Biological Function of Melatonin in the Gut and Its Systemic Effects in Swine Production: A Review

Abstract

Melatonin, an indoleamine hormone, not only regulates circadian rhythms but also exhibits antioxidant, anti-inflammatory, neuroprotective, and metabolic regulatory properties. It has attracted significant global research attention due to its well-documented potential in cancer prevention, anti-aging effects, alleviating insomnia, and mitigating metabolic disorders. In recent years, the functional significance of intestinal melatonin has emerged as a focal point, driven by the discovery that its concentration in the gut exceeds that found in the pineal gland by at least 400-fold. In addition, melatonin has been widely studied in animal husbandry for its potential to enhance growth performance, improve reproductive outcomes, and maintain intestinal homeostasis. This review article summarizes the localization, synthesis, and biological functions of melatonin in the gut, along with the latest research advances and their application in swine production. This review is expected to serve as a reference for the potential application of melatonin as an experimental nutritional intervention in livestock production and to outline future research directions.

1. Introduction

Melatonin (N-acetyl-5-methoxytryptamine) is recognized as a ubiquitous hormone found in animals, plants, and microbes [1]. In the United States, melatonin is classified as a dietary supplement by the Food and Drug Administration (FDA). In animals, melatonin was first detected in the bovine pineal gland in 1958 [2]. Extensive research conducted over the past six decades has established that melatonin plays a pivotal role in regulating diverse physiological functions, including biological rhythms [3,4], immune response modulation [5,6], reactive oxygen species (ROS) clearance [7,8], and energy metabolism homeostasis [9]. In addition, emerging evidence suggests that melatonin may possess therapeutic potential in inhibiting or delaying the development of various diseases, such as gastro-intestinal diseases, cardiovascular disorders, neurodegenerative diseases, and cancer [10,11]. Melatonin is mainly secreted by the pineal gland, a process regulated by the light/dark cycle of the suprachiasmatic nucleus. However, it is also produced by other organs, including the retina [12], skin [13], placenta [14], thymus [15], and gastrointestinal tract (GIT) [16,17]. The intestine serves not only as a crucial site for nutrient digestion and absorption, but also functions as the largest immune organ within the animal organism [18]. Notably, melatonin concentration in the rat gut (upper duodenum) is reported to be up to 400 times higher than that in the pineal gland [19], strongly suggesting that melatonin plays a critical role in maintaining gut homeostasis. Furthermore, studies indicate that insufficient melatonin levels are closely associated with the progression of gastrointestinal disorders [20,21]. Therefore, elucidating the functional mechanisms and roles of melatonin within the gut is imperative for animal growth and health.

Recent studies in animal production have explored the nutritional regulatory effects of dietary melatonin supplementation in various livestock species, including swine [22,23,24], poultry [25,26], ruminants [27], and aquatic animals [28,29]. In particular, research findings in swine production have demonstrated that melatonin exhibits therapeutic potential in mitigating oxidative stress and inflammation induced by pregnancy [23,30], mycotoxin contamination [22], pesticide exposure [24], and bacterial infections [31]. In this review, we summarize the localization, synthesis, and biological functions of melatonin in the gut, along with recent research progress on its application in swine production. The literature cited in this review was identified through comprehensive searches in electronic databases including PubMed, Web of Science, and Google Scholar, covering publications primarily from 1990 to 2025. Key search terms included “melatonin”, “gut”, “swine/pig”, “intestinal health”, “antioxidant”, and “microbiota”. This review synthesizes evidence from studies in swine, as well as from rodent, human, and cell culture models where swine-specific data are limited, to provide a comprehensive mechanistic basis for understanding melatonin’s biological functions in the gut. These insights aim to provide essential references for advancing melatonin research and its practical implementation in animal husbandry.

2. Localization and Synthesis of Melatonin in the Gut

2.1. Melatonin Localization

Autoradiographic analysis of the distribution of melatonin binding sites in the duck intestinal wall revealed that maximal binding was localized to the mucosa and intestinal villi [32]. In subcellular distribution analyses, the maximum melatonin binding was detected in the nuclear fraction, followed by the microsomal, mitochondrial, and cytosolic fractions [33,34]. Enteroendocrine cells, particularly enterochromaffin cells in the intestinal mucosa, are the primary source of melatonin in the gut [35]. However, species-specific regional variations exist in melatonin distribution along the mammalian GIT. In rats, melatonin levels were observed to be significantly elevated in the jejunal and ileal regions compared to other segments of the intestinal tract [17]. Pigs exhibit markedly elevated melatonin levels in the cecum and colon, while cows show no anterior–posterior concentration differences [36].

2.2. Melatonin Synthesis

No photoperiod-dependent cyclical melatonin secretion was observed in the gut, contrasting with the typical circadian secretion pattern of melatonin exhibited by the pineal gland [37]. Melatonin concentrations in the gut are independent of the pineal gland production, as pinealectomy has no effect on intestinal melatonin concentrations in rats and pigs [17,38]. Furthermore, the concept of GIT melatonin production was further confirmed following the identification of melatonin-synthesizing enzymes in the rat gut [39].

Previous studies have demonstrated that dietary tryptophan can increase plasma melatonin levels [40,41]. Studies in rats and chicks have shown that the tryptophan-induced increase in circulating melatonin levels remains unaffected by pinealectomy; however, this increase was almost abolished by partial ligation of the portal vein, further suggesting independent synthesis and secretion of melatonin in the gut [42]. The biosynthesis of melatonin in enterochromaffin cells initiates through the tryptophan metabolic pathway [43]. First, tryptophan undergoes hydroxylation catalyzed by tryptophan hydroxylase (TPH), generating the intermediate 5-hydroxytryptophan (5-HTP). Subsequently, aromatic L-amino acid decarboxylase (AADC) converts 5-HTP into the serotonin (5-hydroxytryptamine, 5-HT) via decarboxylation. 5-HT undergoes sequential enzymatic modifications: arylalkylamine N-acetyltransferase (AANAT) first catalyzes the acetylation of serotonin to generate N-acetylserotonin (NAS), which is subsequently O-methylated by acetylserotonin O-methyltransferase (ASMT) to form melatonin (Figure 1).

Mitochondria are proposed to be major sites of melatonin synthesis [44,45]. The melatonin synthetic enzymes and melatonin membrane receptors were also identified in mitochondria [46]. Melatonin exerts multiple important functions at the mitochondrial level, such as modulating mitochondrial metabolism, promoting mitochondrial fusion, and alleviating mitochondrial oxidative stress [47]. In dysfunctional mitochondria, melatonin production is compromised. Under pathological conditions, such as in tumor cells, the metabolic shift from mitochondrial glucose oxidation to glycolysis results in decreased acetyl coenzyme A levels [48]. As acetyl coenzyme A serves as a critical substrate for the AANAT enzyme involved in mitochondrial melatonin synthesis, its reduction may plausibly account for diminished intracellular melatonin levels [49,50]. Given that mitochondria are the major resources of ROS generation, melatonin synthesis in mitochondria is thought to play a critical role in maintaining mitochondrial homeostasis.

2.3. Regulatory Effects of Gut Microbiota on Melatonin Production

The gut microbiota plays an important role in regulating tryptophan metabolism. Recent evidence suggests that intestinal microbiota and related metabolites could be another critical factor influencing the synthesis and secretion of melatonin in the gut [51]. Both gut microbial dysbiosis and germ-free conditions have been demonstrated to alter local and systemic melatonin levels in mice [20,52]. Puerariae Lobatae Radix-resistant starch has been reported to induce the gut microbiota to generate melatonin, which protected against ischemic stroke in rats [53]. Roseburia hominis enhances melatonin synthesis in the rat intestine and pheochromocytoma cell line BON-1 via its metabolites, propionate and butyrate, by activating the AANAT pathway through phosphorylation of the cAMP-response element-binding protein (CREB) [54]. Liu et al. (2024) revealed that gut microbiota-mediated activation of the TLR2/4-MyD88-NF-κB signaling cascade in colonic epithelial cells directly enhanced AANAT expression, thereby increasing colonic melatonin production [55]. Additionally, gut microbiota-derived short-chain fatty acids (e.g., butyrate) and bacterial tryptophan-derived indole metabolites enhance 5-HT synthesis [56], functioning as critical precursors for melatonin production. This mechanism is further validated in a sow model, where dietary fiber supplementation enriches SCFA-producing microbiota, elevates 5-HT concentrations in colonic mucosa and serum, and upregulates colonic TPH1 mRNA expression [57]. An in vitro study also reported that butyrate increased melatonin expression in BON-1 cells in a dose-dependent manner [58]. However, the mechanisms underlying the involvement of intestinal microbiota in melatonin biosynthesis remain to be fully elucidated.

3. Melatonin Receptors in the Gut

Melatonin exerts its effects through its membrane and nuclear receptors, as well as receptor-independent pathways [5]. Melatonin receptors (MT), particularly the G protein-coupled receptor subtypes MT1 and MT2 (encoded by the genes MTNR1A and MTNR1B, respectively), are widely distributed throughout the GIT and mediate diverse physiological functions through their specific interactions [16]. Another melatonin binding site, MT3, is a cytosolic enzyme called quinone reductase 2, which exhibits detoxifying properties [59]. Furthermore, melatonin can also bind to nuclear receptors of the retinoid Z receptor (RZR) subfamily and the retinoid-related orphan receptor (ROR) subfamily, which includes three subtypes (α, β, γ) [16]. It should be noted, however, that the evidence for direct, high-affinity binding of melatonin to these nuclear receptors remains inconclusive. Emerging data have challenged the view that melatonin acts as a direct ligand for ROR receptors, suggesting that melatonin may not be a physiological ligand for this nuclear receptor subfamily [60,61]. Although the nuclear receptors as melatonin receptors remains controversial, melatonin has been shown to regulate the transcriptional activity of ROR, at least via indirect action [60]. Early studies also suggested that nuclear RZR/RORα receptor is involved in the antitumor effect of melatonin on mouse colon cancer [62], but whether this reflects direct ligand–receptor interaction or indirect signaling crosstalk is still debated. As such, while nuclear receptors may contribute to melatonin’s biological effects, their role as direct physiological targets of melatonin requires further experimental validation.

Melatonin receptor expression exhibits region-specific distribution patterns along the GIT, with notable species-specific variations. In the duck gut, melatonin binding site densities varied significantly, following the descending order: ileum and jejunum > duodenum and colon > cecum > esophagus [32]. Western blotting results showed that MT2 receptor protein levels in the rat colon were higher than those in the duodenum and pancreas, with no evidence of circadian variation in any of these tissues [63]. Interestingly, short-term fasting increased MTNR1A mRNA levels in the subepithelial layer of both the rat small and large intestines, whereas prolonged fasting maintained elevated MTNR1A expression exclusively in the distal colon, with levels returning to normal in the small intestinal segments [64]. In addition, emerging evidence further links melatonin receptors to interactions with gut microbiota. Fecal microbiota transplantation in germ-free rats induced a significant increase in colonic melatonin receptor expression, an effect likely mediated by short-chain fatty acids (SCFAs) [65]. Another study conducted in zebrafish larvae revealed that Lactobacillus rhamnosus supplementation increased the abundance of melatonin receptors [66].

4. Biological Functions of Melatonin in the Gut

Melatonin, as a highly lipophilic molecule, can penetrate various biological barriers and target subcellular compartments such as mitochondria and nuclei. Its mechanisms of action involve both direct molecular interactions and receptor-mediated signaling pathways. Given its dual intracellular/extracellular presence and multi-receptor signaling, melatonin exhibits multifaceted regulatory roles in the gut [16,67].

4.1. Antioxidant Properties of Melatonin

Maintaining homeostatic balance between free radical generation and elimination is critical for host health, as disruption of this equilibrium triggers oxidative stress [68]. Oxidative stress within the gut significantly impairs the structural and functional integrity of the intestinal mucosal barrier. Mechanistically, mitochondrial DNA (mtDNA) damage in intestinal epithelial cells induces aberrant generation of ROS through electron transport chain uncoupling, thereby exacerbating mitochondrial bioenergetic dysfunction [69]. Melatonin acts as a powerful antioxidant through its ability to either directly neutralize ROS or increase the activity of antioxidant enzymes [70]. Melatonin has been demonstrated to be a highly effective scavenger of free radicals and reactive oxygen intermediates, including nitric oxide, hydroxyl radicals, singlet oxygen, and peroxynitrite [71]. In addition, numerous studies have demonstrated that melatonin enhances the activity of endogenous antioxidant enzymes, such as superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GSH-Px), which form the core enzymatic defense system against oxidative stress [72,73,74]. Melatonin also suppresses pro-oxidative enzymes, further promoting a favorable redox balance in the gut [75]. Furthermore, the interactions of melatonin with ROS generate several metabolites, such as N-acetyl-N-formyl-5-methoxykynuramine (AFMK) and N-acetyl-5-methoxykynuramine (AMK), which are recognized to exhibit even stronger radical-scavenging effects [47]. Evidence in swine: strong.

4.2. Anti-Inflammatory Properties of Melatonin

Melatonin plays a complex and multifaceted role as a modulator of the immune system. Inflammation, a series of cellular and molecular events, serves as a defense mechanism against infection. Accumulating evidence has demonstrated that melatonin is involved in the treatment of inflammation-related gut diseases. Many anti-inflammatory functions of melatonin in the gut can be attributed to its inhibition of the NF-κB pathway [20,72]. Cyclooxygenase (COX) is the rate-limiting enzyme for synthesizing inflammatory mediators, with COX-2 being particularly important in inflammation through its production of prostaglandins. In rat models of colitis induced by 2,4,6-trinitrobenzene sulfonic acid (TNBS), melatonin exerted beneficial effects by reducing colonic prostaglandin E2 (PGE2) and nitric oxide (NO) levels, as well as downregulating the expression of inducible nitric oxide synthase (iNOS) and COX-2 in colonic mucosa [76]. In a similar study, melatonin inhibited COX-2 expression and NF-κB activation in IL-1β-stimulated Caco-2 cells [6]. In addition, melatonin improved intestinal Th17/Treg balance through activation of the AMPK/SIRT1 pathway in necrotizing enterocolitis mouse model [77]. Esposito et al. (2008) also demonstrated that melatonin prevented colitis in rats by reducing the activities of matrix metalloproteinases (MMP)-9 and MMP-2, an effect associated with the inhibition of tumor necrosis factor-α (TNF-α) production [78]. Xiong et al. (2022) reported that melatonin suppressed the activation of the NLRP3 inflammasome and TLR4 signaling pathway in neonatal mice with necrotizing enterocolitis [79]. Collectively, these studies highlight melatonin’s ability to target multiple inflammatory pathways, suggesting its potential as a therapeutic strategy for gut inflammation-related disorders.

Recent studies have demonstrated that disruption of host–microbial interactions is associated with intestinal inflammation [80,81]. The proliferation of harmful bacteria and their subsequent secretion of enterotoxins in the GIT can compromise the intestinal mucosal barrier and trigger inflammatory responses [82,83]. In vivo studies revealed that melatonin combated bacterial infections through multiple molecular pathways, including modulation of NF-κB signaling, regulation of TLR2/4 receptors, and scavenging of ROS [84]. Notably, emerging evidence suggests that melatonin’s protective effects may be partially mediated through its interactions with gut microbiota [5]. However, further investigations are required to elucidate the mechanisms and actions of melatonin in gut–microbiome–immune axis. Evidence in swine: moderate.

4.3. Microbial Regulation

Melatonin and gut microbiota exhibit a complex bidirectional relationship characterized by mutual regulatory mechanisms. Emerging evidence reveals that gut microbiota actively regulate melatonin synthesis within the GIT (Section 2.3), while melatonin conversely exerts significant influence on microbial composition and ecological balance [85].

Accumulating evidence has demonstrated that melatonin exerts regulatory effects on gut microbiota composition, both under normal physiological conditions and across diverse pathological conditions. Exogenous melatonin administration has been shown to enhance the relative abundances of Bacteroides, Alistipes [86], and Akkermansia [87], while reducing the Firmicutes/Bacteroidetes ratio in mice fed high-fat diets [87]. Liu et al. (2022) reported that melatonin ameliorated aflatoxin B1-induced intestinal microbiota dysbiosis in mice by reducing the relative abundances of Desulfovibrio, Clostridium_XIVa, and Lactobacillus [88]. Additionally, in other mouse models of stress such as weaning stress [89], heat stress [90], and sleep deprivation [20], exogenous melatonin administration has been shown to enhance intestinal homeostasis by promoting beneficial bacterial proliferation and suppressing pathogenic colonization. Moreover, recent studies have manipulated endogenous melatonin production through overexpression or knockdown of rate-limiting enzymes in melatonin synthesis, to investigate its potential influence on gut microbiota composition. Li et al. (2021) reported that ASMT-overexpressing sheep had a significantly lower abundance of microbial genes associated with infectious diseases [91]. In contrast, in AANAT-knockout mice with endogenous melatonin deficiency, the animals exhibited gut microbiota dysbiosis accompanied by increased intestinal permeability and systemic inflammation [92].

Melatonin not only influences intestinal microbiota composition but also regulates its rhythmic variations. Recently, a study demonstrated that the swarming activity of Enterobacter aerogenes increased in response to melatonin, exhibiting a rhythmic expression pattern with an approximately 24 h period [93]. Another study indicated that a high-fat diet in mice disrupted circadian rhythms in intestinal bacteria, while melatonin administration restored diurnal rhythmicity of some specific microbiota [94]. Undoubtedly, the intricate interplay between intestinal microbial communities and gut melatonin represents a fascinating field to be explored. Evidence in swine: moderate.

4.4. Mitochondrial Regulation

The primary role of mitochondria is to generate adenosine triphosphate (ATP) for mammalian cells through oxidative phosphorylation. The intestinal epithelium exhibits a substantial demand for ATP to maintain gut homeostasis and function [95]. Mitochondrial dysfunction is associated with a variety of gastrointestinal disorders, such as inflammatory bowel disease (IBD), irritable bowel syndrome (IBS), and colorectal cancer [96]. Mitochondria are the primary sites of melatonin synthesis, which in turn is crucial for maintaining normal mitochondrial function [97]. Melatonin plays a crucial role in protecting mitochondria through several mechanisms, such as alleviating mitochondrial oxidative stress [98], promoting mitochondrial dynamics and mitophagy [99,100], improving mitochondrial energy production [98,101], and downregulating mitochondrial apoptosis [102]. Evidence in swine: limited.

4.5. Other Biological Functions

In addition to its protective role in gastrointestinal tissues, melatonin also regulates gastrointestinal motility and secretory functions [16]. In the human colonic T84 cells, melatonin has been suggested to regulate chloride secretion [103]. Melatonin has also been reported to regulate intestinal epithelial proliferation and migration, digestive enzyme activity, and nutrient transport in the chicken small intestine [25,26]. A study in a rat model indicated that low-dose melatonin accelerated intestinal transit, whereas high doses exerted the opposite effect [104]. This dose-dependent pattern is intriguing, and appears to involve crosstalk with other enteric hormones. For instance, melatonin has been implicated in the modulation of cholecystokinin (CCK)-induced intestinal motor activity. In rats, pinealectomy abolishes the characteristic long-lasting excitomotor effect of CCK on the ileum [105]. During the light phase, administration of a high dose of melatonin (1000 μg/kg) reduces the duration of CCK-induced excitomotor effects on the duodenum and jejunum, but not on the ileum. Conversely, blockade of melatonin receptors by the antagonist during the dark phase restores the CCK’s effect to its daytime pattern [106]. These findings suggest that melatonin can modulate intestinal responsiveness to other neuroendocrine signals in a dose- and region-dependent manner, adding another layer of complexity to its regulatory role.

Moreover, melatonin can influence other organs by protecting the intestinal tract. For example, melatonin ameliorated intestinal ischemia/reperfusion-induced neuroinflammation and cognitive dysfunction in rats by inhibiting the microglial TLR4/MyD88 pathway [107]. Another study in rats demonstrated that melatonin prevented intestinal barrier dysfunction and reduced bacterial translocation, thereby decreasing pancreatic-related infections [108]. Additionally, melatonin alleviated cadmium-induced hepatic inflammation and lipid metabolism dysregulation in mice by restoring intestinal barrier integrity and modulating gut microbiota composition [109]. Melatonin also inhibited hepatic trimethylamine N-oxide (TMAO) production and lipid dysmetabolism in aged mice by reducing Campylobacter jejuni-mediated deconjugation of tauroursodeoxycholic acid [110]. Evidence in swine: limited.

5. Application of Melatonin in Swine Production

Melatonin’s diverse biological functions, such as antioxidant activity, microbial community regulation, and anti-inflammatory effects, provide a strong scientific basis for its application in animal production. This section summarizes the current state of knowledge regarding melatonin use in swine production and discusses its potential mechanisms, with key studies listed in

Table 1. Summary of melatonin supplementation studies in swine production.

Animal	Experimental Context	Melatonin Dose	Duration & Route	Main Outcomes	Proposed Mechanisms	References
Suckling piglets	Normal physiological conditions	10 mg/day	21 days, oral administration	No effect on growth performance; increased jejunal nNOS and Claudin-1 expression; increased serum proline concentration.	Improved intestinal neural development and barrier integrity.	[111]
Weaned piglets	Normal physiological conditions	50 mg/kg diet	35 days, dietary	Increased BW; reduced roaming/fighting behavior; increased resting and feeding time.	Reduced energy expenditure on non-productive activities.	[112]
Weaned piglets	Normal physiological conditions	5 mg/kg BW	23 days, dietary	No effect on growth performance; increased skeletal muscle fiber hypertrophy; improve mitochondrial function and decrease fat deposition in muscle.	Improved mitochondrial function; regulated lipid metabolism.	[113]
Pigs (5–6 months old)	stereotaxic surgery with the removal of the pineal gland	10 mg/kg BW	4 months, in drinking water	No effect on growth performance, intestinal morphology or intestinal mucosal barrier function; increased melatonin levels in the GIT and intestinal contents, altered the structure of the gut microbiota.	Melatonin synthesis in the GIT is independent of the pineal gland in pigs.	[38]
Weaned piglets	T-2 toxin challenge	5 mg/kg diet	14 days, dietary	Improved growth performance; enhanced intestinal barrier function; reduced colonic oxidative stress and inflammation.	Nrf2 pathway activation; NF-κB inhibition; SCFA synthesis promotion; microbiota remodeling.	[22]
Weaned piglets	Diquat-induced oxidative stress	2 mg/kg diet	21 days, dietary	Increased ADG and ADFI; reversed diquat-induced barrier disruption; decreased serum MDA; increased serum CAT and SOD; increased colonic IL-17, IL-10 mRNA expression.	Antioxidant enzyme enhancement; anti-inflammatory effects.	[24]
Weaned piglets	T-2 toxin challenge	5 mg/kg diet	14 days, dietary	Increased serum immunoglobulin and PCV2 antibody concentrations; alleviated spleen and thymus oxidative damage.	Enhanced immune function; reduced apoptosis.	[114]
Pigs (approximately 30 kg)	Acute pancreatitis	10 mg/kg BW	2 h after pancreatitis induction, intravenous bolus	Reduced acinar necrosis, fat tissue necrosis, and edema of pancreatic tissue.	Anti-inflammatory effects.	[115]
Growing-finishing pigs	Gastric ulcer condition	10 mg/kg diet	20 kg to market weight, dietary	Decreased severity of ulcers and feed intake.	Reduced bile acid concentration in the stomach digesta.	[116]
Growing pigs	Normal physiological conditions	5 mg/kg diet	28 days, dietary	Increased apparent digestibility of crude protein and dry matter.	Modulated gastrointestinal motility.	[117]
Sows	Heat stress	1 mg/kg/day	From weaning to day 10 post-mating, dietary	Increased litter size and litter weight; improved serum progesterone level.	Improved luteal function.	[118]
Sows	Normal physiological conditions	2 mg/kg diet	Day 90 of gestation to farrowing, dietary	Increased litter size, birth survival rate, birth weight, weaning weight and weaning survival rate.	Nrf2 pathway activation; suppressed placental oxidative stress.	[30]
Sows	Normal physiological conditions	36 mg/day	From mating to farrowing, dietary	Increased placental weight, decreased the percentage of piglets born with weight < 900 g.	Improved placental antioxidant status; reduced inflammation; enhanced mitochondrial function.	[23]
Sows	Normal physiological conditions	20 mg/day	Mid to late gestation, dietary	Enhanced myoblast differentiation; improved postnatal performance.	Modulated circadian-related genes and myogenic genes in offspring muscle.	[119]
Gilts	Normal physiological conditions	36 mg/day	Day 25–50 of gestation, dietary	Increased fetal weight at day 50 of gestation.	-	[120]
Gilts	Different housing temperature and lighting conditions	5 mg/day	Follicular and early luteal developmental phases, dietary	Improved estrus duration and embryo survival.	Modulated follicle functions and estrogen production; anti-oxidant effects.	[121]
Gilts & parity 1 sows	Seasonal infertility	3 mg/day	Gilts: 21 days, starting before insemination at third estrus, oral; Parity 1 sows: 21 days, starting 2 d before weaning, oral	Gilts: Increased follicle number, no effect on conception, farrowing, or litter size. Parity 1 sows: reduced estrus.	Responses to melatonin in pigs influenced by parity, duration of administration and environmental factors before breeding.	[122]

.

5.1. Growth Performance

In recent years, a growing number of studies have investigated the application of melatonin in pig production. However, the interpretation of its effects on production performance is complicated by substantial variability in experimental outcomes, which may be critically influenced by dosing strategies. As summarized in Table 1, melatonin doses range widely from 1 mg/kg/day to 50 mg/kg diet, in addition to fixed daily doses in sows. This variability necessitates careful consideration of whether these regimens represent physiological supplementation or pharmacological intervention. This distinction remains challenging to establish due to the absence of plasma melatonin measurements in most studies. Furthermore, the use of inconsistent dosing strategies, namely mg/kg of diet versus mg/kg body weight (BW), leads to large discrepancies in actual daily intake across experiments, further confounding cross-study comparisons. Therefore, differences observed in experimental outcomes may not only reflect the varying biological contexts of the studies (e.g., presence or absence of stressors) but also likely represent underlying dose-dependent effects of melatonin.

A recent study demonstrated that dietary supplementation with 50 mg/kg melatonin enhanced growth performance in weaned piglets by reducing energy expenditure on non-productive activities such as roaming and fighting, and increasing resting and feeding behavior [112]. Zha et al. (2024) reported that the addition of melatonin at 2 mg/kg to the diet increased average daily gain (ADG) and feed conversion efficiency in weaned piglets [24]. However, several experiments indicated that melatonin administration failed to affect the growth performance of piglets at similar or higher doses [38,111,113]. The difference between studies might be attributed to complex factors beyond just dosage, diet composition, and the duration of melatonin administration. For example, studies suggest that growth promoters exhibit greater efficacy in livestock exposed to environmental stressors, nutritional deficiencies, and infections [123,124,125]. T-2 toxin, one of the most toxic trichothecene mycotoxins, is widely distributed in food crops and livestock feed and can adversely affect animal health [126]. Wu et al. (2025) reported that dietary melatonin supplementation increased the ADG and average daily feed intake (ADFI), and decreased the feed-to-gain ratio (F/G) and fecal scores in weaned piglets fed T-2 toxin-contaminated diets [22]. In diquat-induced oxidative stress models, dietary supplementation with 2 mg/kg melatonin improved the ADG and ADFI of piglets [24]. These studies demonstrated that dietary melatonin supplementation in piglet diets could enhance their resilience to adverse stress factors and improve growth performance. In addition, environmental factors, particularly lighting regimens and photoperiod, may modulate the efficacy of exogenous melatonin, as gut melatonin synthesis is not photoperiod-dependent. Unfortunately, most swine studies fail to report or standardize photoperiod and light intensity, making cross-study comparisons difficult. Furthermore, the choice of performance metrics matters. Chen et al. (2023) found that melatonin did not alter the growth performance of weaned piglets, but it enhanced skeletal muscle growth and muscle fiber hypertrophy, improved mitochondrial function, and reduced fat deposition in muscle [113]. Beyond these experimental factors, cross-study comparisons are further complicated by differences in housing conditions and baseline health status, both of which can influence responsiveness to melatonin supplementation. Furthermore, differences in study duration and statistical power across studies may also contribute to inconsistent outcomes, with some investigations potentially underpowered to detect significant differences.

Research on the effects of melatonin on the growth performance of growing-finishing pigs is limited. It was reported that dietary melatonin supplementation at 10 mg/kg reduced feed intake in growing-finishing pigs with gastric ulcers, while no differences were observed in BW, ADG, or F/G [116]. The addition of melatonin may reduce gastrointestinal motility by slowing the digesta transit rate, which could allow more nutrients to be absorbed and consequently improve feed efficiency.

5.2. Gut Physiology and Microbiota

The intestines perform multiple physiological functions, including nutrient absorption, regulation of water–electrolyte balance, production of protective mucins and immunoglobulins, and maintenance of a selective barrier against pathogens and toxic antigens [127]. Young piglets require substantial amounts of nutrients to sustain both intestinal epithelium development and differentiation, as well as physiological functions [128]. A recent study demonstrated that melatonin supplementation significantly increased serum proline concentrations, with a tendency toward increased concentrations of histidine, serine, glycine, glutamate, threonine, alanine, and lysine compared to the control group [111]. Ayles et al. (1999) also found that melatonin increased the apparent digestibility of crude protein and dry matter in the diets of growing pigs [117].

The integrity of the intestinal morphological structure serves as the basis of nutrient digestion and absorption and is essential for maintaining the intestinal barrier function. Dietary supplementation with melatonin at 2 mg/kg increased jejunal villus height and the villus height to crypt depth ratio, while reducing crypt depth in the duodenum [24]. In addition, melatonin supplementation reversed the diquat-induced decrease in villus height and the villus height to crypt depth ratio in the jejunum and ileum of weaned piglets [24]. Wu et al. (2025) discovered that melatonin supplementation effectively ameliorated T-2 toxin-induced intestinal barrier disruption in piglets by increasing the number of goblet cells and enhancing the expression of tight junction proteins (ZO-1, Occludin, Claudin-1) and MUC2 [22]. Additionally, this study demonstrated that melatonin exerted its positive effects on intestinal health by stimulating the Nrf2 pathway, inhibiting the NF-κB signaling pathway, and promoting SCFA synthesis. An in vitro study further demonstrated that melatonin mitigated deoxynivalenol-induced intestinal barrier impairment in IPEC-J2 cells by enhancing the antioxidant system and inhibiting autophagy [129]. Moreover, oral melatonin significantly improved intestinal neural development and barrier integrity as evidenced by the increased nNOS and Claudin-1 expressions in suckling piglets [111].

The relationship between gut microbiota and host health has been extensively studied. Specifically, the gut microbiota plays a crucial role in regulating porcine health and growth performance by modulating nutrient absorption, maintaining mucosal barrier integrity, and orchestrating immune system functions [130]. Melatonin has been shown to improve intestinal health by regulating the gut microbiota and their related metabolites [20,89]. Dietary melatonin supplementation induced alterations in the intestinal microbiota of piglets, with an increase in Actinobacteria abundance and a decrease in Selenomonadales abundance within the ileal digesta [111]. Zheng et al. (2024) found that piglets fed melatonin-supplemented diets exhibited an increased relative abundance of Actinobacteriota and Desulfobacterota, which are responsible for SCFA production [38]. In addition, melatonin alleviated T-2 toxin-induced intestinal injury by remodeling gut microbiota, as evidenced by a decrease in pathogenic genera such as g_norank_f_T34, and an increase in beneficial genera including Lactobacillus, Enterorhabdus, and Romboutsia [22]. However, it remains unclear whether microbiota shifts are a causal mediator of melatonin’s effects. Establishing causality in pigs will require more targeted experimental designs, such as: (1) antibiotic depletion to test whether microbiota absence abolishes melatonin’s effects; (2) fecal microbiota transplantation from melatonin-treated donors to determine whether the “melatonin-shaped” microbiota alone can transfer health benefits; and (3) supplementation with key metabolites to assess whether they recapitulate melatonin’s effects. Overall, these studies demonstrated that melatonin promoted intestinal health by enhancing nutrient absorption, preserving intestinal barrier integrity, and modulating microbiota composition along with its derived metabolites. Determining the causal direction and mechanisms governing these microbe–host interactions remains a key research priority.

5.3. Antioxidant Capacity and Immune Status

Melatonin’s most prominent function is its antioxidant activity, which has been proven to alleviate oxidative stress in pig production. In piglets exposed to T-2 toxin, melatonin supplementation significantly decreased colonic malondialdehyde (MDA) levels, increased GSH content, enhanced CAT and SOD activities, and upregulated the Nrf2/HO-1 pathway in the colon [22]. Melatonin also decreased serum MDA levels, increased CAT and SOD activities, and enhanced mRNA expression of MnSOD and CuZnSOD in the colon of diquat-treated piglets [24]. In addition, the balance between ROS production and the antioxidant defense system plays a critical role in ensuring optimal in vitro embryonic development [131]. Yao et al. (2020) reported that melatonin exhibited protective effects against zearalenone-induced defects during early porcine embryonic development by alleviating oxidative stress, enhancing mitochondrial function, and reducing autophagy and apoptosis [132]. Niu et al. (2020) reported that melatonin administration mitigated rotenone-induced impairment of porcine embryo development by reducing ROS generation and enhancing mitochondrial biogenesis [133].

The underdeveloped gastrointestinal immune system of weaned piglets increases their susceptibility to pathogenic factors, leading to diarrhea and elevated mortality rates [134]. Du et al. (2022) reported that Escherichia coli (ETEC) infection reduced intracellular melatonin production in porcine macrophages, while melatonin pretreatment alleviated ETEC-induced macrophage death and enhanced bacterial clearance in these cells [31]. In weaned piglets, dietary melatonin supplementation alleviated the diquat-induced inflammatory response by increasing colonic IL-4 and IL-10 mRNA expression [24]. Wu et al. (2025) found that melatonin effectively ameliorated T-2 toxin-induced intestinal inflammatory responses by decreasing colonic mRNA expression levels of pro-inflammatory cytokines and suppressing the NF-κB pathway in the colon [22]. Ning et al. (2024) further demonstrated that melatonin alleviated the T-2 toxin-induced reduction in serum immunoglobulin and porcine circovirus type 2 (PCV2) antibody concentrations in weaned piglets, thus enhancing immune function [114]. In pigs with acute pancreatitis, melatonin treatment alleviated inflammatory responses by decreasing pancreatic tissue damage including acinar necrosis, adipose necrosis, and edema [115]. These results indicated that melatonin exhibited great potential in modulating immune function in pigs, especially during immune challenges such as infections caused by bacteria and mycotoxin exposure.

5.4. Reproductive Performance

Beyond its direct effects on the GIT, melatonin exerts systemic endocrine effects that influence reproductive outcomes. Although the primary focus of this review is on gut biology, the reproductive benefits of melatonin are also discussed, as they may stem from two potential pathways: on one hand, improved gut health (e.g., enhanced intestinal barrier function, reduced systemic inflammation), and on the other, direct effects on reproductive tissues.

The reproductive benefits of melatonin are closely linked to the enhancement of endometrial receptivity. Bae et al. (2020) reported that the expression of MTNR1A and MTNR1B mRNAs in both the trophectoderm and uterine luminal epithelium increased progressively during early pregnancy [135], suggesting a potential role of melatonin in pregnancy establishment and maintenance. Mechanistically, melatonin stimulated conceptus-uterine crosstalk via receptor-mediated activation of SIRT1/PI3K/MAPK signaling pathways, critical for embryo implantation and placental development [135]. Qin et al. (2025) revealed that melatonin supplementation in sows with high backfat thickness during early pregnancy improved embryo implantation rates by activating the MT2/PI3K/LIF signaling pathway [136]. This aligns with findings by Wang et al. (2021) where dietary melatonin administration from weaning until the 10th day after estrus mating increased litter size and serum progesterone levels in heat-stressed sows [118]. The stimulatory effect of melatonin on progesterone secretion could be attributed to the upregulation of cholesterol side-chain cleavage enzyme (P450scc) and steroidogenic acute regulatory protein (StAR) expression in luteal cells [137], highlighting melatonin’s endocrine-modulating capacity.

Notably, melatonin exerts pleiotropic effects that significantly influence placental function and pregnancy outcomes [138]. Accumulating evidence suggests that melatonin serves as a promising therapeutic candidate for managing pregnancy complications, including preeclampsia, preterm birth, and low birth weight [139,140]. Dobbins et al. (2024) discovered that maternal dietary melatonin modulated circadian-related gene expression in pre- and postnatal skeletal muscle of offspring and enhanced myoblast differentiation during growth, thus improving postnatal performance [119]. Dearlove et al. (2017) found that maternal melatonin supplementation during early gestation increased fetal weight on day 50 of gestation [120]. Similarly, a recent study showed that late gestation melatonin supplementation exhibited increased birth weight, weaning weight and weaning survival rate of piglets [30]. The beneficial effects of melatonin might be closely related to ameliorating placental antioxidant status, inflammatory response, and mitochondrial dysfunction [23]. Furthermore, an in vitro study demonstrated that melatonin effectively protected H₂O₂-treated porcine trophectoderm cells from oxidative stress by reducing intracellular ROS generation, suppressing apoptosis, and modulating antioxidant-related gene expression [141]. These results suggest that melatonin may be an effective supplement to improve fetal growth and placental function.

Additionally, high temperatures and shifting photoperiods during the summer and early fall are associated with reduced fertility in sows, manifesting as delayed puberty, extended wean-to-estrous intervals, lower pregnancy rates, and conception failures [142,143]. Ramírez et al. (2009) reported that polymorphisms in the MTNR1A gene significantly influence the seasonal reproduction of pigs [144]. Gilts administered melatonin during the follicular and early luteal developmental phases exhibited prolonged estrous duration and enhanced embryo survival rates, particularly during summer under varying room temperatures and lighting regimens [121]. Arend et al. (2019) discovered clear evidence of seasonal fertility failures in gilts and sows; however, melatonin treatment failed to improve litter traits in gilts and even reduced estrus in parity 1 sows [122]. The efficacy of melatonin treatment in pigs’ seasonal infertility appears to be influenced by factors such as parity, duration of treatment administration, and variations in pre-breeding light exposure and thermal conditions, which may collectively account for observed differences in response to melatonin therapy.

6. Conclusions and Perspectives

Gut melatonin is derived from multiple potential sources, including diet, enterochromaffin cells, intestinal microbiota, and microbial metabolites (Figure 2). Melatonin plays a pivotal role in maintaining gut health through its antioxidant, anti-inflammatory, and microbiota-modulating properties. Gut melatonin regulates redox balance, immune responses, and barrier function via receptor-dependent (MT1/MT2) and receptor-independent pathways, while also interacting bidirectionally with gut microbiota. However, the precise mechanisms by which gut-derived melatonin exerts its physiological effects, particularly its complex interplay with the gut microbiota, are still being elucidated. Further validation is required before these mechanistic insights can be translated into targeted therapeutic strategies. Notably, given that mitochondria serve as the primary sites of melatonin synthesis, the biological significance of mitochondrial melatonin interactions warrants systematic investigation.

Figure 3 provides an integrative overview of the cascade from melatonin signaling in the gut to the production outcomes observed in swine. In swine production, dietary melatonin supplementation has demonstrated considerable potential: it improves growth performance under stress conditions, enhances intestinal barrier function, and mitigates oxidative damage. This pattern implies that melatonin may function as a stress mitigator that helps pigs maintain performance and health under adverse conditions. In addition, melatonin positively influences reproductive outcomes by improving embryonic development and placental function. However, several challenges need to be addressed to facilitate the practical application of melatonin: (1) the optimal dosage of melatonin for different physiological stages (e.g., weaning, growing–finishing, gestation) requires further refinement; (2) timing of administration may also influence efficacy; pigs exhibit circadian rhythms, and whether daytime versus nighttime feeding affects outcomes remains unknown; (3) melatonin is light-sensitive, and its stability in feed under commercial storage conditions has yet to be systematically evaluated; (4) residue concerns, although melatonin is an endogenous molecule with a favorable safety profile, nonetheless merit evaluation given increasing consumer attention to food safety; (5) economic viability hinges on a clear cost–benefit advantage. Field trials that weigh supplementation costs against tangible improvements, including reduced morbidity, enhanced growth, and better reproductive performance, are essential to support adoption by the swine industry; and (6) the regulatory status of melatonin as a feed additive remains unresolved. In many major pork-producing regions, melatonin is regulated as a hormone or veterinary drug rather than a simple feed additive.

In summary, while the scientific evidence for melatonin’s benefits in swine is compelling, its path to commercial application is contingent upon regulatory classification and demonstration of economic viability. Until these issues are addressed, melatonin should be regarded as an experimental intervention with therapeutic potential rather than a standard production tool.