The Applications of Nanocellulose and Its Modulation of Gut Microbiota in Relation to Obesity and Diabetes

Abstract

Obesity and type 2 diabetes are closely linked and often referred to as diabesity. Therapies of diabesity include improving intestinal health and reducing intake of fat and sugars. Diagnosis of diabesity-related metabolic disorders would involve monitoring of glucose and other factors. Nanocellulose, also known as cellulose nanomaterials, is emerging as a potential material for various applications. It has unique properties, such as high surface area, biodegradable, biocompatibility and tunable surface chemistry. In this review, we initially provided a brief description of differently produced nanocellulose and their potential applications in different areas, including therapeutics and diagnostics, by focusing on obesity and diabetes. Then, the uptake, absorption, distribution, metabolism and excretion of nanocellulose were discussed. Further, the mechanisms of nanocellulose in modulating diabesity were summarized by emphasizing the role of gut microbiota. Finally, we discussed gut microbiota-related health effects of nanocellulose, both beneficial and detrimental. It was found that the interactions between nanocellulose and gut were complex, with alterations of microbial composition, metabolic activity, and the immune functions both locally and systemically. There seemed to be many beneficial changes following short-term exposure to nanocellulose (e.g., increased beneficial bacteria and decreased pathogenic ones); however, some of these effects were no longer seen after long-term consumption. Importantly, long-term nanocellulose consumption may be associated with certain detrimental health effects, e.g., malnutrition and its associated neurotoxicity, although additional studies are needed to substantiate such health implications. This information is critical for developing safe and effective nanocellulose derivatives that can be applied in food and medicine as well as to harness the benefits of the gut microbiota.

1. Introduction

Nanocellulose has recently gained attention in many applications, including therapeutics and diagnostics, due to its unique characteristics, such as nanoscale sizes, functional properties, high renewability, abundance and biocompatibility [1]. For example, nanocellulose shows some promises in managing obesity by reducing fat absorption [2]. The close association between increasing body mass index and the risk of chronic diseases has made reducing obesity and diabetes, often referred to as diabesity, a medical priority. In this regard, nanocellulose is also being explored for its potential applications in diabetes management, particularly in diabetic wound healing [3] and glucose monitoring [4]. In addition, nanocellulose delays starch digestion through interacting with the digestive enzymes, e.g., the pancreatic amylase, which can be beneficial for diabetes as starch, is broken down into glucose to increase blood sugar levels [5].

Nanocellulose is derived from cellulose, a polysaccharide consisting of chains of glucose monomers. The production of nanocellulose was estimated at 39.6 tons in 2018, which is continuously increasing [6]. Nanocellulose is defined as cellulose that has at least one dimension under 100 nm [7,8]. Different cellulose sources can be used to obtain nanocellulose, such as algae, tunicate (a marine invertebrate), lignocellulosic (wood, crop residues, etc.) or bacteria. The different sources and isolation processes result in nanocellulose with various sizes, shapes and physicochemical properties. Bacterial nanocellulose (BNC), also called microbial cellulose, is a biopolymer synthesized by a few bacteria (Figure 1), such as those in the Acetobacter, Achromobacter, Bacillus, Komagataeibacter, Pseudomonas and Salmonella genera [9]. This process involves the bacteria producing cellulose strands and then assembling into a complex network of nanofibers. In addition to BNC, there are three other main types of nanocellulose (Figure 1): cellulose nanofibril (CNF), and its derivatives 2,2,6,6,-tetramethylpiperidine-1-oxyl oxidized (TEMPO)-CNF and cellulose nanocrystals (CNCs).

CNF is created by mechanically grinding down cellulose until at least one dimension is less than 100 nm, e.g., a width of 4–20 nm and a length of 500–2000 nm (Figure 1). This allows CNF to have some characteristics as high surface area, high Young’s modulus, very low coefficient of thermal expansion, low weight and low density [10]. They are chargeless, with long and flexible nanofibrils entangled into a network structure [1]. CNF with higher purity and uniformity can be produced from fiber slurry through chemical treatment. CNF can be processed into TEMPO-CNFs, which are oxidized polysaccharides with sodium carboxyl groups being introduced onto the surfaces of cellulose elementary fibrils [11]. TEMPO-CNF therefore has negative surfaces and can be well dispersed during polar adhesion [1]. CNC, also called cellulose nanowhiskers or nanocrystalline cellulose, is produced by acid hydrolysis of cellulose to remove lignin, hemicellulose and amorphous regions. They become cylindrical, elongated rods with high crystallinity and purity, possessing improved mechanical and thermal properties, hydrophilicity and adsorption capacity [1]. CNC has a diameter of 2–20 nm and a length of 100–600 nm and generally possesses a negative surface charge due to the presence of sulfate or carboxylate groups introduced during their production. CNC can be dispersed stably in water because of its abundant hydroxyl groups.

Depending on the intended application, cellulose can be extracted in different sizes. The insoluble micron-scale cellulose fibers and crystals, e.g., microcellulose, have long been added to processed foods as fillers and thickeners, and they have been designated by FDA as generally regarded as safe (GRAS) for ingestion. However, due to its dramatically reduced size and sometimes altered charges, further research and safety studies are needed before nanocellulose can be granted GRAS status [12]. Indeed, the wide application of nanocellulose has raised concerns about its potential effects on human health. Thus, we conducted a comprehensive review of the current literature that describes the health effects of nanocellulose and their related mechanisms by focusing on obesity and diabetes. In addition, gut microbiota plays a crucial role in both obesity and diabetes, and studies have shown that nanocellulose may affect both diseases by changing the diversity and composition of the gut microbiota [13,14,15,16]. In the past few years, several studies have been conducted to determine the modulatory effects of nanocellulose on gut microbiota. It is timely to conduct a review of the current literature on “The applications of nanocellulose and its modulation of gut microbiota in relation to obesity and diabetes”.

2. Methods

Literature searches of different nanocellulose and their effects were carried out in various platforms, including Google Scholar, MEDLINE, PubMed and the Directory of Open Access Journals, for all studies from the earliest available indexing year through 15 July 2025. We used various search terms corresponding to nanocellulose exposure, gut microbiome/microbiota, obesity and diabetes, including, but not limited to, “nanocellulose theranostics”, “nanocellulose toxicity”, “cellulose nanocrystals”, “cellulose nanofibrils”, “nanocellulose microbiome/microbiota”, and “nanocellulose metabolism”.

3. Applications of Nanocellulose—Focusing on Obesity and Diabetes

The prevalence of obesity has been increasing at an alarming rate, and it is characterized by excessive fat accumulation in the body [17]. Currently, the most effective treatment to help obese people lose weight is bariatric surgery; however, the procedure can have serious complications and long-lasting side effects [18]. Obesity is often associated with hyperlipidemia, a condition characterized by high levels of lipids in the blood, including cholesterol (hypercholesteremia) and triglycerides (hypertriglyceridemia). Triglycerides are used for energy production by the organism and excess triglycerides are stored in fat tissue. Triglycerides found in food are the primary form of dietary fat, and they are broken down into fatty acids and glycerol for absorption in the small intestine. The first phase in the intestinal absorption of triglycerides is emulsification, a process of breaking down large fat globules into smaller fat droplets. Bile salts from the liver act as emulsifying agents to surround fat droplets and prevent them from re-clumping. Cholesterol is essential for making hormones, vitamin D and substances that help digestion, and our body can produce all the needed cholesterol. However, cholesterol can be found in animal-based foods like eggs, meat and cheese. High levels of cholesterol, particularly in the form of low-density lipoprotein (LDL), can be detrimental because it can build up in the arteries, forming plaque and leading to atherosclerosis. Obesity can negatively impact cholesterol levels, often leading to higher levels of LDL and triglycerides, and lower levels of high-density lipoprotein, e.g., the “good” cholesterol. This combination increases the risk of heart disease and stroke [19,20].

Diets high in dietary fiber, such as cellulose, have been associated with reduced risks of many non-communicable diseases, including diabetes and obesity [21,22,23]. Nanocellulose, with its nanoscale structure and large surface area, can interact with lipids in the digestive system to slow its digestion and reduce its absorption (Figure 2). It has been reported that CNF could delay lipid digestion in in vitro digestion systems [24]. Using an intestinal monolayer and mucus model, Lin et al. showed that nanocellulose may reduce hypercholesteremia by enhancing the viscosity of digesta, adsorbing cholesterol and reducing bile acid permeation [25]. In rats, it has been shown that CNF binds and traps ingested triglycerides and prevents them from being broken down into fatty acids by lipases [2]. Rats fed with heavy cream containing 1% CNF absorbed 36% less triglycerides than rats fed heavy cream alone. Further studies suggested two primary mechanisms for this effect: (1) coalescence of fat droplets on CNF, resulting in a reduction in available surface area for lipase binding, and (2) sequestration of bile salts, causing impaired interfacial displacement of proteins at the lipid droplet surface and impaired solubilization of lipid digestion products [2]. Chen et al. reported that CNF at the dose of 30 mg/kg body weight could decrease fat absorption in jejunum, the main section of gut responsible for fat absorption, and attenuate Western diets (WD)-induced fatty liver in C57BL/6 mice [26]. The dose of 30 mg/kg BW/day was equivalent to the amount that a 25 g mouse consumed 0.75 mg CNF every day and corresponded to a 2.1 g daily consumption of CNF by a 70 kg adult human. This level is approximately half of the calculated cellulose intake in the UK and Scandinavian populations [26,27]. Nagano and Yano [14] studied the effects of CNF intake on obesity in high-fat diet (HFD)-fed mice, and it was found that the weight gain in HFD-fed mice was suppressed following 0.2% CNF treatment. In addition, the accumulation of epididymal and subcutaneous fat in HFD-fed mice was reduced. In a separated study, Nagano and Yano [28] showed that CNF intake and exercise together suppressed increases in body weight and fat mass in HFD-fed mice.

Obesity is a common risk factor for diabetes since obesity is often associated with insulin resistance [29]. It has been reported that 62% adults with type 1 diabetes were affected by overweight or obesity, compared to 64% without diabetes and 86% adults with type 2 diabetes [30]. The beneficial effects of soluble dietary fibers on glycemic control are associated with their viscosity [6,14]. Similarly, nanocellulose has a unique feature of dispersing in water to produce a dispersion of high viscosity, and consumption of nanocellulose might attenuate diabetes and related diseases (Figure 2). Using a series of artificial digestive solutions, CNF was shown to decrease starch digestion and glucose release by directly absorbing glucose and retarding amylolysis without affecting the enzyme activity [31]. On the other hand, Chen et al. reported that CNF delayed the starch digestion through interacting with the digestive enzymes, e.g., the pancreatic amylase [5]. In vivo, Nagano and Yano [28] showed that CNF intake and exercise together improved glucose tolerance in HFD-fed mice. Chen et al. reported that CNF could lower the non-fasting blood glucose levels in WD-fed mice of both sexes [26]. In addition, Xu et al. showed that there were significant decreases in diabetes incidence following CNF intake in WD-fed female NOD mice [16], a model for type 1 diabetes.

Probiotics show promises in potentially managing obesity and diabetes by influencing gut microbiota and improving insulin sensitivity [32]. However, they are sensitive to acidic environments, high temperatures, and high oxygen concentrations. To increase their viability, nanocellulose might help encapsulate probiotics to protect them from environmental stresses (Figure 2). Several encapsulation techniques can be used to encapsulate probiotics involving nanocellulose, including spray drying, freeze drying, emulsification and extrusion. Baek et al. showed that a shell consisting of shellac and CNC nanocomplex protected the yeast to have high viability and retention under simulated gastrointestinal conditions and display favorable muco-adhesion in the intestinal environment [33]. On the other hand, TEMPO-CNF has abundant carboxylate groups and outstanding mechanical strengths, serving as a cross-linking agent and nanofiller to enhance the properties of encapsulated probiotics [34,35]. Nanocellulose can also be used as a cryoprotectant in the process of freeze drying to prevent the formation of ice crystals by attaching to the surface of probiotics to form a viscous layer [36,37]. Last but not the least, nanocellulose is also being explored for encapsulating probiotics in the process of spray drying, a preferred method for probiotic products due to its high efficiency, low cost, simplicity, flexibility and controllable particles size [38].

Nanocellulose is also being explored for other areas of applications in diabetes management (Figure 2), e.g., in the acceleration of diabetic wound healing [39]. TEMPO-CNF has shown promises in managing diabetic wounds due to their flexibility, biocompatibility, drug release capacity and the presence of a physical barrier that supports cell migration towards the wound bed [3]. Recently, Ma et al. reported that an aldehyde-modified nanocellulose-based scaffold with silk fibroin-loaded cerium oxide nanoparticles displayed excellent porosity, water absorption, air permeability, water retention, controlled degradability, antibacterial activity and antioxidant properties [40]. Further, this scaffold dressing could reduce inflammation at the wound site of diabetic mice and promote collagen deposition, angiogenesis and re-epithelialization, with its efficacy surpassing the current commercially available membrane dressings and medical PELNAC dressings [40]. Nanocellulose can also be incorporated into glucose sensors to enhance their sensitivity and enable more accurate glucose level measurements. For example, Hossain et al. reported that nanocellulose-coated paper diagnostic device used a color change to indicate glucose concentration [4]. In addition, nanocellulose-based systems are being investigated for oral insulin delivery, potentially overcoming the limitations of subcutaneous injections and creating glucose-responsive nanomedicine that can release insulin only when needed, mimicking the natural function of pancreatic β cells [41].

There are many foods, such as pudding and gravy, which require emulsifiers to combine liquids that typically do not mix (like oil and water) into a stable, uniform mixture. However, studies have suggested that there is a positive correlation between the consumption of emulsifiers that are currently widely used on the market, such as lecithin, and inflammatory diseases such as colitis and inflammatory bowel disease [42]. Further, a mouse study indicated that lecithin might promote adipocyte differentiation and lipid accumulation, which is a potential contributor to weight gain [43]. Thus, replacing currently available emulsifiers with nanocellulose would dramatically alleviate many health issues if it retains similar benefits as the prebiotic cellulose. In addition to being a potential emulsifier, the use of nanocellulose as a food additive has been explored in ice cream, mayonnaise and yogurt among other food products. In a 30-day exposure study in which CNF at 7, 14 or 21% were directly added to a regular AIN-93 diet and fed to rats, the blood glucose levels, lipid profiles and liver histology did not show any significant changes [44]. However, there was a trend for an increased weight over experimental time with some losses of mineral nutrients [44].

While the research on the applications of nanocellulose is promising, it is important to understand that much of the work is still in the early stages. Importantly, studies have found certain detrimental effects associated with exposure to nanocellulose. For example, decreased lean body mass and dysregulated glucose homeostasis were observed following the CNF treatment in mice, likely because CNF non-specifically inhibited intestinal absorption [26]. Xu et al. showed that depressive-like symptoms and some inflammatory cytokines (e.g., KC) were increased following long-term CNF consumption in male mice [15]. Khare et al. also reported that the expression of several cytokines in ileal mucosa, including IL-5, IL-7, IL-10 and IL-18, was upregulated by oral CNF treatment in male rats [45]. Further, the expression of following genes was significantly downregulated by CNF [45]: (1) two claudins (Cldn2 and Cldn3) that regulate intestinal barrier function and form paracellular channels for small cations and water, (2) two gap junction proteins (Gja3 and Gjd2) that are involved in paracellular transport, (3) integrins Itga2, Itga3 and Itgav that regulate paracellular permeability, (4) one of the major cadherins (Cdh1) that is involved in maintenance of tight junctions, and (5) the adherens junction protein Nectin 2 (Pvrl2). Thus, further studies are needed to fully understand the long-term effects of nanocellulose on human health and to optimize its use in different settings.

4. Uptake, Absorption, Distribution, Metabolism and Excretion of Nanocellulose

As discussed above, nanocellulose can be potentially applied in many areas, including biomedical applications (e.g., drug delivery, tissue replacements, biomedical devices), food industry, biosensors, water pollutant removal, paper industries and as a filler in food packaging [1]. Thus, in addition to the oral route, humans can also be exposed by inhalation or contacting nanocellulose-containing materials [46]. Nanocellulose may also be released to wastewater to enter ambient waters and disposed of in landfill to biodegrade or at a composting facility, making aquatic and environmental exposures possible [47]. However, there is a lack of information on uptake, absorption, distribution, metabolism and excretion of nanocellulose through routes other than the oral exposure (Figure 3). Following oral consumption, nanocellulose is mechanically broken down into smaller particles through chewing. The surface charge of nanocellulose can affect its interaction with salivary proteins, which can be positively, negatively or neutrally charged. Uncharged nanocellulose may not interact with salivary proteins, however, it is subjected to the shear forces between the oral mucosa and the teeth [48]. The negatively charged nanocellulose may not be able to attach to the negatively charged mucus layer because of the repelling interaction.

Similarly to dietary fibers, nanocellulose is difficult to be digested in the human gastrointestinal tract because of lacking the enzymes to degrade β-glycosidic bonds between glucose monomers. Furthermore, the mucus secretion rate is high and nanocellulose is unlikely to pass through the mucus gel layer to reach the intestinal epithelium [49]. In the stomach, where the pH is between 1 and 3, nanocellulose may have a reduced repulsion interaction with the mucus solution, leading to agglomeration following mixing with gastric acid. Moreover, high ionic strength in the stomach may result in a charge-shielding phenomenon that can diminish the electrostatic repulsion force, and thus further induce the aggregation of nanocellulose and extend the standing time there [50]. Following the gastric phase, nanocellulose enters the small intestine. While the absorption of nanocellulose may occur through transcytosis in the M-cells of the Peyer’s patches in the gut-associated lymphoid tissues, most nanocellulose would be trapped in the mucus layer and thus are unable to reach the bottom enterocytes [51]. Lin et al. also reported that CNC could not penetrate the small intestine lining due to increasing viscosity of mucin solutions [25]. Muñoz-Juan et al. showed that BNC could be taken up and excreted by worms without crossing the intestinal barrier [52]. Taken together, the absorption of nanocellulose in the upper GI tract is limited or negligible. While passing through the large intestine, however, the undigested nanocellulose can be fermented by the bacteria in the colon through anaerobic degradation. The remaining residues, including unfermented nanocellulose, are excreted in feces.

Short-chain fatty acids (SCFAs), including acetate, butyrate and propionate, are the major metabolites of anaerobic fermentation of nanocellulose. Because of the generation of SCFAs, colonic contents become acidic [49]. SCFAs regulate the physiological functions of the gut, local and systemic immunity and energy metabolism of the organism [53]. Both butyrate and propionate are important energy sources for colon cells, participating in the gluconeogenesis pathway and inhibiting fat and cholesterol production; acetate can enhance intestinal contraction and promote intestinal peristalsis [49]. Further, it was reported that consumption of acetate improved glucose tolerance and protected against obesity [54]. In constipated mice, Wang et al. reported that CNC at the dosage of 100 mg/kg/day increased the contents of SCFAs by 25%, and that of butyrate and acetate by 65% and 24%, respectively [55]. Nsor-Atindana et al. showed that the size of microcrystalline cellulose affected cecal fermentation, with the smallest particle size yielding the highest concentration of SCFAs [53]. Further, increased aspect ratio also improved the fermentability of nanocellulose and increased the production of SCFAs [56]. However, Khare et al. reported that CNF had few effects on the fecal metabolome in rats, with significant changes in only 10 of 366 metabolites measured [45]. In addition, none of the altered metabolites were SCFAs except for isobutyrate, which was significantly increased by CNF in the presence of cream [45]. Interestingly, comparable levels of SCFAs were also reported in mice fed with high-cellulose or low-cellulose diet [57]. In addition, Wen et al. have reported that a high-cellulose diet decreases asthma symptoms by altering the composition of the intestinal microbiome; however, this mechanism is thought to be independent of SCFAs and may involve the regulation of lipid metabolism [58].

5. Effects of Nanocellulose on Gut Microbiota and Their Relations to Obesity and Diabetes

The gut microbiota, a complex community of microorganisms residing in the digestive tract, plays a significant role in the development and progression of obesity and diabetes. Dysbiosis, or an imbalance in the gut microbiota composition, is linked to these metabolic diseases in different ways. First, it affects energy harvest, inflammation, glucose homeostasis and insulin resistance and interacts with bile acids and medicines (e.g., metformin). It may also change endotoxemia, gut permeability, hormonal balance, expression of lipogenesis genes and the proportion of brown adipose tissue [59,60]. By serving as an important substrate to gut microbiota, dietary fiber can exert its effects on host immunity and microbial community, and reduce hyperlipidemia, obesity and diabetes [54,61]. Cellulose, a linear homopolymer of β-glucopyranose units linked to each other by 1,4-glycosidic bonds, is a well-known dietary fiber that reduces inflammation underlying various diseases such as colitis. It increases beneficial bacteria in the gut, e.g., Akkermansia genus, the number of goblet cells and crypt length [57]. In animal studies, nanocellulose showed efficacy in alleviating constipation and ulcerative colitis by modulating gut microbiota [55,62]. Therefore, it is important to understand how nanocellulose affects gut microbiota, and consequently, how it modulates human health, e.g., obesity and diabetes. In the past few years, several studies, both in vitro and in vivo, have been conducted on the modulatory effects of nanocellulose on gut microbiota. In this part of the review, four sections will be included to discuss the effects of CNF, CNC, BNC and other nanocellulose on gut microbiota, respectively, and their relationship to health by focusing on obesity and diabetes.

5.1. Effect of Cellulose Nanofibril on Gut Microbiota

5.1.1. Effect of Cellulose Nanofibril on Gut Microbiota in Males

Nagano and Yano [14] conducted a study in HFD-fed male C57BL/6N mice (

Table 1. Summary of gut microbiome studies on cellulose nanofibril in males.

Nanocellulose	Model/Diseases	Model/Population	Exposure Window/Period	Dose/Concentration	Routes of Administration	Diet/Medium	Effects	Reference
CNF	Obesity and related diseases	Male C57BL/6N mice	7 weeks	0.1–0.2 wt%	Oral via drinking water	High-fat diet (HFD) without cellulose	CNF at 0.2% increased bacterial diversity. HFD-induced increases in the relative abundances of Streptococcaceae and Rikenellaceae were reversed by CNF. Further, 0.2% CNF intake increased the abundance of Lactobacillaceae.	[14]
CNF	Obesity/Gut microbiota	Male C57BL/6 mice	30 days	30 mg/kg	Gavage daily	Western diet	CNF treatment altered the β diversity. Further, there were changes in taxonomic features and predicted functional contents, which were mostly beneficial.	[15]
CNF	Type 1 diabetes	Male NOD mice	2 months	30 mg/kg	Gavage daily	Western diet	No significant differences were found in either the α diversity or the β diversity. However, there were changes in taxonomic features and predicted functional contents. Many of these changes have been associated with beneficial health effects.	[15]
CNF	Type 1 diabetes	Male NOD mice	6 months	30 mg/kg	Gavage daily	Western diet	No significant differences were found in either the α diversity or the β diversity. However, there were some beneficial changes in taxonomic features. Further, changes in the predicted functional contents were mostly beneficial.	[15]
CNF	Obesity and related diseases	Male C57BL/6N mice	7 weeks	0.2 wt%	Oral via drinking water	HFD without cellulose	Exercise and CNF intake together increased Eubacteriaceae.	[28]
CNF	Gut microbiota	Male Wistar Han rats (13 wks old)	5 weeks	1%	Gavage (Twice per week)	PicoLab Rodent Diet 5053	CNF ingestion altered microbial diversity and reduced the abundances of pathogenic bacteria. However, the numbers of live Bifidobacterium were also decreased. Nonetheless, CNF had few effects on the fecal metabolome.	[45]

), and it was found that oral administration of CNF at 0.2%, but not at 0.1%, for seven weeks increased bacterial diversity and induced changes in the gut microbiota composition. The principal component analysis showed a shift in the microbiota composition at the phylum level resulting from feeding HFD, which was reversed by CNF intake at 0.2%. In addition, HFD-induced increases in the relative abundances of Streptococcaceae and Rikenellaceae were reversed upon intake of CNF [14]. It has been reported that the increases in Streptococcaceae and Rikenellaceae correlated with HFD-induced obesity and diabetes [63,64]. Some strains of Streptococcaceae could induce mild inflammation [65], and the presence of Alistipes, a genus in Rikenellaceae, correlated with type 2 diabetes in humans [66]. Further, CNF intake increased the abundance of Lactobacillaceae [14]. Different Lactobacillus strains have been used as probiotics to modulate gut microbiota composition in the context of obesity. Oral feeding of L. curvatus HY7601 and L. plantarum KY1032 to HFD-fed mice shifted the gut microbiota composition and reduced obesity [67]. Dietary supplementation with L. paracasei, L. rhamnosus and B. animalis also attenuated HFD-induced weight increase and modulated the gut microbiota composition [68]. However, Dahiya et al. reported an increase in the relative abundance of Lactobacillus in obese subjects and this increase correlated with obesity [69]. In a separate study, Nagano and Yano [28] showed that the combination of exercise and CNF intake increased Eubacteriaceae. Interestingly, Eubacterium siraeum could suppress fat deposition in HFD-induced obese pigs [70].

Khare et al. [45] conducted a study in 13-week-old male Wistar Han rats to evaluate the effects of CNF (1%) on gut microbiota following gavage (twice per week) for five weeks (Table 1). It was found that CNF ingestion altered microbial diversity and reduced the relative abundance of pathogenic bacteria. For example, both Porphyromonas spp. and Tannerella spp. were reduced [45]. Porphyromonas spp. are often pathogenic in the context of oral and periodontal health [71] and have been linked to obesity and diabetes [72,73,74]. Tannerella spp. are also associated with periodontitis, obesity and diabetes [75]. In addition, the population of Bacteroides acidifaciens was also lowered by CNF [45]. While Bacteroides acidifaciens might prevent obesity and improve insulin sensitivity in mice [76], a human study showed that the only significantly different gut microbiota was an increased genus Bacteroides in an autism cohort [77]. However, the numbers of live Bifidobacterium, a type of probiotic that is known to produce SCFAs, were also decreased by CNF ingestion [45]. Further, the population of Coprococcus catus, a bacterium notable for production of large quantities of SCFAs (e.g., propionic and butyric), was also reduced. Nonetheless, CNF had few effects on the fecal metabolome [45]. In fact, isobutyrate, an isomer of butyrate that is known for anti-obesity and anti-inflammatory properties, was significantly increased by CNF in the presence of cream [45]. There is evidence that isobutyrate can reshape the intestinal microbiota to increase the relative abundance of probiotics, especially the Lactobacillus strains [78]. Further, the level of isobutyrate may increase after bariatric surgery along with butyrate and propionate to have beneficial effects on weight management and insulin sensitivity [79].

Xu et al. [15] studied the effects of CNF at 30 mg/kg on gut microbiota in male C57BL/6 mice on a Western diet following one-month treatment by daily gavage (Table 1). Western diets are characterized by high animal protein, sugar, starch and fats, but a low fiber content [80]. CNF treatment had no significant effects on α diversity, while the β diversity was significantly altered. At the phylum level, CNF exposure significantly increased the abundance of Firmicutes, while decreased that of Bacteroides. Further, there were increases in the Clostridiaceae family, the Allobaculum and Turicibacter genera and the perfringens species, while decreases in the Clostridiales order, the S24-7 (also known as Muribaculaceae) and Gemellaceae families, the Anaerotruncus, Dorea, Acinetobacter and Gemella genera, and gnavus and uniformis species [15]. Many taxonomic alterations caused by CNF consumption might be beneficial for health as many anti-inflammatory taxonomic features were identified. For example, CNF consumption caused an increase in Allobaculum, which is associated with active utilization of glucose [81] and prevention of obesity and insulin resistance [82]. On the other hand, Muribaculaceae are linked to obesity, inflammation and metabolism [83]; Acinetobacter baumannii complex infection is associated with increased mortality in diabetic patients [84]; Anaerotruncus are strongly linked with obesity and gestational diabetes [85,86]; and the abundance in Dorea is higher in overweight/obese subjects [87].

The Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) algorism predicts the functional composition of metagenome associated with the microbial community from its 16S sequence profile [88]. When the PICRUSt was used to predict changes in functional contents, CNF treatment in male C57BL/6 mice decreased fatty-acid biosynthesis, lipid biosynthesis proteins, lipopolysaccharide biosynthesis proteins, sphingolipid metabolism, glycerolipid metabolism, glycosphingolipid biosynthesis pathways (lacto- and neolactorseries and ganglioseries) and glycosphingolipid biosynthesis (globoseries). On the other hand, CNF treatment increased pathways of propanoate metabolism, synthesis and degradation of ketone bodies and fatty acid metabolism [15]. In terms of carbohydrate processing functions, CNF treatment increased fructose and mannose metabolism, as well as glyoxylate and dicarboxylate metabolism, and decreased glycosaminoglycan degradation and lipopolysaccharide biosynthesis [15]. These findings are consistent with a previous report that subchronic exposure to CNF in the WD-fed C57BL/6 mice led to reduced fatty liver and blood glucose levels [26]. Decreases in globoseries glycosphingolipid biosynthesis by CNF might also be beneficial as the globoseries glycosphingolipids play a role in diabetic nephropathy by enhancing Toll-like receptor 4-mediated inflammation [89].

In male NOD mice following two months of daily treatment with CNF (30 mg/kg) by gavage when maintained on a Western diet (Table 1), no significant differences were found in either the α diversity or the β diversity [15]. However, there were increases in the VHS-B5-50 class, the Aerococcaceae family, the Epulopiscium genus and the ultimum species, and decreases in the YS2 and Desulfovibrionales orders, the Clostridiaceae family, the Bacteroides, Flavobacterium, Sporosarcina and Inquilinus genera. Many of these changes have also been associated with beneficial health effects such as increases in Epulopiscium and decreases in the Desulfovibrionales order and the Inquilinus genus. Studies have shown that increases in the abundance of Epulopiscium were linked to weight loss, reduced appetite and changes in eating behavior following bariatric surgery due to their role in the digestion and fermentation of carbohydrates [90]. In the Desulfovibrionales order of bacteria, Desulfovibrio desulfuricans has been associated with obesity and related conditions like non-alcoholic fatty liver disease [91], and Desulfovibrio has been linked to both type 1 and type 2 diabetes [92]. The YS2 order has been associated with cognitive dysfunction in mice [93], while the Inquilinus genus is associated with pulmonary diseases such as cystic fibrosis [94]. The PICRUSt analysis predicted decreases in glycosphingolipid biosynthesis, glyoxylate and dicarboxylate metabolism, protein folding and associated processing and arginine and proline metabolism, while increases in gluconeogenesis and dioxin degradation [15]. There is evidence that glycosphingolipids contribute to the development of insulin resistance and obesity. For instance, the ganglioside GM3 can induce insulin resistance in adipocytes and inflammation in adipose tissue [95]. Arginine has shown promise in reducing white fat mass and improving insulin sensitivity in obese animals [96], and the arginine and proline metabolism pathway is enriched in subjects with obesity and metabolic syndrome [97]. In addition, there is evidence that the glyoxylate pathway may contribute to fat-induced hepatic insulin resistance [98] and that dicarboxylic acids can counteract the metabolic effects of the Western diet by boosting energy expenditure to protect against obesity and type 2 diabetes [97,98,99,100,101].

After the male NOD mice consumed CNF for six months while being maintained on a Western diet (Table 1), neither the α diversity nor the β diversity was significantly changed [15]. The altered taxonomic features after six months of daily CNF consumption included increases in the Bacillaceae and Lachnospiraceae families, Photobacterium and Sporosarcina genera and the gnavus species, and decreases in the RF32 and YS2 orders, the S24-7 family, the Bacteroides and Parabacteroides genera, the uniformis and producta species [15]. Increases in the family Bacillaceae might be beneficial since many members of the Bacillaceae family produce anti-microbial peptides [102]. A decreased Bacteroides genus, which was observed in male NOD mice after CNF treatment for both two and six months, might also be beneficial as discussed earlier that the only significantly different gut microbiota was an increased genus Bacteroides in an autism cohort [77]. The PICRUSt analysis predicted that there were increases in the glycerolipid metabolism, biosynthesis of unsaturated fatty acids and pentose phosphate pathway, along with decreases in glycan degradation, amino sugar and nucleotide sugar metabolism and glycosphingolipid biosynthesis of both globoseries and ganglioseries [15]. An increased glycerolipid metabolism is considered beneficial as triacylglycerols are a major form of stored energy in the body, and their accumulation in adipose tissue is a key feature of obesity [103]. An increased level of unsaturated fats may prevent fat mass gain [104]. A decreased glycan degradation can be beneficial as some degradation products may contribute to inflammation and metabolic dysfunction [105]. Additionally, amino sugar and nucleotide sugar metabolism pathways are enriched in subjects with obesity and metabolic syndrome [97]. Decreases in globoseries glycosphingolipid biosynthesis by CNF might also be beneficial as discussed earlier since the globoseries glycosphingolipids play a role in diabetic nephropathy [89].

5.1.2. Effect of Cellulose Nanofibril on Gut Microbiota in Females

Xu et al. [16] reported the effects of CNF at 30 mg/kg on gut microbiota in female C57BL/6 mice on a Western diet following one-month treatment by gavage (

Table 2. Summary of gut microbiome studies on cellulose nanofibril in females.

Nanocellulose	Model/Diseases	Model/Population	Exposure Window/Time	Dose/Concentration	Routes of Administration	Diet/Medium	Effects	Reference
CNF	Obesity/Gut microbiota	Female C57BL/6 mice	30 days	30 mg/kg	Gavage daily	Western diet	The β diversity was altered. In addition, there were many beneficial changes in taxonomic features. The seemingly beneficial effects of CNF intake are supported by pathway analysis.	[16]
CNF	Type 1 diabetes	Female NOD mice	2 months	30 mg/kg	Gavage daily	Western diet	The β diversity was altered. In addition, there were changes in taxonomic features and predicted functional contents (e.g., increases in biosynthesis of unsaturated fatty acids).	[16]
CNF	Type 1 diabetes	Female NOD mice	6 months	30 mg/kg	Gavage daily	Western diet	The α diversity was increased. Further, the genus Enterococcus was increased, and the histidine metabolism was predicted to increase.	[16]

). While no significant differences were found in the α diversity, β diversity was significantly altered. Following CNF treatment, there were increases in the Lactobacillales and Pasteurellales orders and the Streptococcaceae, Planococcaceae and Pirellulaceae families as well as the Coprobacillus, Propionigenium and Lactococcus genera [16]. As discussed earlier, the probiotic Lactobacillus strains have been used to modulate gut microbiota composition in the context of obesity [67,68]. The genus Coprobacillus has been identified as being more abundant in healthy individuals compared to obese individuals [106]. Propionigenium might have protective properties against metabolic illness and obesity by producing propionate [107]. Lactococcus strains, particularly Lactococcus lactis, have shown potential in combating obesity through regulating lipid metabolism and modulating adipogenesis [108]. On the other hand, CNF decreased the Firmicutes phylum, the Bacteroidales order, the Micrococcaceae and S24-7 families, the Bacteroides and Clostridium genera and the acidifaciens species [16]. As discussed earlier, one study has shown that the only significantly different gut microbiota was an increased genus Bacteroides in an autism cohort [77]. The seemingly beneficial effects of CNF intake in female C57BL/6 mice are supported by pathway analysis. Using PICRUSt, fatty acid biosynthesis, propanoate metabolism and butanoate metabolism were predicted to increase following CNF treatment, along with decreases in galactose metabolism, glycosphingolipid biosynthesis-ganglioseries and glycosaminoglycan degradation [16]. The propanoate metabolism and butanoate metabolism would increase the production of the SCFAs butyrate and propionate, respectively. On the other hand, galactose metabolism is a biochemical process that converts galactose into glucose [109], which would increase blood sugar levels. Indeed, galactose metabolism pathway is enriched in subjects with obesity and metabolic syndrome [97].

Following exposure to CNF (30 mg/kg) for two months in female NOD mice while being maintained on a Western diet (Table 2), no significant changes were observed in α diversity, but there was a significant differnce in β diversity. Further, there were significant increases in the family Clostridiaceae, and the genera SMB53, Prevotella and Clostridium and the species muciniphila [16]. The genus Provotella was known for producing beneficial SCFA acetate [110], while increases in the species A. muciniphila have been associated with decreases in obesity, diabetes, inflammation and metabolic disorders in humans [111]. On the other hand, following CNF treatment for two months in female NOD mice, there were decreases in the families Desulfovibrionaceae, Enterobacteriaceae and Lachnospiraceae, the genera Enterovibrio, Roseburia and Bilophila and the species gnavus and producta [16]. As discussed earlier, Desulfovibrio has been associated with obesity and non-alcoholic fatty liver disease and both type 1 and type 2 diabetes [91,92]. Enterobacteriaceae have also been linked to obesity, with some species like Enterobacter cloacae potentially inducing obesity and insulin resistance, particularly in high-fat diet conditions [112]. Enterovibrio, belonging to the Enterobacteriaceae family, exhibits an abnormal proliferation in conditions like obesity [113]. Many species in the Lachnospiraceae family are associated with intra- and extraintestinal diseases such as IBS and depression, despite producing SCFAs [114]. The genus Bilophila has been associated with high-fat diet exposure and bile-resistance [115]. Further, PICRUSt predicted that there would be significant increases in fatty acid biosynthesis, lipid biosynthesis proteins, biosynthesis of unsaturated fatty acids and glycerophospholipid metabolism following CNF treatment for two months in female NOD mice [16]. Increases in biosynthesis of unsaturated fatty acids are considered beneficial in reducing fat accumulation as discussed earlier [104].

After six months of treatment with CNF (30 mg/kg) in female NOD mice (Table 2), there was a significant increase in α diversity but not in β diversity [16]. Further, the genus Enterococcus was significantly increased. The relationship between Enterococcus and obesity is complex. Some Enterococcus species, particularly Enterococcus faecalis and Enterococcus faecium, have been studied for their potential roles in both promoting and mitigating obesity and related metabolic disorders. For example, E. faecalis AG5 has shown promise in improving glucose metabolism and insulin sensitivity [116], while others, such as E. faecium B6, have been linked to non-alcoholic fatty liver disease in obese individuals [117]. In addition, the Enterococcus genus is known to contain major causative agents of healthcare-associated infections [83], and it can cause infections in individuals with diabetes, particularly in the context of diabetic foot infections [118]. When PICRUSt was conducted, histidine metabolism was predicted to increase [16]. While histidine intake may help prevent obesity and improve insulin sensitivity and glucose control, certain gut bacteria can convert histidine into imidazole propionate, which may contribute to insulin resistance [119]. Further, histidine can promote glucose synthesis through activation of the gluconeogenic pathway [120]. Nevertheless, after six months of treatment with CNF in female NOD mice, there were significant decreases in the Mac-3⁺ macrophage and CD4⁺CD25⁺ T-cell populations in spleen, and the serum concentration of LIX (C-X-C motif chemokine 5, a small proinflammatory chemokine), suggesting that long-term CNF consumption in female NOD mice might decrease inflammation [16]. Higher LIX levels have been observed in type 2 diabetes mellitus patients [121].

5.2. Effect of Cellulose Nanocrystals on Gut Microbiota

Wang et al. [55] reported that CNC at the dosage of 100 mg/kg/day could effectively restore the dysbiosis and reverse the reduced richness and evenness and the Firmicutes/Bacteroidetes ratio in constipated mice (

Table 3. Summary of gut microbiome studies on cellulose nanocrystals.

Nanocellulose	Model/Diseases	Model/Population	Exposure Window/Period	Dose/Concentration	Routes of Administration	Diet/Medium	Effects	Reference
CNC	Obesity	Female C57BL/6 mice	30 days	30 mg/kg	Gavage daily	Western diet	The α diversity was significantly altered. Further, there were changes in taxonomic features that were mostly beneficial.	[16]
Microcrystalline cellulose of nanometric size (125 nm)	Prebiotics/Gut microbiota	Male Wistar rats (5 wks old)	14 days	250 mg/kg	Gavage (Twice daily)	Normal (AIN-93G) diet	There were increased SCFA yields as well as Bifidobacterium counts when compared to both control and the micro scale size cellulose.	[53]
CNC	Constipation	Female ICR mice	5 days	50–150 mg/kg	Gavage daily	Standard chow	CNC at the dosage of 100 mg/kg/day could effectively restore the disordered gut microbiota mediated by constipation. Further, the contents of SCFAs were increased.	[55]
CNC	Ulcerative colitis	Female C57BL/6J mice	7 days	50–200 mg/kg	Gavage daily	Standard chow	CNC at the dosage of 200 mg/kg/day could effectively restore the disordered gut microbiota mediated by ulcerative colitis. Further, six selected beneficial bacteria, including Akkermansia, Odoribacter, Lactobacillus, Prevotellaceae_UCG-001, Anaerotruncus and Roseburia, were significantly increased.	[62]

). Further, CNC treatment reversed constipation-mediated decreases in several strains, including Lactobacillus, Prevotellaceae_UCG-001 and Anaerotruncus. Additionally, CNC treatment also significantly increased the relative abundances of Bacteroides, Alloprevotella, Ruminococcaceae_UCG-014, and Ruminiclostridium_9 [55]. As discussed earlier, CNF intake also increased the abundance of Lactobacillaceae in HFD-fed male C57BL/6N mice [14] and the Lactobacillales order in female C57BL/6 mice on a Western diet [16]. Further, the probiotic Lactobacillus strains have been used to modulate gut microbiota composition in the context of obesity [67,68]. Similarly to CNC, CNF intake also increased the genus Prevotella in female NOD mice while maintained on a Western diet [16], and the genus Provotella was known for producing beneficial SCFA acetate [110]. In contrast, Alloprevotella has a complex relationship with obesity, with some research showing reduced abundance in individuals with obesity, while other studies link increased abundance of Prevotella genus (which includes Alloprevotella) to potentially better weight loss outcomes on certain high-fiber diets [122]. For example, Alloprevotella rava is lower in subjects with obesity, and its abundance may also correlate with certain types of dyslipidemia [122]. Similarly, Ruminococcaceae has been implicated in both obesity promotion and obesity resistance. Some studies suggest that certain Ruminococcaceae species are associated with reduced body weight and improved metabolic health [123], while others indicate a link between specific Ruminococcaceae and increased risk or development of obesity [124]. In terms of Ruminiclostridium, the precise role of this genus is not uniformly associated with obesity. Some studies indicate Ruminiclostridium species, such as Ruminiclostridium 9, are linked to an increased risk of obesity, while others suggest antibiotics that deplete Ruminiclostridium can prevent obesity [125,126].

In another study, Wang et al. [62] showed that CNC at the dosage of 200 mg/kg/day could effectively reverse the reduced richness and restore the dysbiosis in mice with ulcerative colitis (Table 3). Six selected bacteria were significantly increased by CNC treatment, including Akkermansia, Odoribacter, Lactobacillus, Prevotellaceae_UCG-001, Anaerotruncus and Roseburia [62]. Akkermansia can increase the number of goblet cells and maintain/increase the thickness of the mucus layer [111]. Odoribacter can induce the differentiation of T helper cell 17 to alleviate the inflammation in colon [127]. As discussed earlier, increases in both Lactobacillus and Prevotella are considered beneficial [67,68,110]. Roseburia is also a genus of beneficial gut bacteria associated with a lean body composition and various positive health outcomes [128].

Xu et al. [16] also studied the effects of CNC at 30 mg/kg on gut microbiota in female C57BL/6 mice on a Western diet following one-month treatment by gavage (Table 3). The α diversity, but not β diversity, was significantly altered. Further, there were increases in the order Chroococcales, the families Christensenellaceae, Ruminococcaceae and Desulfovibrionaceae, and the genera Bilophila, Lactococcus and Bacteroides [16]. Christensenellaceae was significantly enriched in individuals with a normal BMI (18.5–24.9) compared to obese individuals [129]. The increase in Ruminococcaceae is consistent with the report of Wang et al. [55] that CNC treatment significantly increased the relative abundance of Ruminococcaceae_UCG-014. The genus Lactococcus, which has potentials in reducing obesity [108], was also increased by CNF in female C57BL/6 mice [16]. On the other hand, CNC treatment in female C57BL/6 mice decreased the phylum Firmicutes, the families Coriobacteriaceae, F16 and Staphylococcaceae, and the genera Allobaculum and Clostridium [16]. Staphylococcaceae includes the common pathogen Staphylococcus aureus, so a decreased abundance would likely be beneficial. In terms of Coriobacteriaceae, high-fat diets can increase its abundance [130], suggesting a reduction in this family might also be beneficial [131]. However, some studies also showed it to be associated with normal weight and a potential benefit for glucose metabolism [132,133].

Finally, Nsor-Atindana et al. showed that microcrystalline cellulose of nanometric size (125 nm) at the 250 mg/kg dose significantly increased the counts of Bifidobacterium in 5-week-old male Wistar rats [53].

5.3. Effect of Bacterial Nanocellulose on Gut Microbiota

BNC has been proposed to use as an encapsulating material to improve the survivability of probiotics against the gastrointestinal condition [134]. Shipelin et al. [12] studied the effects of oral administration of BNC on some of the cecal microbiome populations (

Table 4. Summary of gut microbiome studies on bacterial nanocellulose.

Nanocellulose	Model/Diseases	Model/Population	Exposure Window/Period	Dose/Concentration	Routes of Administration	Diet/Medium	Effects	Reference
BNC	Toxicology	Male Wistar rats	6 weeks	1–100 mg/kg	Diet	AIN-93M	The most changes in the state of the microbiome were observed at the 10 mg/kg dose of BNC. There were increases in the number of hemolytic aerobes and Lactobacilli, simultaneously with a decrease in Enterococci.	[12]
BNC	Obesity	Male ICR mice (4 weeks old)	9 weeks	15%	Diet	Standard chow	It seemed that Romboutsia and Eubacterium were increased while Faecalibaculum was decreased following BNC consumption.	[135]
BNC	Obesity	Male ICR mice (4 weeks old)	9 weeks	15%	Diet	HFD	High-fat diet significantly decreased gut microbiota diversity, which could be reversed by consuming BNC. Decreases in Firmicutes, F/B ratio and (F + P)/B ratio, and an increased Bacteroidetes were observed following BNC consumption.	[135]

). By inoculating the rat cecum contents in tenfold dilutions on specific nutrient media, it was found that BNC at the dose of 1 mg/kg significantly increased the number of CFUs of total aerobes. At the dose of 10 mg/kg, an increase in the number of hemolytic aerobes and Lactobacilli, simultaneously with a decrease in Enterococci. At the dose of 100 mg/kg, the most noticeable alteration was a decrease in the content of molds. Therefore, the most changes in the state of the microbiome were observed at the 10 mg/kg dose of BNC [12].

Using the high-throughput sequencing, Han et al. [135] found that Romboutsia and Eubacterium were increased while Faecalibaculum was decreased following dietary BNC consumption (15%) when male ICR mice were fed a standard chow (Table 4). There is evidence that Romboutsia lituseburensis JCM1404 supplementation can improve lipid metabolisms in obese rats [136]. In contrast, Faecalibaculum rodentium, which is increased by a high-sugar diet, can accelerate obesity [137]. Further, a high-fat diet significantly decreased gut microbiota diversity in male ICR mice, which could be reversed following BNC consumption [135]. BNC treatment in HFD-fed mice decreased Firmicutes and increased Bacteroidetes, and further analysis suggested that BNC increased Eubacterium and reversed HFD-mediated increases in the populations of Coprococcus, Roseburia, Blautia, Ruminiclostridium, Anaerotruncus and Intestinimonas [135]. Intestinimonas genus bacteria, particularly Intestinimonas butyriciproducens, are linked to obesity [138]. As discussed earlier, Nagano and Yano [28] showed that the combination of exercise and CNF intake increased Eubacteriaceae in mice, and Eubacterium siraeum could suppress fat deposition in HFD-induced obese pigs [70]. Similarly to BNC treatment, the populations of Coprococcus catus and Roseburia were also reduced by CNF [16,45]; however, Roseburia and Ruminiclostridium were increased by CNC [55]. Blautia is involved in carbohydrate metabolism, and the association between the genus Blautia and obesity is complex and appears to be species dependent. Some studies found beneficial roles for certain Blautia species (like B. wexlerae and B. hansenii) in reducing fat accumulation and improving metabolic health, while other research has noted high levels of the genus in obese individuals or a positive correlation with BMI in some populations [139,140]. As discussed earlier, Anaerotruncus are strongly linked with obesity and gestational diabetes [85,86], which can be decreased by CNF [15] and BNC [135], but increased by CNC [55,62].

5.4. Other Studies of Nanocellulose Derivatives on Gut Microbiota

There are some studies, both in vivo and in vitro, on gut microbiota following exposure to other nanocelluloses (

Table 5. Summary of gut microbiome studies on other nanocellulose derivatives.

Nanocellulose	Model/Diseases	Model/Population	Exposure Window/Period	Dose/Concentration	Routes of Administration	Diet/Medium	Effects	Reference
TEMPO-CNF	Obesity	Female C57BL/6J mice	30 days	30 mg/kg	Gavage	Western diet	No significant alterations were found in either α or β diversity. However, there were many taxonomic features that were significantly affected. These changes were mostly beneficial.	[16]
Variable sized microcrystalline cellulose	In vitro/Prebiotics	Fecal matter from healthy donors	24 h	1%	Batch-culture fermentation	Fermentation medium	The production of acetate, butyrate and propionate was significantly increased by the size reduction.	[53]
Four types of cellulose derivatives	In vitro	Fecal matter from healthy donors	0–36 h	10 mg/mL	Batch-culture fermentation	Fermentation medium	TEMPO-CNF had the highest aspect ratio and produced the highest amount of total SCFAs. TEMPO-CNF and TEMPO-CNC exhibited an opposite effect on the gut microbiota composition at the phylum level.	[56]

). Using fecal matter from healthy donors, Nsor-Atindana et al. showed that the production of acetate, butyrate and propionate was significantly increased by reducing the size of microcrystalline cellulose in an in vitro fermentation study, with the smallest particle size cellulose yielding the highest concentration of SCFAs [53]. Another study investigated the in vitro fermentation profiles of four types of cellulose derivatives, including microcrystalline cellulose (MCC) with a size 246 μm in length and 40 μm in diameter, TEMPO-CNF with a size 1033 nm in length and 4.1 nm in diameter, TEMPO-oxidized nanocrystalline cellulose (TEMPO-CNC) with a size of 253 nm in length and 8.5 nm in diameter, and soluble carboxymethyl cellulose [56]. TEMPO-CNF had the highest aspect ratio and produced the highest amount of total SCFAs (approximately 20 mM). Thus, reduced particle size and increased aspect ratio improved the fermentability of cellulose derivatives and increased the amount of SCFAs after fermentation [56]. Surprisingly, the fermentable cellulose derivatives TEMPO-CNF and TEMPO-CNC exhibited an opposite effect on the gut microbiota composition at the phylum level. The relative abundance of Bacteroidetes phylum was increased with TEMPO-CNF treatment but decreased with TEMPO-CNC treatment, while the relative abundance of Firmicutes was increased with TEMPO-CNC treatment but decreased with TEMPO-CNF treatment [56].

Xu et al. [16] reported the effects of TEMPO-CNF at 30 mg/kg on gut microbiota in female C57BL/6 mice on a Western diet following one-month treatment by gavage (Table 5). No significant alterations were found in either α or β diversity. However, there were many taxonomic features that were significantly affected. The Chroococcales, Pasteurellales and CAB-I orders, the Christensenellaceae, Streptococcaceae, Enterobacteriaceae and Desulfovibrionaceae families, and the Blautia, Lactococcus, Epulopiscium and Bacteroides genera were significantly increased [16]. The Chroococcales and Pasteurellales orders were also increased by CNC and CNF, respectively, in the same study [16]. As discussed earlier, Christensenellaceae, which was significantly enriched in individuals with a normal BMI compared to obese individuals [129], was also increased by CNC [16]. Epulopiscium, which was linked to weight loss, reduced appetite and changes in eating behavior following bariatric surgery [90], was also increased by CNF [15]. On the other hand, the Firmicutes phylum, the Micrococcaceae, F16 and Coriobacteriaceae families, and the Bifidobacterium, Clostridium and Allobaculum genera were significantly decreased by TEMPO-CNF [16]. Interestingly, as discussed earlier, TEMPO-CNF also decreased the relative abundance of Firmicutes in an in vitro study [56]. Further, CNF also decreased the Firmicutes phylum and the Micrococcaceae family in female C57BL/6 mice [16]. CNC treatment in female C57BL/6 mice also decreased the phylum Firmicutes, the families Coriobacteriaceae and F16 and the genera Allobaculum and Clostridium [16]. In addition, BNC also decreased Firmicutes in male ICR mice [135]. On the other hand, CNF increased the abundance of Firmicutes in male C57BL/6 mice [15].

In summary, nine microbiota studies were identified for CNF with six conducted in males (Table 1) and three in females (Table 2). For CNC (Table 3) and BNC (Table 4), four and three studies were found, respectively. In addition, there are three more microbiota studies identified for other nanocellulose derivatives (Table 5). Based on these reports, it can be concluded that gut microbiota changes were mostly beneficial in terms of diabesity reduction. While the alterations in the diversity of gut microbiota following nanocellulose consumption might be dependent on the time of exposure, sex, strains/species and types of nanocellulose, there are some common findings among these studies in terms of taxonomic alterations. For example, CNF exposure decreased the phylum Bacteroides in male C57BL/6 mice, and the Bacteroides genus was also decreased in male NOD mice following CNF exposure for two and six months [15]. Further, the Bacteroidales order, the Bacteroides genus and the Bacteroides acidifaciens species were decreased by CNF in female C57BL/6 mice [16]. Bacteroides acidifaciens was also lowered by CNF in male Wistar Han rats [45]. On the other hand, Bacteroides genus was increased by CNC [16,55] and TEMPO-CNF [16]. In addition, BNC also increased phylum Bacteroidetes [135]. For Bifidobacterium, it was decreased by CNF ingestion [45] and by TEMPO-CNF consumption [16]. Further, the relative abundance of both Bifidobacterium bifidum and Bifidobacterium longum were decreased by both TEMPO-CNF and TEMPO-CNC treatments in vitro [56]. However, it was increased by CNC [53].

The role of Clostridiaceae in obesity is complex, with some species like Clostridium butyricum being potentially beneficial as probiotics that can alleviate obesity [141], while others, such as Clostridium ramosum and Clostridium sporogenes, may promote obesity by increasing nutrient absorption or fat deposition [142]. For cellulose nanofibrils, there were increases in the Clostridiaceae family and decreases in the Clostridiales order in CNF-treated male C57BL/6 mice, while CNF treatment for two months in male NOD mice decreased the Clostridiaceae family [15]. In female C57BL/6 mice, CNF decreased the Clostridium genus [16]. In female NOD mice following two months of CNF intake, there were significant increases in the family Clostridiaceae and the genus Clostridium [16]. The diversity and abundance of Clostridiaceae in the gut microbiota, particularly the loss of beneficial Clostridia species in favor of others like Desulfovibrio, can be negatively associated with obesity in humans and animal models [143]. In this context, CNF was shown to decrease Desulfovibrionales order in male NOD mice following two months of exposure [15]; similarly, CNF treatment for two months in female NOD mice also decreased the family Desulfovibrionaceae [16]. However, CNC as well as TEMPO-CNF treatment for one month in female C57BL/6 mice on a Western diet increased the Desulfovibrionaceae and decreased the genus Clostridium in female C57BL/6 mice [16]. Therefore, further studies are needed to understand why various types of nanocellulose affect gut microbiota differently.

6. Possible Detrimental Effects Associated with Nanocellulose-Mediated Changes in Gut Microbiota

A comparison of the effects of different nanocellulose on the gut microbiota was surprising, and it seems that CNF treatment produced the most changes [16]. As discussed above, most of the modulations of gut microbiota and associated metabolites and pathways by CNF seem to be beneficial, especially in alleviating diabesity. However, studies have also suggested that some detrimental effects are likely. For example, the species R. gnavus was increased by CNF in male NOD mice following six months of CNF exposures [15]. R. gnavus produces an inflammatory polysaccharide [144] and has been associated with increased depression [145]. Following CNF treatment for two months in NOD females, there was an increase in the SMB53 genus, which has been associated with obesity [146,147], and the abundance of SMB53 positively correlated with the 4 h fasting blood glucose levels [16]. Following CNF treatment in female C57BL/6 mice for one month, there were increases in the Pasteurellales order and the Streptococcaceae and Planococcaceae families [16]. As discussed earlier, the increases in Streptococcaceae correlated with HFD-induced obesity and diabetes [63]. Certain species of bacteria in the Pasteurellales order have potential associations with obesity and related metabolic health [148], and a higher relative abundance of Planococcaceae has been observed in individuals with obesity [149]. High levels of isobutyrate, which was observed in CNF-treated rats [42], may be associated with an increased risk of gestational diabetes [150]. Furthermore, CNF administration in female NOD mice for 2 months was predicted to increase fatty acid biosynthesis, which was also predicted to increase in female C57BL/6 mice following treatment with CNF for one month [16]. There is evidence that defects in glucose uptake and storage may be related to abnormal fat metabolism, e.g., elevated blood fatty acid concentrations [151]. PICRUSt also predicted that there would be significant increases in glycerophospholipid metabolism following CNF treatment for two months in female NOD mice [16]. Glycerophospholipid metabolism plays a significant role in obesity, with alterations in these lipid molecules potentially contributing to insulin resistance and other metabolic dysfunctions associated with obesity [152]. Gluconeogenesis, which could increase blood glucose levels, was also predicted to increase in male NOD mice after two months of daily treatment with CNF [15].

On the other hand, GABA-producing Bacteroides uniformis, known to alleviate inflammation associated with obesity [153,154], was decreased after CNF treatment in male C57BL/6 mice following a 1-month treatment and in male NOD mice following a 6-month treatment [15]. S24-7, a major commensal bacteria family, was also decreased by CNF in the 1-month male C57BL/6 mouse study and the 6-month male NOD mouse study [15], and in the 1-month female C57BL/6 mouse study [16]. A previous study indicated that the decrease in S24-7 was associated with IBS [155]. In male NOD mice following six months of CNF exposures and female NOD mice following two months of CNF exposures, a decrease in Blauntia producta was observed [15,16]. Blauntia producta belongs to the Lachnospiraceae family and can alleviate inflammatory and metabolic diseases with its antibacterial activity [156]. The order RF32, which negatively correlates with depression [157], was significantly decreased in the male NOD mice consumed CNF for six months [15]. In addition, there were decreases in the Flavobacterium genus and the Sporosarcina genus in male NOD mice following 2 months of CNF exposures [15]. The Flavobacterium genus produces metabolite quercetin, a polyphenol and antioxidant [158], while the Sporosarcina genus shows promise as a probiotic [159]. Further, there were significant decreases in Roseburia, a lean-associated genus [131], in NOD females following CNF treament for two months, and the abundance of Roseburia negatively correlated with the 4 h fasting blood glucose levels [16]. Additionally, while CNF-mediated decrease in the species R. gnavus in female NOD mice might be beneficial for Crohn’s disease, its abundance negatively correlated with the 4 h fasting blood glucose levels [16].

The long-term CNF exposure study indeed suggested some adverse health effects. In male NOD mice, CNF treatment for six months significantly increased immobility time in the tail suspension test, indicating an increased depressive-like symptoms [15]. Interestingly, consumption of cellulose alone as the dietary fiber could also evoke intestinal abnormalities and enhance anxiety in male ICR mice [160]. In support that inflammation plays a crucial role in the development of depression-like symptoms [161], the inflammatory cytokines, including KC, IL-3 and IL-12p70, were increased following six months of CNF exposures in male NOD mice [15]. In female NOD mice, CNF treatment for six months also produced some adverse effects. For example, CNF-mediated decreases in Desulfovibrionaceae and Enterovibrio negatively correlated with the 4 h fasting blood glucose levels. In addition, during the 2-month insulin tolerance test, the 4 h fasting blood glucose levels of the CNF treatment group were significantly higher than that of the control group [16]. Similarly to this study, a previous study in C57BL/6 mice has shown that the CNF treatment for six weeks produced higher fasting blood glucose levels than the control groups and increased insulin resistance [26]. As insulin resistance is mainly due to an inability of insulin to appropriately promote glucose uptake in skeletal muscle, it was evident that CNF-treated female NOD mice exhibited signs of insulin resistance following CNF treatment for two months [16]. Chen et al. have shown that CNF non-specifically reduced intestinal absorption and decreased blood glucose levels in both male and female C57BL/6 mice [26]. Further, using a serial of artificial digestive solutions, it was shown that CNF decreased starch digestion and glucose release by directly absorbing glucose [31]. Therefore, CNF-mediated non-specific decrease in intestinal absorption might lead to nutritional risks after long-term exposure.

It should be noted that some of the changes in gut microbiota mediated by CNC and BNC and other nanocellulose derivatives may also be harmful. For example, Bilophila, which has been associated with high-fat diet exposure and bile resistance [115], was increased by CNC in WD-fed female C57BL/6 mice [16]. Anaerotruncus, which was also increased by CNC [55,62], was strongly linked with obesity and gestational diabetes [85,86]. The family Desulfovibrionaceae was increased by CNC in female C57BL/6 mice, and unfortunately members in this family such as Desulfovibrio have been associated with obesity and related conditions like non-alcoholic fatty liver disease and diabetes [91,92]. CNC treatment in female C57BL/6 mice also decreased the genera Allobaculum [16]; however, Allobaculum is associated with active utilization of glucose [81] and prevention of obesity and insulin resistance [82]. In terms of BNC, its treatment reduced the population of Roseburia [135], which is a common gut probiotic that can synthesize lactic acid and SCFAs and promote colonic motility [162]. For TEMPO-CNF, its treatment increased Pasteurellales order, the Streptococcaceae, Enterobacteriaceae and Desulfovibrionaceae families [16]. As discussed earlier, certain species of bacteria in the Pasteurellales order have potential associations with obesity and related metabolic health [148], the increases in Streptococcaceae correlated with HFD-induced obesity and diabetes [63,64] and the Enterobacteriaceae have also been linked to obesity [112].

7. Conclusions and Future Directions

Nanocellulose, including CNF, CNC, BNC and other nanocellulose derivatives (e.g., TEMPO-CNF), is emerging as a potential material for many applications. As a potential dietary supplement, nanocellulose has found its utilization in the prevention of obesity and diabetes. With its nanoscale structure and large surface area, nanocellulose can interact with lipids in the digestive system to delay lipid digestion and reduce fat absorption. By interfering with the digestive enzymes, it can also delay starch digestion and reduce glucose absorption. Further, nanocellulose might help encapsulate probiotics to protect them from environmental stresses. Nanocellulose has also been explored for its potential applications in diabetes management, particularly in diabetic wound healing and glucose monitoring. Additionally, nanocellulose-based systems are being investigated for oral insulin delivery. While the research on the applications of nanocellulose is promising, it is important to realize that much of the work is still in the early stages. Importantly, studies have found certain detrimental effects associated with exposure to nanocellulose. The absorption of nanocellulose in the upper GI tract is limited or negligible, but they can be fermented by the bacteria in the colon through anaerobic degradation. Nanocellulose may affect diabesity by changing the diversity and composition of the gut microbiota. It was found that the interactions between nanocellulose and gut were complex, with alterations of microbial composition, metabolic pathways, and immune functions both locally and systemically. These changes might be dependent on the time of exposure, sex, strains/species and types of nanocellulose. Short-chain fatty acids are the major metabolites of anaerobic fermentation of nanocellulose, which can be affected by both particle size and aspect ratio. There seemed to be many beneficial changes in the gut microbiota in terms of diabesity reduction following short-term exposure to nanocellulose; however, some of these effects were no longer seen after long-term consumption. Importantly, long-term nanocellulose consumption might be associated with certain detrimental health effects, e.g., malnutrition and its associated neurotoxicity, although additional studies are needed to substantiate such health implications.

Nanocellulose can be applied and further engineered and potentially modified and developed as therapeutic agents to restore a healthy microbiome balance by promoting beneficial bacteria and/or inhibiting harmful ones. For example, Yu et al. reported that citric acid-crosslinked carboxymethyl CNF exhibited a robust expansion capacity [163]. Its treatment in mice reduced food intake and delayed digestion rate, mitigated diet-induced obesity and insulin resistance, and ameliorated inflammation. Additionally, its supplementation enriched probiotics such as Bifidobacterium and decreased the relative abundances of deleterious microbiota expressing bile salt hydrolase, which led to increased levels of conjugated bile acids and decreased intestinal FXR signaling to stimulate the release of GLP-1 [163]. Despite many positive aspects, the health/safety, environmental, and certain regulatory issues related to nanocellulose production and usage are still debatable among scientists and lawmakers. Since the measurable toxicities associated with short-term use of nanocellulose are less apparent, more long-term in vivo studies in different animal models and human studies are needed to assess the safety and efficacy of nanocellulose-based agents and underlying mechanisms. As nanocellulose uptake alters gut microbial populations and intestinal function to affect human health, further studies of nanocellulose on intestinal health, and consequently, overall health effects, and its interactions with probiotics should be conducted. For instance, research is needed to fully understand the mechanisms underlying the complex interactions between nanocellulose and the gut microbiota, and their influences on the gut–brain axis and gut–immune axis and potentially impacting brain-related diseases. Finally, studies are needed to optimize the use of nanocellulose in clinical settings, especially in the presence of probiotics. This information is critical for developing safe and effective nanocellulose derivatives that can be applied in therapeutics and diagnostics as well as to harness the benefits of the gut microbiota.