polyphenols”, “phenolic compounds”, “circular bioeconomy”, “bioactive compounds”, “phytochemicals”, “antioxidant activity”, “biotransformation”, “gut microbiota metabolism”, “bioavailability”, “in vitro digestion”, “functional foods”, and “nutraceutical applications.”

Peer-reviewed articles published in English were prioritized. Emphasis was placed on studies providing mechanistic insight into biological activity, metabolic fate, and physiological effects, including in vitro, in vivo, and, when available, human studies.

As a narrative review, no formal systematic review protocol (e.g., PRISMA) was followed, and study selection was guided by relevance to the review’s scope. Therefore, the synthesis is not exhaustive, and some relevant studies may not have been included, potentially introducing selection bias.

2. Chemical Composition and Bioactive Profiles

2.1. Phenolic Composition of Chestnut By-Products

The chestnut industry (C. sativa) generates tonnes of waste with a significant negative impact on the environment and the sector’s economy [32,33], particularly as it is a rapidly growing industry in which shell production is a major by-product [34]. In fact, in recent decades, the expansion of the chestnut processing sector has significantly increased the volume of by-products generated throughout the production chain. In the field, after harvesting, residues such as leaves, pruning, and burs are left behind, with harmful consequences for the crop, as they promote pest infestations [35,36]. Processing chestnuts also generates waste in the form of inner and outer shells, which are typically discarded during post-harvest and food processing. This results in significant economic losses [37]. According to Aschemann-Witzel et al. [38], if this waste can be reused, it is regarded as a by-product with market value. This emerging approach, based on circular thinking principles, offers a promising framework for effectively addressing current economic and environmental challenges [4]. It facilitates the development of new value chains in relevant sectors such as nutraceuticals, pharmaceuticals, and cosmetics. Thus, considering the tissues that constitute the by-products of chestnut cultivation, these are structurally distinct and accumulate various classes of phenolic compounds. Their composition reflects both the protective function of these matrices and the plant’s metabolic responses to biotic and abiotic stresses.

Chestnut shells have emerged as matrices of high scientific and industrial interest. Studies conducted by Amato et al. [37] concluded that the most abundant biomolecules in the extract obtained from shells were reducing sugars (65%) and polyphenols (26%) (

Table 1. Content of biomolecules and phenolic compounds, and their percentage composition in the chestnut shells extract. Adapted from [37].

Biomolecules			Phenolic Compounds
Total Polyphenols (μg GAE/mL)	Proteins (μg BSAE/mL)	Reducing Sugars (μg GE/mL)	Non-Tannin Polyphenols (μg GAE/mL)	Total Tannins (μg GAE/mL)	Hydrolysable Tannins (μg GAE/mL)	Condensed Tannins (μg GAE/mL)
2385.27	813.00	6009.45	632.19	1753.08	59.86	1693.22
(26%) *	(9%) *	(65%) *	(26%) **		(3%) **	(71%) **

GAE: Gallic Acid Equivalents; BSAE: Bovine Serum Albumin Equivalents; GE: Glucose Equivalents; *—percentage composition of the quantified biomolecule classes; **—distribution percentage of the polyphenol classes.

).

Other studies have also demonstrated the richness of these by-products in polyphenols, identifying ellagic acid, protocatechuic acid, pyrogallol, and gallic acid [34], as well as vitamins E and amino acids [39]. These compounds are widely recognized for their benefits to human health due to their marked antioxidant, antimicrobial, and anti-inflammatory activity [40,41,42,43].

Silva et al. [14] and Hu et al. [11] demonstrated that chestnut shells are particularly rich in hydrolysable tannins, simple phenolic acids, and flavan-3-ols. The inner part of the chestnut shell reveals a distinct phenolic signature, rich in catechin, epicatechin, and their oligomeric forms [6,21]. These flavan-3-ols contribute not only to antioxidant activity but also to potential anti-ageing and photoprotective effects, which have sparked growing interest from the food, cosmetics, and pharmaceutical industries [3].

Hydrolysable tannins, especially ellagitannins, represent the predominant phenolic class in chestnut burs and shells. Compounds such as castalagin, vescalagin, and pedunculagin, as well as their derivatives, are consistently reported as major constituents [10,13]. Ellagic acid has been associated with strong antioxidant capacity, antimicrobial potential, and anti-inflammatory properties, which reinforces its relevance for the development of functional ingredients [10,11].

Gallic acid, another key phenolic acid found in chestnut residues, occurs both in free form and as part of gallotannins, contributing to the overall antioxidant potential of these matrices [11].

Furthermore, the chemical characterisation of chestnut shells conducted by various researchers confirms a lignocellulosic matrix characterised by high levels of lignin and structural carbohydrates (

Table 2. Chemical composition of chestnut shells, leaves, and burs as reported in the literature.

Component	Content Reported	References
Klason lignin	31.9% (shells)	[35]
	41.7% (shells)	[45]
	44.6% (shells)	[46]
	44.3% (shells)	[36]
	37.5% (leaves)	[36]
	22.6% (burs)	[36]
Acid-soluble lignin	3.7% (shells)	[46]
Cellulose/Glucan (as glucose)	19.2% cellulose (shells)	[35]
	20.6% glucan (shells)	[46]
	28.4% glucose (shells)	[45]
	14.9% glucan (shells)	[36]
	16.5% glucan (leaves)	[36]
	34.4% glucan (burs)	[36]
Xylan (as xylose)	10.5% (shells)	[46]
	7.9% (shells)	[45]
	10.4% (shells)	[36]
	11.7% (leaves)	[36]
	21.4%(burs)	[36]
Rabinan	3.0% (shells)	[46]
Uronic acids	6.0% (shells)	[46]
Total sugars	33.8% (shells)	[35]
Total monosaccharides	32.7% (shells)	[45]
Glucose (monosaccharide)	19.2% (shells)	[45]
Xylose (monosaccharide)	6.5% (shells)	[45]
Galactose	Detected (shells)	[35]
	2.8% (shells)	[45]
Arabinan	2.91% (shells)	[36]
	2.97% (leaves)
	3% (burs)
Arabinose	Detected (shells)	[35]
	2.2% (shells)	[45]
Mannose	Detected (shells)	[35]
Cellobiose	0.3% (shells)	[45]
Essential amino acids (predominant)	Arginine: 355–721 mg/100 g; Leucine: 159–246 mg/100 g (shells)	[44]
Non-essential amino acids (predominant)	Glutamic acid: 268–484 mg/100 g; Aspartic acid: 268–484 mg/100 g (shells)	[44]
Other amino acids	Tyrosine, threonine, phenylalanine (shells)	[44]

). Rodrigues et al. [44] report that the high carbohydrate content is associated with the presence of dietary fibres, namely cellulose, hemicellulose, and lignin, and varies with the different geographical regions where these by-products are produced. Therefore, the chemical composition of these residues reinforces their potential for recovery in biochemical, biotechnological, and functional applications.

The chemical composition of chestnut burs and leaves differs substantially from that of the shell, highlighting their distinct potential for biochemical and biotechnological applications. According to Costa-Trigo et al. [36], burs have a moderate lignin content of 22.6%, while leaves exhibit a higher lignin content of 37.5%, reflecting the structural and physiological roles of these tissues. Burs, as protective outer structures, balance rigidity with accessibility to polysaccharides, whereas leaves require structural reinforcement alongside higher metabolic activity. Thus, burs and leaves represent complementary residues in the chestnut agro-industry. Burs, with high cellulose and hemicellulose content and lower lignin, are promising for carbohydrate-focused biotechnological applications [36], while leaves, rich in lignin and potentially bioactive phenolics, are more suited for functional ingredient extraction [13]. The compositional differences underscore the importance of residue-specific valorization strategies to optimize the recovery of chemicals and functional molecules from chestnut by-products.

2.2. Phenolic Composition of Grapevine By-Products

Grape pomace is mainly composed of skins, seeds, stems, and residual pulp [47]. It is characterized by a high phenolic content due to incomplete extraction during the winemaking process, with approximately 60–70% of the total phenolic compounds originally present in grape berries remaining in the pomace [48,49]. The composition of grape pomace depends on both the grape variety and the technological parameters applied during winemaking.

Studies have shown that anthocyanins, particularly 3-O-glycosides of malvidin, petunidin, cyanidin, peonidin, and delphinidin, are the dominant phenolic compounds in grape pomace skin. Flavan-3-ols, such as catechins and proanthocyanidins, are mainly found in grape pomace seed, whereas grape pomace stems are particularly rich in proanthocyanidins and stilbens. Overall, anthocyanins, catechins, flavonol glycosides, phenolic acids, and stilbenes constitute the main phenolic compounds present in grape pomace [50,51,52,53,54], as summarised in

Table 3. Main phenolic compounds found in grape pomace.

Phenolic Class	Pomace Fraction (Skins/Seeds)	References
Anthocyanins	Mainly skins (red grapes)	[50]
Flavan-3-ols (Catechin and epicatechin)	Mainly seeds; also skins	[50,55]
Proanthocyanidins	Seeds > skins	[51,55]
Flavonols	Skins	[50]
Phenolic acids	Skins and seeds	[52]
Stilbenes	Mainly skins	[50,56]

.

The distribution of polyphenols within the grape berry is uneven, with approximately 60–70% of total soluble phenolics in the seed, 28–35% in the skin, and only 10% in the pulp [57]. The main flavan-3-ols units identified in grapes (stems, skins, and seeds) include catechin, epicatechin, gallocatechin, epigallocatechin, and their gallate derivatives, which constitute the units of condensed tannins, also known as proanthocyanidins (procyanidins and prodelphinidins) [58,59,60].

Grape seed proanthocyanidins form a complex mixture primarily of procyanidins and procyanidin gallates. Procyanidins are oligomeric and polymeric flavanols, composed of catechin and epicatechin units. Procyanidin gallates contain epicatechin acylated with gallic acid as an additional flavanol unit. These flavanol units are linked through interflavanic bonds at the 4–8 or 4–6 positions [58,59]. Flavan-3-ols, in their monomeric, oligomeric, and polymeric forms, together with their gallate derivatives, represent the major phenolic compounds in grape seeds, although they are also present in grape skins, stems, and pomace.

In grape skins, the principal constitutive units of proanthocyanidins (procyanidins and prodelphinidins) were catechin, epicatechin, epicatechin gallate, and epigallocatechin [61]. Grape skins are mainly characterized by the presence of flavonols and anthocyanins [60], whereas grape stems are particularly rich in stilbenes and proanthocyanidins [62,63]. White grape skins contain significantly lower levels of phenolic compounds than those of red grape skins due to their absence of anthocyanins [58,59].

Phenolic acids identified in grape seeds, skins, stems, and pomace occur either in free form or conjugated with sugar moieties and/or tartaric acid [64]. Gallic acid is the most frequently quantified phenolic acid in grape seeds of various grape varieties [60], whereas ethyl and methyl gallate have been reported in pomace and seeds following fermentation [65]. Ellagic acid has also been identified in grape seeds and pomace [66]. Hydroxycinnamic acids, including p-coumaric, caffeic, and ferulic acids, are predominantly found in grape skins, where they occur as cis/trans isomers conjugated with tartaric acid, forming compounds such as feruloyltartaric, caffeoyltartaric, and coumaroyltartaric acid [58,59,67,68].

Anthocyanidins are inherently unstable compounds and are therefore rarely found in nature in their free aglycone form. Instead, they predominantly occur as glycosylated derivatives, known as anthocyanins, which are the main pigments responsible for the coloration of grape skins. Among these, malvidin-based anthocyanins are the most abundant, particularly in red V. vinifera grape varieties [69]. In addition to non-acylated forms, grape skins commonly contain acylated anthocyanins, including acetylated, caffeoylated, and coumaroylated derivatives [60,65,69].

2.3. Other Bioactives—Vitamin E, Fibers, and Minor Components

While phenolic compounds represent the most extensively studied class of bioactives in chestnut [11,13] and vine [70,71,72] by-products, increasing evidence demonstrates that these matrices also contain a diverse array of other biologically relevant compounds, including vitamins, dietary fibres, minerals, organic acids, and nitrogenous compounds [44,73,74,75,76]. These constituents play a pivotal role in modulating the overall bioactivity, technological functionality, and nutritional value of chestnut [77] and grapevine residues [76]. Importantly, their interaction with phenolics within complex plant matrices may significantly influence bioaccessibility, gastrointestinal biotransformation, and physiological effects [78,79].

Vitamin E, encompassing tocopherols and tocotrienols, is one of the most relevant lipophilic antioxidant systems in plant-derived foods. Its primary biological role is to protect polyunsaturated fatty acids in cellular membranes from oxidative degradation, thereby preserving membrane integrity and preventing lipid peroxidation chain reactions [80,81]. In the context of chestnut by-products, several studies have reported the presence of vitamin E isoforms, particularly α-tocopherol, in shells, burs, leaves, and flowers [77,82]. Although chestnut residues are not lipid-rich matrices, the detection of tocopherols is of considerable interest, as these compounds may act synergistically with phenolic antioxidants to enhance oxidative stability and biological efficacy [83].

In grapevine by-products, vitamin E is predominantly associated with grape seeds, pomace, and, to a lesser extent, vine leaves, reflecting the higher lipid content of these tissues [76]. Grape seed oils are particularly rich in α-tocopherol, α-tocotrienol, and γ-tocotrienol, which contribute to their remarkable oxidative stability and nutritional quality [84]. The coexistence of tocopherols with phenolics may result in additive or synergistic antioxidant effects, as tocopherols are capable of interrupting lipid oxidation, while phenolics regenerate oxidised tocopherol radicals [85,86]. This antioxidant network is highly relevant for nutraceutical formulations aimed at mitigating oxidative stress, chronic inflammation, and cardiometabolic disorders [87].

Dietary fibre constitutes one of the most abundant and technologically relevant fractions of chestnut and vine by-products. Chestnut residues are characterised by a lignocellulosic matrix rich in insoluble fibres, including cellulose, hemicellulose, and lignin, alongside smaller proportions of soluble polysaccharides [45,88,89]. From a technological perspective, these fibres exhibit high water-holding capacity, oil-binding properties, and structural stability, making them attractive ingredients for fibre-enriched foods and nutraceutical formulations [45,90].

Beyond their physicochemical properties, chestnut fibers may act as effective carriers of phenolic compounds, protecting them during gastrointestinal digestion and enabling their gradual release in the colon [13,79]. This carrier function is particularly relevant for hydrolysable tannins and fibre-bound phenolics, which generally exhibit limited absorption in the small intestine but undergo extensive microbial biotransformation in the colon, thereby forming bioactive metabolites such as urolithins [31,91]. Upon reaching the large intestine, fibre-bound phenolic compounds are extensively metabolized by the resident gut microbiota through a broad range of enzymatic reactions, including deglycosylation, dehydroxylation, α- and β-oxidation, dehydrogenation, demethylation, decarboxylation, C-ring fission, and cleavage into lower-molecular-weight phenolics [92].

Grapevine by-products, especially grape pomace and skins, are widely recognised as rich sources of both insoluble and soluble dietary fibers, frequently associated with polyphenols covalently or non-covalently bound to the cell wall matrix [93,94]. This association confers significant prebiotic potential, as fibre-bound phenolics reach the colon largely intact, where they are metabolised by the gut microbiota. The resulting metabolites, together with short-chain fatty acids produced during fibre fermentation, have been linked to improved intestinal barrier function, modulation of immune responses, and reduced systemic inflammation [95,96].

In addition to vitamins and fibres, chestnut and vine by-products contain a broad spectrum of minor bioactive constituents that further enhance their nutritional and functional relevance. Mineral elements such as potassium, calcium, magnesium, iron, and zinc have been reported in chestnut shells and grape pomace, contributing to electrolyte balance, enzymatic activity, and bone health [97,98]. Although their bioavailability may be influenced by fibre-phenolic interactions, these minerals can meaningfully contribute to the micronutrient profile of derived nutraceutical ingredients.

Organic acids, including malic, citric, and tartaric acids—particularly abundant in grape-derived matrices—play important roles in antioxidant defence, metal chelation, and modulation of gastrointestinal pH, potentially affecting mineral solubility and microbial metabolism [75,99]. Furthermore, chestnut by-products have been shown to contain free amino acids and small peptides, including arginine, glutamic acid, and aspartic acid, which may contribute to nitrogen metabolism, antioxidant activity, and metal-chelating capacity [97,100].

Collectively, the presence of vitamin E, dietary fibres, and minor bioactive compounds reinforces the concept that chestnut and vine by-products should be regarded as multifunctional matrices rather than simple sources of isolated compounds. These constituents interact at multiple levels, influencing digestive stability, microbial biotransformation, and systemic bioactivity. Their combined antioxidant, prebiotic, and metabolic effects strongly support the valorisation of these by-products as sustainable raw materials for the development of next-generation nutraceuticals and functional foods, fully aligned with circular bioeconomy principles and emerging consumer demands for natural, health-promoting ingredients.

2.4. Analytical Methods—LC–MS, HPLC, and Folin–Ciocalteu, Among Others

Comprehensive characterization of bioactive compounds in chestnut and vine by-products requires the integration of complementary analytical techniques. High-performance liquid chromatography (HPLC) is widely employed for the separation and quantification of metabolites in several agrifood matrices [18], including chestnut shells, burs, and leaves [101,102] and grape pomace, stems, and seeds [103,104], thereby enabling reliable detection of minor or structurally complex compounds. For example, studies have identified compounds such as ellagic acid, castalagin, and vescalagin using HPLC coupled with UV detection, providing insight into both nutritional and functional properties [105,106]. Similarly, HPLC is commonly used to analyze vine by-products, including grape pomace, seeds, and stems, for flavan-3-ols (e.g., catechin, epicatechin), proanthocyanidins, anthocyanins, and stilbenes such as resveratrol [107]. For instance, HPLC–UV and HPLC–diode array detection (DAD) have been employed to quantify anthocyanin profiles in red grape skins, while HPLC coupled with fluorescence or mass spectrometry has been used to detect minor polyphenols in grape seed extracts [108]. HPLC is not only useful for direct quantification but also for monitoring biotransformation processes, such as fermentation or enzymatic treatments, where precursor compounds may be converted into more bioactive or bioavailable forms [109]. By providing both qualitative and quantitative information, HPLC enables the assessment of the functional potential of these agro-industrial by-products and guides their application in functional foods, nutraceuticals, and natural preservatives. Coupling HPLC with mass spectrometry (LC–MS) enhances sensitivity and structural resolution, enabling elucidation of metabolic pathways and identification of novel bioactives with potential nutraceutical relevance [110].

Table 4. Examples of representative metabolites detected in chestnut and vine by-products using HPLC.

By-Product	Metabolite Class	Representative Compounds	Detection Method	Objective	References
Chestnut shells & burs	Hydrolysable tannins	Ellagic acid, Castalagin, Vescalagin	HPLC–UV/HPLC–DAD	Quantification of tannin content for functional applications	[105,111]
Chestnut leaves, buds, shells, and stems	Flavonoids, phenolic acids	Quercetin, Kaempferol, Gallic acid	HPLC–DAD	Used for antioxidant and nutraceutical studies	[13,111]
Grape pomace (skins)	Anthocyanins	Malvidin-3-O-glucoside, Delphinidin-3-O-glucoside	HPLC–DAD/HPLC–MS	Profiled for colorant and antioxidant potential	[112]
Grape seeds	Flavan-3-ols, Proanthocyanidins	Catechin, Epicatechin, Procyanidin B1	HPLC–Fluorescence/HPLC–MS	Monitored for functional food and nutraceutical applications	[113,114]
Grape stems	Stilbenes, Phenolic acids	Resveratrol, Piceid, Caftaric acid	HPLC–UV/HPLC–MS	Studied for antimicrobial and cardiometabolic bioactivity	[115,116]
Grape shoots	Stilbenes, Phenolic acids	Trans-resveratrol, ε-viniferin	HPLC–quadrupole time-of-flight (QTOF)–mass spectrometry (MS)	Potential Use for Cardiac Health	[117]

presents some representative metabolites detected in chestnut and vine by-products using HPLC.

Complementary to chromatographic techniques, spectrophotometric assays, particularly the Folin–Ciocalteu method, provide a rapid and comparative measure of total phenolic content (TPC) across by-products, processing conditions, and biotransformation treatments. For example, Galiñanes et al. [118] showed that the Folin–Ciocalteu assay can be effectively used to estimate total phenolic content in chestnut by-products. The same observation was recently reported by Vicente-Zurdo et al. [18], who found that TPC is a screening proxy prior to compound-specific profiling by HPLC analysis. In the winery by-products of V. vinifera cv. Riesling, the F–C method showed substantial TPC (432–665 mg gallic acid equivalents per g of extract) across pomace, stems, and vine leaves [119], illustrating the substantial potential of TPC as a method for rapid comparison of phenolic richness across different matrices. Although it is non-specific, this assay is useful for the preliminary screening of raw materials and for assessing the impact of processing before undertaking more detailed, compound-specific analyses.

Several studies, such as those by Hawrył et al. [120] and Vidal-Casanella et al. [121], have shown that combining chromatographic techniques (HPLC or LC–MS/MS) for the identification and quantification of individual phenolic compounds with total phenolic content (TPC) and antioxidant or functional assays offers a more comprehensive evaluation of antioxidant potential and biological activity in plant extracts or food matrices. When used together, HPLC or LC–MS data and TPC measurements can be correlated with antioxidant activity and biological effects, leading to a more nuanced understanding of functional potential. Collectively, these complementary analytical approaches provide a robust framework for linking chemical composition with bioactivity, thereby supporting the valorization of agro-industrial by-products as sustainable sources of health-promoting ingredients. Such correlations are essential for interpreting how compounds identified and quantified by chromatographic methods contribute to the antioxidant effects observed in spectrophotometric assays. Moreover, this integrated strategy enables detailed chemical profiling and insight into transformation processes, while also informing the development of functional foods and nutraceuticals and highlighting areas for further methodological refinement and future research.

3. Bioaccessibility and Biotransformation

Phenolic compounds are integral components of the human diet, and understanding their bioaccessibility and bioavailability is essential for evaluating their biological potential [122]. Bioaccessibility is the fraction of a compound released from the food matrix during gastrointestinal digestion that becomes available for intestinal absorption and biotransformation by the gut microbiota, thereby exerting biological activity [47].

Bioavailability encompasses the entire process of gastrointestinal digestion, absorption, metabolism, tissue distribution, and subsequent biological effects. From a nutritional perspective, it describes the proportion of an ingested nutrient or bioactive compound that reaches the systemic circulation and is available for physiological functions. Bioavailability is therefore a key determinant of nutritional effectiveness, as not all ingested bioactive compounds are efficiently absorbed or utilized by the organism [123].

Most phenolic compounds occur naturally as esters, glycosides, and polymers, which are not directly absorbable in vivo. Their absorption depends on enzymatic activity and microbial metabolism in the gastrointestinal tract, which transform these compounds into glucuronated, methylated, and sulfated derivatives with altered physicochemical and functional properties [124]. Anthocyanins, for instance, are poorly absorbed in their native form and are rapidly metabolized into absorbable metabolites after ingestion [125]. Their bioaccessibility is influenced by structural features such as hydrophilicity, hydroxylation pattern, and sugar moieties. Gut microflora typically cleaves glycosidic bonds and degrades anthocyanidin structures; however, anthocyanidins bound to pentoses or acyl groups exhibit greater resistance to degradation [126].

Similarly, oligomeric and polymeric phenolic compounds undergo depolymerisation into lower-molecular-weight compounds, thereby enhancing absorbability [124]. Procyanidins, in particular, readily break down into flavan-3-ol monomers, such as catechin and epicatechin, which are subsequently absorbed and metabolised [127]. These extensive transformations make the identification of in vivo metabolites particularly challenging.

Grapevine by-products, including vine shoots, stalks, and grape pomace [128], are especially rich sources of polyphenols, dietary fiber, and other bioactive compounds with potential health benefits. Similarly, chestnut by-products, such as outer and inner shells, leaves, and burs, are rich in bioactive compounds, particularly phenolics [74,129,130,131]. However, despite the well-documented bioactivity of these polyphenols, their bioaccessibility must be considered when evaluating their biological effects [132].

3.1. Static and Dynamic In Vitro Gastrointestinal Digestion Models

Simulated gastrointestinal digestion is commonly performed using a standardized in vitro model that reproduces the oral, gastric, and intestinal phases of human digestion. In vitro methods that simulate gastrointestinal digestion are widely used to investigate the effects of this process on food components [133]. These in vitro digestion models are generally classified as either static or dynamic [134].

Static digestion models simulate the different stages of human digestion in closed systems under constant conditions. During each phase, samples are incubated with simulated digestive fluids at fixed temperature, pH, enzyme activity, and incubation time. To reduce inter-study variability, the INFOGEST network proposed a standardized static digestion protocol in 2014, defining oral, gastric, and intestinal phases based on physiological parameters [135]. Although static models are widely used due to their simplicity, low cost, and reliable endpoint accuracy, they may be inadequate when digestion kinetics are of critical importance [135,136,137].

In contrast, dynamic digestion models more accurately replicate the physiological complexity of human digestion by incorporating continuous flow, controlled secretion of digestive fluids and enzymes, pH regulation, peristalsis movements, and gastric emptying [138]. These models enable time-resolved analyses and provide a closer approximation to in vivo conditions. Representative systems include the TNO gastrointestinal model (TIM) [139], the Simulator of the Human Intestinal Microbial Ecosystem (SHIME) [140], and the Gastrointestinal Simulator developed at CIAL (Spain). These systems offer multi-compartmental simulations of the gastrointestinal tract with real-time monitoring of key physiological parameters [141].

Importantly, the biological efficacy of grape-derived bioactive compounds depends not on their total concentration in the raw material, but on their bioaccessibility and biotransformation during gastrointestinal digestion. In this context, in vitro digestion models are widely used to evaluate the fate of grape-derived bioactive compounds under simulated gastrointestinal conditions. Notably, the bioaccessibility of polyphenols from grape skin and seed depends on the grape variety [31].

For example, Fleschhut et al. [142] demonstrated that glycosylated and acylated anthocyanins are rapidly degraded by intestinal microflora following anaerobic incubation with a human faecal suspension. The major stable products of anthocyanin degradation are phenolic acids derived from the B–ring of the anthocyanin structure.

Similarly, in vitro simulated gastrointestinal digestion has been applied to chestnut shells to assess their bioaccessibility, bioactivity, and metabolic profile [79]. During digestion, increases were observed in total phenolic and flavonoid contents, antioxidant and antiradical activities, radical-scavenging capacity, and acetylcholinesterase inhibition. These findings suggest that chestnut shells are a promising nutraceutical ingredient with antioxidant and neuroprotective potential [79].

3.2. In Vivo Metabolism and Biological Effects of Grape and Chestnut Polyphenols

In several animal studies and human intervention trials, polyphenol metabolites have been analyzed in blood and urine samples collected at various time points following the ingestion of grape pomace polyphenol extracts, with the aim of characterizing the in vivo metabolic profiles and temporal dynamics of these compounds [143,144]. Although numerous studies have demonstrated that consumption of grape pomace phenolics is associated with a range of health benefits in both humans and animal models, the specific compounds responsible for these effects remain largely unidentified. This uncertainty arises from the extensive metabolic transformations that these phenolics undergo after ingestion, resulting in the presence of both parent compounds and a diverse array of metabolites that may be distributed throughout the body and collectively contribute to the observed biological effects [47].

Polyphenolic extracts from grape skin pomace have been widely reported to exert beneficial health effects. Several studies have highlighted their potential role in the prevention and management of metabolic syndrome [145], through mechanisms such as improvements in endothelial function and vascular protection [146], modulation of bile acid metabolism and lipid homeostasis [147], and the attenuation of obesity and insulin resistance [32]. In addition, grape pomace polyphenols have been shown to influence the composition and activity of the gut microbiota [148]. These health benefits are largely attributed to polyphenol-derived metabolites formed in vivo, which may exert biological activity both locally within the gastrointestinal tract and systemically following absorption.

The in vivo bioactivity of chestnut shells has also been demonstrated in rats, with enhanced antioxidant enzyme activities, reduced lipid peroxidation, and no histopathological alterations [149]. These effects are largely attributed to the phenolic composition of chestnut shells, highlighting their potential applications in nutraceuticals [43,150].

3.3. Metabolic Pathways of Grape and Chestnut-Derived Phenolic Compounds

Grape-derived phenolic compounds undergo extensive biotransformation following ingestion, and this metabolic processing critically influences their bioavailability and potential health effects. During absorption in the small intestine and liver, phenolic glycosides are initially hydrolyzed to release their corresponding aglycones, a process mediated by endogenous enzymes such as lactase phloridzin hydrolase and cytosolic β-glucosidases, as well as by microbial β-glucosidases in the gut [151,152,153,154].

Once released, these aglycones undergo phase II conjugation reactions, which represent the main metabolic pathways for phenolic compounds in humans [155]. The major conjugation pathways include methylation, glucuronidation, and sulfation, which increase the solubility of phenolics, facilitating their systemic circulation and subsequent elimination [156]. As a result, conjugated metabolites often predominate in the circulation, and their biological activity may differ substantially from that of the original compounds.

Phenolic compounds that escape absorption in the upper gastrointestinal tract can reach the colon, where microbial enzymatic activities further hydrolyze and degrade complex structures into smaller phenolic acids. These metabolites can then be absorbed through the colonic epithelium and undergo further metabolism, contributing to systemic exposure to grape-derived phenolics [157].

Similarly, daily oral intake of a polyphenol-rich chestnut shell extract, composed of phenolic acids, followed by hydrolysable tannins, flavanols, and lignans, with sulfated conjugates predominating in the kidneys, significantly enhanced the in vivo antioxidant response. These findings provide the first evidence supporting the use of a nutraceutical extract from chestnut shells, recovered via an emerging green technology, as a promising source of antioxidant compounds [106].

4. Biological Activities and Functional Effects of Chestnut and Grapevine By-Products

4.1. Antioxidant Activity—Mechanistic Insights (ROS Scavenging, Enzyme Modulation)

4.1.1. ROS-Scavenging Activity: General Mechanisms and Process Overview

Reactive oxygen species (ROS) are byproducts of normal metabolism and stress that function in cell signaling at low levels but cause oxidative damage when present in excess. To maintain redox balance, cells use coordinated antioxidant defenses. Enzymatic systems—such as superoxide dismutase, catalase, glutathione peroxidase, and ascorbate peroxidase—convert ROS into less harmful molecules, while auxiliary enzymes recycle antioxidants. Non-enzymatic antioxidants, including ascorbate, glutathione, tocopherols, carotenoids, and phenolic compounds, directly scavenge ROS and regenerate oxidized enzymes. ROS sensing activates these defenses through redox-sensitive signaling pathways. Antioxidant activity is compartmentalized within organelles like mitochondria, chloroplasts, and peroxisomes, and damaged biomolecules are repaired or degraded by specialized enzymes [158,159,160,161,162,163,164] (Figure 1).

During our literature search, no direct comparative studies were found that specifically measure or contrast reactive oxygen species (ROS) synthesis between chestnut (Castanea spp.) and grapevine (V. vinifera). Available work instead describes general, largely conserved plant ROS-generation systems, as well as some grapevine-specific case studies. Both plants share the same ROS synthesis machinery. Across angiosperms, ROS are produced in the same main organelles and via similar enzyme systems: (i) organellar by-products of metabolism such as chloroplasts (photosynthetic electron transport), mitochondria (respiratory chain), peroxisomes, and, to a lesser extent, endoplasmic reticulum and cell walls generate superoxide and hydrogen peroxide during normal metabolism and under stress [161,166,167,168,169]; (ii) plasma-membrane NADPH oxidases like Respiratory Burst Oxidase Homologs (RBOHs), a family of specialized membrane-bound enzymes in plants that function as the primary generators of ROS and generate apoplastic superoxide/H₂O₂ bursts that function in development, stress signaling, and systemic “ROS waves” [161,170,171,172] and (iii) Other ROS-producing enzymes such as peroxidases, oxalate/amine oxidases, lipoxygenases among others [161,166,171,173,174]. These architectures are considered broadly conserved across vascular plants, so chestnut and grapevine are expected to share similar basic ROS synthesis routes [161,166,167,168,170].

Regarding grapevine, several studies characterize ROS dynamics and associated pathways. Hydrogen cyanamide and pruning cause a rapid, transient increase in ROS (H₂O₂ and related species) and NO in grapevine buds; this correlates with dormancy release and is accompanied by induction of ROS-generating genes (e.g., VvRBOH isoforms, peroxidases) and ROS-scavenging genes (APX, CAT, SOD, AOX, among others) [173]. Also, at véraison, grape berry skin shows controlled accumulation of H₂O₂ and singlet oxygen in cytosol and plastids, along with increased catalase activity and lipoxygenase-mediated galactolipid peroxidation, indicating a “developmentally programmed ROS signal” [174].

Under salt and alkali stress, grapevine leaves increase superoxide, H₂O₂, and lipid peroxide levels and strongly induce superoxide dismutase, peroxidase, catalase, ascorbate–glutathione cycle enzymes, and flavonoid biosynthesis, indicating a tightly regulated ROS–antioxidant response [175]. Moreover, excess light elevates ROS and induces photoinhibition in grape leaves; grapevine mitigates ROS-mediated damage via VvHY5/VvBEE1-regulated stilbene synthase [176]. In contrast, in shade-drying, ROS accumulation and membrane lipid peroxidation drive browning; exogenous ATP maintains ROS-scavenging enzymes (SOD, peroxidase, GPX, GST, GR), limiting ROS buildup [177].

These data indicate that grapevine uses canonical ROS sources, along with a rich specialized-metabolite antioxidant network (flavonoids, stilbenes such as resveratrol), to modulate ROS in organs such as buds, leaves, and berries [172,173,174,176].

The current papers analysed do not report that any study in the retrieved set measures ROS production rates, RBOH activity, or organellar ROS fluxes in chestnut, nor do they compare these with those in grapevine; therefore, it is not possible to state whether chestnut has higher or lower basal or stress-induced ROS synthesis than grapevine, or whether specific ROS sources dominate differently.

4.1.2. ROS-Scavenging Activity by Chestnut and Grapevine By-Products

Chestnut and vine by-products, such as shells, burs, and chestnut leaves, as well as grape pomace and grape leaves, are rich in polyphenols and flavonoids, making them promising sources of natural antioxidants. Recent research highlights their ability to scavenge reactive oxygen species (ROS) and modulate antioxidant and browning-related enzymes, supporting their use in food preservation, nutraceuticals, and functional foods.

Chestnut by-products demonstrate vigorous ROS-scavenging activity, primarily due to their high content of phenolic acids (e.g., gallic acid, ellagic acid) and flavonoids. These compounds neutralize free radicals and reduce oxidative damage in cell and tissue models [129,178,179,180]. Both in vitro and in vivo studies confirm that chestnut shell extracts maintain antioxidant activity after digestion and can upregulate endogenous antioxidant enzymes, protecting against lipid peroxidation and cellular oxidative stress [149].

Chestnut by-products and their extracts modulate key enzymes involved in ROS metabolism and browning. Treatments with chestnut extracts or by-products enhance activities of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and ascorbate peroxidase (APX), while suppressing polyphenol oxidase (PPO) and phenylalanine ammonia-lyase (PAL), leading to reduced ROS accumulation and browning in plant tissues [181,182,183,184]. These effects are observed in both food preservation (e.g., water chestnut and chestnut storage) and in cell-based antioxidant assays [129,149].

The antioxidant effects of chestnut by-products are attributed to high levels of polyphenols (catechin, gallic acid, ellagic acid, taxifolin) and tannins in chestnut by-products [129,178,179]. These compounds not only scavenge ROS but also modulate cellular signaling pathways, reduce inflammation, and protect against oxidative stress-induced cell death [129,149,179].

Vine by-products contain both enzymatic and non-enzymatic antioxidants. Key enzymatic antioxidants include superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), glutathione reductase (GR), and peroxidases, which directly neutralize ROS such as superoxide and hydrogen peroxide [159,161,172,184,185,186]. Non-enzymatic antioxidants—such as polyphenols, flavonoids, carotenoids, ascorbate, and tocopherols—act as electron donors, reducing ROS to less harmful molecules and supporting enzymatic defenses [159,161,172,184,185].

ROS scavenging enzymes are tightly regulated at transcriptional and post-translational levels, often in response to grapevine environmental stress. Signaling pathways involving mitogen-activated protein kinases (MAPK), calcium, and phytohormones (e.g., abscisic acid, auxin) modulate the expression and activity of these enzymes [159,184,185,186,187,188]. Enhanced enzyme activity is a hallmark of stress-tolerant plant genotypes and is crucial for maintaining redox homeostasis [184,185,186].

Flavonoids and other specialized metabolites in vine by-products not only scavenge ROS but also modulate stress-responsive gene expression, contributing to improved plant resilience and potential health-promoting effects when consumed [161,172,188]. These compounds are upregulated during stress and developmental processes, reinforcing antioxidant defenses.

Although important, ROS-scavenging enzymes may lose activity after processing and digestion. Plant antioxidant enzymes are important in living plants, but they are generally not expected to remain catalytically active after typical food processing and passage through the human gastrointestinal tract [189]. Phenolics remain partly bioaccessible during food processing and passage through the human GI tract, but their antioxidant capacity usually decreases during intestinal digestion, even though some assays (e.g., ORAC) may increase for specific fractions [190,191].

Health effects of vine and chestnut by-products are therefore attributed mainly to non-enzymatic antioxidants rather than direct ROS scavenging by intact plant SOD, CAT, APX, GR, or peroxidases.

4.2. Anti-Inflammatory and Metabolic Effects—Molecular Targets (NF-κB, Cytokines, Glucose/Lipid Metabolism)

Chestnut by-products (leaves, burs, shells, inner skin, flowers) are rich in ellagitannins, flavonoids, and polar lipids, which consistently modulate NF-κB-driven inflammation. Nuclear factor-κB (NF-κB) is a family of transcription factors that activate genes involved in a wide range of biological processes, including immune responses, inflammation, cell growth, and survival [192].

As shown by Cerulli et al. [129] and Pozzoli et al. [111], methanolic extracts of burs and leaves from C. sativa reduced NF-κB activation and NO production in LPS-stimulated THP-1 monocytes, likely via synergy between tannins, other phenolics, and polar lipids. Multiple chestnut by-products (bud, bur, wood, pericarp, episperm) inhibited NF-κB-driven transcription and release of IL-8, CXCL-10, MCP-1, and ICAM in IL-1β/IFN-γ-stimulated Caco-2 intestinal cells [111].

Other authors [193] found that ellagitannins (castalagin, vescalagin) from C. sativa leaves attenuated H. pylori-induced NF-κB signaling and IL-8 release in gastric epithelial cells, thus demonstrating that they can impair inflammatory markers typically elevated during gastritis.

Chestnut inner-shell extract suppressed NF-κB phosphorylation, with downstream reductions in COX-2, iNOS, MMP-9, and type-2 cytokines (IL-4, IL-5, IL-13) in an allergic asthma mouse model [194]. Moreover, flavonoids from chestnut flowers reduced iNOS, COX-2, NO, PGE2, TNF-α, IL-6, IL-1β, and oxidative stress markers in LPS-stimulated macrophages and LPS-induced acute lung injury [195].

Grapevine by-products, including pomace, seeds, and skins, are abundant sources of proanthocyanidins and other polyphenols that effectively modulate NF-κB signaling and its associated pathways. For instance, red grape pomace extracts have been shown to inhibit TNF-α-induced NF-κB activation and subsequent IL-8 production in intestinal reporter cell models. Docking studies suggest a direct interaction between phenolic compounds and the NF-κB–DNA complex [196].

Further investigation of grape seed extracts reveals their capacity to suppress the production of pro-inflammatory cytokines, such as TNF-α and IL-6, and to inhibit iNOS and nitric oxide (NO) production. These extracts also downregulate the phosphorylation of NF-κB and MAPK pathways (including ERK, JNK, and p38) in LPS-stimulated RAW264.7 macrophages [197,198,199].

Notably, wild grape seed procyanidins have been shown to reduce NO, PGE2, reactive oxygen species (ROS), TNF-α, and IL-1β. These compounds also impede the phosphorylation and nuclear translocation of IκBα and NF-κB p65, as well as p38 MAPK activation [200].

Additionally, grape seed proanthocyanidin extract has shown therapeutic promise in alleviating dextran sulfate sodium (DSS)-induced colitis in murine models by reducing colonic levels of TNF-α, IL-6, and IL-1β and increasing IL-10 production through NF-κB inhibition [201].

Lastly, polyphenols derived from white grape pomace have been linked to a reduction in systemic and cardiac levels of TNF-α, IL-6, IL-1β, and iNOS/COX-2-associated damage in myocardial infarction models, mechanisms that are closely associated with the suppression of NF-κB signaling [202,203].

Table 5. Principal inflammatory pathways modulated by chestnut and grapevine by-products. The signal (↓) means that the by-product effect decreases.

	Target/Pathway	By-Products Effects	References
Chestnut	NF-κB (p65, transcription)	Inhibition/phosphorylation reduction	[111,129,193,194,195,204]
	iNOS/NO, COX-2, PGE2	Downregulated, less NO and prostaglandins	[129,194,195,204]
	Cytokines/chemokines	↓ TNF-α, IL-6, IL-1β, IL-4/5/13, IL-8, CXCL-10, MCP-1	[111,193,194,195,204,205]
Grapevine	NF-κB (IκBα, p65)	Inhibited phosphorylation, nuclear translocation	[196,197,198,199,200,201,202,203]
	iNOS/COX-2, NO, PGE2	Downregulated; reduced NO and prostaglandins	[197,198,199,200,201,202,203]
	Cytokines (TNF-α, IL-6, IL-1β, IL-8)	Strongly decreased in serum/tissues/cells	[196,197,198,199,200,201,202,203]

presents the main inflammatory pathways modulated by chestnut and grapevine by-products.

Chestnut by-products have been shown to influence key metabolic processes, including adipogenesis, lipolysis, and digestive enzyme activity. Park et al. [206] demonstrated that inner shell powder from Castanea crenata significantly reduced body, hepatic, and visceral fat mass in high-fat diet mice. This was accompanied by decreases in leptin and LDL cholesterol levels, as well as suppression of adipogenic markers, including Pparγ, C/EBPα, Fabp4, and Fas. Additionally, the study noted the activation of AMPK and ACC pathways.

Liu et al. [207] investigated the effects of polyphenols extracted from Chinese chestnut shells. They found that these compounds improved body weight, lipid profiles, and leptin sensitivity by activating the LEPR–JAK2/STAT3 signaling cascade in the hypothalamus and downregulating key adipogenic factors, including PPARγ, FAS, and LPL, in adipose tissue. Interestingly, earlier work by Youn et al. [208] revealed that extracts from chestnut by-products inhibited adipogenesis in 3T3-L1 cells by downregulating C/EBPβ, C/EBPα, and PPARγ.

Recent studies have also highlighted the potential of supercritical extracts from chestnut shells. Pinto et al. [149] reported that these extracts inhibited α-amylase and lipase activities, suggesting hypoglycemic and hypolipidemic properties. In vivo studies indicated that chestnut shell extracts mitigated elevations in blood glucose and lipids and provided systemic antioxidant protection [149].

Furthermore, Kang et al. [209] found that water chestnut extracts ameliorated hyperlipidemia, non-alcoholic fatty liver disease (NAFLD), and oxidative stress and enhanced AMPK expression in type II diabetic mice.

Multiple grape by-products have demonstrated significant effects on glucose metabolism and lipid homeostasis: (i) Grape seed oils (both red and white) were shown to enhance glucose uptake and glycogenesis in HepG2 cells by up-regulating GLUT2 and GLUT4 transporters, along with key regulatory factors such as HNF1α. Additionally, these oils activated critical signaling pathways involving AMPK, IRS-1, AKT, and PKCζ, facilitating both insulin-dependent and independent translocation of GLUT4 [210]. (ii) In a chronic study involving lean rats, grape seed extract resulted in a marked reduction of serum triglycerides and leptin levels, lowered hepatic cholesterol, and decreased visceral fat accumulation. This was accompanied by AMPK activation, ChREBP inactivation, and a shift toward PPARβ/δ-mediated lipid oxidation [211]. (iii) A polyphenol-rich grape fraction administered to high-fat-fed mice led to significant reductions in adiposity, insulin resistance, hepatic triglyceride levels, and the expression of inflammatory genes associated with white adipose tissue, while also lowering C-reactive protein (CRP) levels. This intervention was correlated with alterations in gut microbiota, notably an increase in Allobaculum. (iv) Grape skin polysaccharides and fresh grape extracts demonstrated inhibitory effects on α-amylase and α-glucosidase activities, suggesting a potential for postprandial glycemic control [212,213]. (v) Extracts from grape seeds and skins, as well as stilbene concentrates, have been shown to improve lipid profiles and enhance the expression of GLUT4 and PPARγ. These extracts also mitigate systemic inflammation in models of metabolic syndrome and diabetes [203,214,215].

4.3. Gut Health and Microbiota Modulation—Prebiotic Effects and Short-Chain Fatty Acid (SCFA) Production

Chestnut by-products have demonstrated significant prebiotic activity and enhance the production of short-chain fatty acids (SCFAs). Several fractions derived from chestnuts serve as effective prebiotic substrates because they resist digestion in the upper gastrointestinal tract, are fermentable by the gut microbiota, and selectively promote the proliferation of beneficial microbial taxa. Non-starch polysaccharides extracted from chestnut kernels (NSPCK) exhibit resistance to both salivary and intestinal enzymatic degradation. They maintain a porous structural integrity, subsequently undergoing fermentation that positively influences lipid profiles and colonic histology in murine models, while simultaneously enriching beneficial bacterial populations and diminishing pathogenic strains [216].

Chestnut non-starch polysaccharides (CNP) have been shown to mitigate DSS-induced colitis in mice by enhancing intestinal tight-junction integrity, elevating antioxidant enzyme activity, decreasing pro-inflammatory cytokine levels, and restoring the richness and balance of microbial communities [217]. Furthermore, chestnut tannic acid derived from C. sativa has been found to increase alpha diversity in weaned piglets, promoting beneficial genera such as Lactobacillus while reducing the presence of potentially pathogenic bacteria, including Prevotella, Slackia, and Collinsella [218], and in mixed human fecal fermentations, extracts of chestnut tannins utilized to enrich food matrices have been shown to enhance the abundance of health-associated genera, such as Akkermansia, Lachnospiraceae, and Ruminococcaceae [219].

Rich extracts from chestnut shells, containing proanthocyanidins and anthocyanins, have been observed to promote beneficial bacterial populations and inhibit pathogens in vitro, supporting their role in gut protection [220].

In Table 6, we summarize the SCFA increments reported for chestnut by-products. These SCFAs are key mediators of prebiotic benefits, supporting epithelial energy metabolism, barrier integrity, and anti-inflammatory signaling [221,222,223,224,225].

Grapevine by-products demonstrate significant prebiotic activity, particularly in enhancing SCFA production, especially when they contain fiber-bound polyphenols. The primary materials under investigation include grape pomace (composed of skins and seeds), stems, canes, and extracts derived from seeds and skins.

Grape pomace is abundant in phenolic compounds and oligosaccharides, many of which can traverse the gastrointestinal tract intact, selectively stimulating beneficial commensals such as Lactobacillus and Bifidobacterium while suppressing various pathogens, thus highlighting its potential as a prebiotic agent [95,226,227].

Enzymatic extracts of pomace containing xylooligosaccharides serve as fermentable substrates for Lactobacillus and Bifidobacterium and remain effective even after simulated digestion. These extracts have also shown to be non-toxic to Caco-2 cells, reinforcing their safety profile [228].

Polyphenol extracts from winery by-products, which include pomace, seeds, stems, canes, and leaves, consistently promote beneficial microbial taxa such as Bacteroides, Eubacterium, Ruminiclostridium, Bifidobacterium, and Lactobacillus, while reducing pathobiont populations in both in vivo and in vitro studies. This modulation of the microbiota aligns well with established prebiotic functionality [95,226,229,230]. Furthermore, a grape pomace seed extract formulated in nanovesicles enhances biofilm formation by Lactobacillus reuteri, indicating its potential utility as a prebiotic [231].

Despite these studies, it remains unclear that most SCFA data are in vitro; human trials rarely quantify SCFAs directly, focusing instead on metabolic or cardiometabolic endpoints [95,226], and the contributions of phenolics versus fiber/oligosaccharides are not fully disentangled [95,226]. Nevertheless, in Figure 2, we illustrate the proposed mechanisms by which chestnut and grapevine by-products are fermented by the gut microbiota, leading to the production of SCFAs, including acetate, propionate, and butyrate. SCFAs interact with the intestinal epithelium and activate G-protein–coupled receptors (GPR41/43) on enteroendocrine L-cells, stimulating the secretion of peptide YY (PYY) and glucagon-like peptide-1 (GLP-1). These signaling pathways contribute to systemic metabolic effects, influencing hepatic lipid metabolism and glucose production; pancreatic β-cell function and insulin secretion; adipose tissue inflammation and energy expenditure; skeletal muscle mitochondrial function and insulin sensitivity; and intestinal gluconeogenesis. Together, these interactions highlight the role of gut microbiota–host crosstalk in mediating the metabolic benefits of plant-derived by-products [40,232,233,234].

Table 6. SCFA increments reported for chestnut and grapevine by-products. The signal (↑) means that the main SCFA effect increases.

	Byproduct	Main SCFA Effects	Model	References
Chestnut	NSPCK polysaccharides	↑ Acetate, propionate, butyrate	Human fecal fermentation	[216]
	Chestnut tannic acid	↑ Acetate, propionate, isobutyrate, valerate, hexanoate; ↑ total SCFA	Piglets	[218]
	Chestnut tannin food extracts	“Booster” of total SCFA; ↑ acetate especially	Human fecal fermentation	[219]
Grapevine	Pinot noir pomace soluble DF	↑ Total SCFA, especially propionate (~53% of total SCFA); also acetate, butyrate	In vitro fecal fermentation	[213]
	Pinot noir pomace insoluble DF	↑ Total SCFA enriches microbiota diversity	In vitro fecal fermentation	[213]
	Enzymatic GP extract (XOS, fiber)	Fermented by Lactobacillus/Bifidobacterium; SCFA and lactic acid production are associated with a pH drop	In vitro fermentation	[226,228]
	Grape seed/skin extracts	↑ SCFA-producing genera (Ruminococcus, Lachnospiraceae NK4A136; Faecalibaculum—butyrate/lactate producer) in rats	In vivo	[229]

Table 6. SCFA increments reported for chestnut and grapevine by-products. The signal (↑) means that the main SCFA effect increases.

	Byproduct	Main SCFA Effects	Model	References
Chestnut	NSPCK polysaccharides	↑ Acetate, propionate, butyrate	Human fecal fermentation	[216]
	Chestnut tannic acid	↑ Acetate, propionate, isobutyrate, valerate, hexanoate; ↑ total SCFA	Piglets	[218]
	Chestnut tannin food extracts	“Booster” of total SCFA; ↑ acetate especially	Human fecal fermentation	[219]
Grapevine	Pinot noir pomace soluble DF	↑ Total SCFA, especially propionate (~53% of total SCFA); also acetate, butyrate	In vitro fecal fermentation	[213]
	Pinot noir pomace insoluble DF	↑ Total SCFA enriches microbiota diversity	In vitro fecal fermentation	[213]
	Enzymatic GP extract (XOS, fiber)	Fermented by Lactobacillus/Bifidobacterium; SCFA and lactic acid production are associated with a pH drop	In vitro fermentation	[226,228]
	Grape seed/skin extracts	↑ SCFA-producing genera (Ruminococcus, Lachnospiraceae NK4A136; Faecalibaculum—butyrate/lactate producer) in rats	In vivo	[229]

5. Applications in Food and Nutraceuticals

5.1. Functional Food Development—Incorporation in Bakery, Snacks, and Beverages

Chestnut, grape, and vine by-products have attracted growing interest due to their high phytochemical and other bioactive compound content, which may confer health benefits [19,75,76,236], making them promising sources of natural food ingredients. Chestnut by-products have been used in functional infusions and plant-based beverages [111], whereas grape by-products—skins, seeds, and stems—have been incorporated into juices, smoothies, teas, and fermented drinks, thereby enhancing phenolic content and antioxidant activity [237]. Moreover, grape pomace can be fermented to produce cellulose, hydrolytic enzymes, and exopolysaccharide (exo-PG), which are used in the food, paper, and pulp industries, as well as in winemaking [238]. Incorporating these by-products into commonly consumed foods offers a practical strategy to improve dietary quality, support circular economy principles, reduce waste, and generate value-added functional ingredients. Their use in bakery products, snacks, and beverages represents a scalable approach to functional food development, integrating nutritional enhancement with environmental sustainability [239]. The abundant polyphenols and bioactive phytochemicals in these materials have been linked to reduced oxidative stress and inflammation, as well as protective effects against chronic diseases, including cardiovascular disorders, type 2 diabetes, and certain cancers [240]. These attributes make them attractive as nutraceutical ingredients, whether in concentrated extracts or powders, for incorporation into a range of food products.

5.2. Nutraceutical Formulations—Extracts, Encapsulation, and Delivery Systems

The increasing interest in chestnut and vine by-products as sources of bioactive compounds has driven the development of nutraceutical formulations to improve stability, bioavailability, and targeted delivery. However, direct application is often constrained by low aqueous solubility, chemical instability, and susceptibility to degradation during food processing and gastrointestinal digestion [241], highlighting the need for advanced formulation strategies. Extract-based formulations remain the most widely adopted nutraceutical approach for chestnut and vine by-products. Aqueous, hydroalcoholic, and, increasingly, green extraction techniques—such as ultrasound-assisted, microwave-assisted, and pressurized liquid extraction—are employed to concentrate phenolic compounds while reducing solvent consumption and environmental impact [213,242].

Encapsulation technologies have become essential for enhancing the stability, functionality, and applicability of bioactive compounds. Micro- and nanoencapsulation approaches—including spray drying, freeze drying, coacervation, and lipid-based systems—have been extensively investigated for their ability to protect sensitive phenolic compounds from oxidation, light, and thermal degradation [213,243,244]. Beyond stabilization, encapsulation can mitigate undesirable sensory attributes, such as bitterness and astringency, commonly associated with phenolic-rich extracts [245,246]. Advanced delivery systems are also being explored to enhance the bioaccessibility and bioavailability of phenolic compounds from chestnut [246] and grapevine by-products [247,248]. In this context, lipid-based carriers, including nanoemulsions and liposomes, have shown particular promise in improving intestinal absorption of poorly soluble compounds, such as resveratrol and other grape-derived polyphenols [247,249]. Complementarily, biopolymer-based nanoparticles and hybrid systems are being developed to enable controlled release and site-specific delivery along the gastrointestinal tract [250]. Collectively, these strategies help bridge the gap between in vitro bioactivity and demonstrable in vivo health effects. From an industrial perspective, the choice of extraction, encapsulation, and delivery methods requires careful consideration of efficacy, scalability, cost, and regulatory compliance.

5.3. Natural Preservatives and Colorants—Use in Food Preservation and Shelf-Life Extension

Grape and chestnut by-products—including pomace, seeds, stems, shells, and leaves—exhibit potent antimicrobial activity against bacteria, yeasts, and molds, effectively inhibiting microbial growth, delaying oxidation, and preserving sensory quality in food systems [13,251,252,253,254]. These extracts can be applied directly, incorporated into edible coatings, or embedded in active packaging to enhance shelf life and safety [255,256]. Grape skins are rich in anthocyanins, providing natural red-to-purple coloration, while chestnut-derived extracts contribute brown to amber hues due to their high tannin and phenolic content [257,258,259]. Beyond aesthetics, these pigments exhibit antioxidant activity and function as functional colorants with technological and health-promoting benefits. Encapsulation can further improve stability and control release, although formulation challenges—including off-flavors, color changes, and matrix interactions—remain. The dual role of these by-products as preservatives and colorants enables the development of multi-functional, value-added ingredients that support food safety, sensory quality, and sustainability by valorizing agro-industrial residues and reducing reliance on synthetic additives. Future research should optimize extraction and processing, systematically assess sensory impacts, and address regulatory considerations to maximize industrial adoption.

5.4. Safety and Regulatory Considerations—Toxicological Data and European/FAO Guidelines

Although chestnut and grapevine by-products are generally considered safe due to their long history of dietary use, a comprehensive assessment of their bioactive composition, toxicological profile, and regulatory status is essential to support large-scale industrial applications [260]. Current evidence from in vitro assays and animal studies indicates low acute toxicity, with no adverse effects observed at doses relevant to functional foods and dietary supplements [178,261]. Nevertheless, a thorough safety evaluation remains crucial to mitigate potential risks. For nutritional and nutraceutical applications, it is therefore imperative to establish both the bioactivity and safety of these by-products and their extracts prior to consumption.

The European Food Safety Authority (EFSA) regulates the evaluation of concentrated by-product extracts, classifying them as either conventional food ingredients or novel foods based on their formulation and documented history of consumption [262]. The industrial application of plant-derived extracts necessitates rigorous standardization, routine batch testing, and accurate labeling of bioactive constituents, supported by controlled sourcing and optimized processing protocols. For instance, quality control and standardization procedures applied to grape polyphenol dietary supplements have demonstrated consistent polyphenol content and batch uniformity using validated analytical methods, providing a robust foundation for reliable industrial production and regulatory compliance [260].

6. Sustainability and Circular Bioeconomy Perspective

The increasing demand for sustainable agri-food systems has intensified interest in circular bioeconomy strategies aimed at reducing waste, optimising resource use, and generating added value from agro-industrial by-products. In this context, chestnut and vine by-products constitute exemplary cases of underutilised biomass streams that can be successfully integrated into sustainable food systems. Their valorisation aligns with global sustainability agendas, including the European Green Deal, the Farm to Fork Strategy, and the United Nations Sustainable Development Goals (SDGs), particularly those related to responsible consumption and production, climate action, and rural development.

6.1. Waste Valorization—Integration into Sustainable Food Systems

Waste valorisation represents a fundamental pillar of the circular bioeconomy, aiming to convert agro-industrial residues into value-added products while minimising environmental burdens and maximising resource efficiency. In recent years, chestnut and grapevine by-products have emerged as paradigmatic examples of underexploited biomass streams with high potential for integration into sustainable food systems, e.g., in [179,263,264,265]. Traditionally regarded as waste or low-value materials, residues such as chestnut shells, burs, leaves, grape pomace, skins, seeds, stems, and vine shoots are now recognised as rich sources of bioactive compounds, including polyphenols, dietary fibres, vitamins, and other functional constituents [107,236].

The integration of these by-products into sustainable food systems requires a systemic approach encompassing selective recovery, stabilisation, processing, and formulation stages [266,267]. Selective collection and pre-treatment are critical to preserving bioactive integrity and preventing microbial spoilage or oxidative degradation, particularly in highly perishable matrices such as grape pomace [268]. Within this framework, green extraction and processing technologies play a central role in enabling the efficient recovery of high-value compounds while adhering to environmental sustainability principles [269].

Eco-friendly extraction methods, including aqueous extraction, ultrasound-assisted extraction, microwave-assisted extraction, pressurised liquid extraction, and enzyme-assisted treatments, have demonstrated considerable potential to enhance extraction yields, reduce processing times, and minimise the use of organic solvents and energy inputs [270]. These technologies are particularly suitable for chestnut shells and burs, which are characterised by dense lignocellulosic matrices, as well as for grape pomace and vine shoots, where polyphenols and fibres are often bound to cell wall components [107,271,272]. Enzymatic treatments, in particular, facilitate the release of bound phenolics and improve fibre functionality, thereby increasing both bioaccessibility and technological applicability [273,274].

From a food system perspective, the incorporation of chestnut- and vine-derived ingredients into functional foods contributes to the development of shorter, more resilient, and locally anchored value chains. By valorising regionally available by-products, these strategies reduce dependence on imported raw materials and synthetic additives, while fostering the production of clean-label foods enriched with natural antioxidants, dietary fibres, and nutraceutical ingredients [29]. This approach aligns with current consumer trends favouring transparency, sustainability, and naturalness, as well as with regulatory incentives promoting the use of natural food ingredients.

Importantly, waste valorisation strategies also facilitate the reintegration of nutrients and bioactives into the food system, contributing to the closure of material and nutrient loops [275]. Fibre-bound polyphenols and other non-extractable bioactives, often overlooked in conventional processing, can be retained in food formulations, thereby enhancing nutritional density and supporting gut health through microbial fermentation [276,277]. In this sense, chestnut and vine by-products serve not only as sources of isolated compounds but also as multifunctional food ingredients that deliver combined antioxidant, prebiotic, and metabolic benefits.

Overall, the valorisation of chestnut and grapevine by-products within sustainable food systems exemplifies a shift from linear waste management towards circular, resource-efficient models. By integrating green processing technologies, functional ingredient development, and clean-label food innovation, these strategies support environmental sustainability, economic viability, and nutritional enhancement, reinforcing the role of agro-industrial by-products as strategic resources in future food systems.

6.2. Life Cycle and Environmental Benefits—Reduction in Agricultural Waste

From an environmental perspective, the valorisation of chestnut and vine by-products offers substantial life cycle advantages by significantly reducing agricultural waste and mitigating associated environmental burdens. Large quantities of residues are generated annually during harvesting, post-harvest handling, and processing stages. When inadequately managed, these residues may contribute to greenhouse gas emissions through uncontrolled decomposition, promote soil degradation, and create favourable conditions for the proliferation of pests and phytopathogens, ultimately compromising crop health and productivity [278,279].

Life cycle assessment (LCA) studies have consistently demonstrated that converting agro-industrial residues into functional ingredients or bioactive-rich extracts can markedly reduce the environmental footprint of food production systems [280]. These benefits arise from multiple mechanisms, including avoiding waste-disposal impacts, reducing demand for virgin raw materials, and substituting synthetic additives with naturally derived compounds [281,282]. In grapevine systems, for instance, the valorisation of grape pomace through the recovery of polyphenols, dietary fibres, and seed oils has been associated with measurable reductions in carbon footprint, cumulative energy demand, and water use when compared with conventional waste management practices such as composting, landfilling, or incineration [283,284].

Moreover, grapevine by-product valorisation supports a cascading use of biomass, in which high-value compounds are recovered first, followed by the use of residual fractions for energy production or soil amendment. This cascading approach is widely recognised as a best practice within circular bioeconomy frameworks, as it maximises resource efficiency while minimising environmental impacts across the entire life cycle [285,286]. The recovery of fibre-rich fractions, in particular, contributes to carbon sequestration potential and reduces reliance on externally sourced fibre ingredients [287].

Similarly, the valorisation of chestnut residues contributes to improved orchard and forest management practices. The removal and controlled processing of shells, burs, and pruning residues reduce the accumulation of plant debris that may serve as reservoirs for pests and diseases, thereby lowering phytosanitary risks [33,288,289]. This effect can indirectly reduce the need for chemical pesticides and fungicides, thereby contributing to more environmentally sustainable agricultural systems and to reduced chemical inputs. Additionally, the transformation of chestnut by-products into functional ingredients or bio-based materials enhances the overall efficiency of biomass utilisation, ensuring that a greater proportion of the harvested resource contributes to value creation rather than waste generation [179,290].

Overall, the environmental benefits of valorising chestnut and vine by-products extend beyond waste reduction alone. By integrating these residues into circular value chains, it is possible to reduce emissions, conserve natural resources, improve land management practices, and enhance the sustainability performance of agri-food systems. These outcomes strongly support the strategic role of by-product valorisation as a key lever for reducing environmental impacts and advancing climate-smart and resource-efficient food production models.

6.3. Economic and Societal Impacts—Potential for Rural Development and Eco-Innovation

Beyond the environmental dimension, the valorisation of chestnut and vine by-products entails substantial economic and societal benefits, particularly for rural and wine-producing regions where agricultural activity remains a key driver of local livelihoods. These territories are often characterised by fragmented production structures, seasonal employment, and limited economic diversification, which increases their vulnerability to market fluctuations, price volatility, and the growing impacts of climate change [291,292]. In this context, by-product valorisation presents a strategic opportunity to enhance the resilience and competitiveness of local agri-food systems.

The development of value-added products from agro-industrial residues has the potential to stimulate rural economies by creating new income streams, strengthening small and medium-sized enterprises (SMEs), and promoting locally embedded bio-based industries. Chestnut- and grape-derived nutraceuticals, functional food ingredients, bioactive extracts, and natural additives provide opportunities for product differentiation, market diversification, and access to high-value segments, including health-oriented, organic, and premium markets [179,293,294]. Importantly, these activities can be developed in close proximity to production sites, reducing logistical costs and fostering territorial value chains that retain economic value within rural areas.

Eco-innovation plays a pivotal role in enabling this transition, encompassing not only technological advances but also organisational and social innovations that collectively enhance sustainability performance. The adoption of circular business models—such as cooperative processing units, shared biorefineries, and integrated agri-food clusters—facilitates the efficient use of biomass and lowers entry barriers for small producers [295]. Concurrently, advances in green extraction technologies, process intensification, and modular biorefinery concepts enable the cascading use of chestnut and vine biomass, maximising economic returns from multiple product streams while minimising environmental impacts [296].

Beyond direct economic gains, the valorisation of chestnut and vine by-products contributes to broader societal benefits, including knowledge transfer, skills development, and capacity-building for innovation in rural communities. The implementation of circular valorisation strategies often requires interdisciplinary collaboration between farmers, researchers, technology providers, and policymakers, fostering learning processes and strengthening social networks [297]. These dynamics can enhance social cohesion and empower local actors, particularly in marginal or less-favoured agricultural regions.

Ultimately, the integration of chestnut and vine by-products into a circular bioeconomy framework supports a transition towards more resilient, inclusive, and sustainable agri-food systems. By aligning environmental stewardship with economic viability and societal well-being, the valorisation of these agro-industrial residues exemplifies how waste streams can be transformed into strategic resources for sustainable development, contributing simultaneously to rural revitalisation, eco-innovation, and long-term food system resilience.

7. Conclusions

The valorisation of chestnut and grapevine by-products through the recovery of bioactive compounds represents an effective strategy for advancing sustainable agri-food systems within a circular bioeconomy framework. The conversion of these residues into functional ingredients contributes to waste valorisation by reducing the environmental impacts associated with by-product disposal and promoting resource efficiency across the agri-food chain.

A key contribution of this review is the integrated analysis of chestnut and grapevine by-products within a common mechanistic and biotransformation-oriented framework, linking (i) chemical composition, (ii) gastrointestinal bioaccessibility, (iii) microbial metabolism, and (iv) biological activity. By jointly considering these matrices, this work highlights how polyphenol–fibre associations influence colonic delivery, microbial biotransformation, and the modulation of redox, inflammatory, and metabolic pathways. This integrative perspective helps bridge the gap between compositional studies and functional outcomes, supporting a more realistic evaluation of nutraceutical potential.

Chestnut by-products, including shells, burs, and leaves, are characterised by lignocellulosic matrices enriched in phenolic compounds and fibre-associated bioactives, conferring strong antioxidant activity via both direct reactive oxygen species scavenging and modulation of redox- and browning-related enzymes. In vitro and in vivo evidence indicates their capacity to modulate inflammatory and metabolic pathways, particularly NF-κB-centred signalling, cytokine production, and key regulators of lipid and glucose metabolism. In addition, chestnut fibres and fibre-bound phenolics act as fermentable substrates in the colon, promoting beneficial shifts in the gut microbiota and increased short-chain fatty acid production.

Grapevine by-products, notably grape pomace, skins, seeds, and stems, retain high levels of anthocyanins, flavan-3-ols, proanthocyanidins, flavonols, phenolic acids, and stilbenes. These compounds exhibit pronounced antioxidant and anti-inflammatory effects through both direct radical scavenging and regulation of NF-κB/MAPK signalling pathways and metabolic targets involved in glucose and lipid homeostasis. The strong association between polyphenols and dietary fibre in grape pomace further enhances colonic delivery, microbial biotransformation, and prebiotic potential.

However, several limitations and research gaps remain. Much of the available evidence derives from in vitro models or animal studies, with a limited number of well-controlled human trials. Variability in raw material composition arising from cultivar, geographical origin, processing conditions, and extraction methods hampers inter-study comparisons and limits standardisation. Moreover, the bioavailability and metabolic fate of many phenolic metabolites remain insufficiently characterised, and dose–response relationships relevant to realistic dietary intake are often unclear. Long-term safety data, potential interactions with the food matrix or pharmaceuticals, and regulatory aspects of by–product–derived ingredients also require further investigation.

Importantly, the biological efficacy of chestnut and grapevine by-products depends not only on chemical composition but also on bioaccessibility, gastrointestinal stability, and microbial metabolism. The integration of advanced analytical techniques with harmonised in vitro digestion models, omics-based approaches, and well-designed human intervention studies is therefore essential to establish robust composition–function relationships.

Overall, chestnut and grapevine by-products emerge as multifunctional and sustainable resources with significant potential for food and nutraceutical applications. Future research should prioritise standardisation of raw materials and extracts, clarification of bioavailability and metabolite profiles, long-term safety assessment, and clinically relevant trials, which are critical steps for industrial-scale implementation and regulatory approval of functional ingredients derived from agro-industrial by-products.