L-isoleucine regulates the balance of intestinal flora

L-isoleucine, an essential branched-chain amino acid (BCAA) in humans, not only participates in protein synthesis and energy metabolism but has also been found in recent studies to regulate the composition and function of gut microbiota through multiple pathways, playing a potential role in maintaining gut microecological balance. This regulatory effect is closely related to the metabolic characteristics of isoleucine, its impact on the intestinal barrier, and its interaction with microbiota. The specific mechanisms are as follows:

I. Direct Role of Isoleucine as a Microbial Metabolic Substrate

1. Selective Promotion of Beneficial Bacteria Proliferation

Gut microbiota such as Bacteroidetes (e.g., Bacteroides fragilis) and Firmicutes (e.g., Faecalibacterium) can utilize isoleucine as a carbon and nitrogen source. Studies have shown that isoleucine supplementation can increase the number of Akkermansia muciniphila in the mouse intestine by 2-3 times. This bacterium promotes barrier function by degrading the intestinal mucus layer. Meanwhile, isoleucine can reduce the colonization ability of pathogenic bacteria (e.g., E. coli, Salmonella) by inhibiting the expression of their virulence genes.

Mechanism: Isoleucine is metabolized by microbiota to produce short-chain fatty acids (SCFAs), such as propionic acid and butyric acid. Butyric acid, the main energy source for colonic epithelial cells, promotes mucus secretion and tight junction protein expression, indirectly inhibiting pathogenic bacteria adhesion.

2. Regulation of Microbial Metabolic Pathways

Under the action of microbiota, isoleucine can generate α-ketoisocaproic acid (KIC) through the branched-chain amino acid transaminase (BCAT) and branched-chain α-keto acid dehydrogenase (BCKD) pathways, which is then converted into SCFAs such as propionic acid. When isoleucine intake is insufficient, microbial metabolism tends to utilize other amino acids (e.g., tryptophan, tyrosine), possibly leading to an increase in harmful metabolites (e.g., phenols, indoles). Appropriate isoleucine can optimize the microbial metabolic profile and reduce the production of pro-inflammatory substances.

II. Indirect Regulation of Intestinal Barrier and Immunity by Isoleucine

1. Enhancement of Intestinal Physical Barrier Function

Isoleucine can promote the synthesis of tight junction proteins (e.g., ZO-1, occludin) in intestinal epithelial cells by activating the mTOR signaling pathway, reducing intestinal permeability. In mouse experiments, isoleucine deficiency leads to a 40% reduction in the thickness of the intestinal mucus layer, increasing the risk of microbial translocation. Isoleucine supplementation (0.5% dietary addition) restores the integrity of the mucus layer and inhibits the migration of pro-inflammatory bacteria to mesenteric lymph nodes.

2. Regulation of Intestinal Immunity and Microbial Interaction

The isoleucine metabolite KIC can act as a signaling molecule to activate the aryl hydrocarbon receptor (AhR) of intestinal lamina propria T cells, promoting IL-22 secretion. IL-22 induces the expression of antimicrobial peptides (e.g., Reg3γ), selectively inhibiting pathogenic bacteria (e.g., Citrobacter rodentium) while protecting commensal bacteria. In addition, isoleucine can reduce the levels of intestinal pro-inflammatory factors (TNF-α, IL-6), minimizing the disruption of microbial balance caused by excessive immunity.

III. Correlation Between Isoleucine Dosage and Microbial Regulation

1. Bidirectional Regulation Effect of Physiological Dosage

Low dosage (0.1-0.3 g/kg body weight/day): Mainly improves the intestinal environment by promoting SCFA production, with little effect on microbial diversity. For example, healthy adults supplemented with 0.2 g/kg isoleucine daily showed a 15%-20% increase in the abundance of intestinal butyrate-producing bacteria after 4 weeks, without significant changes in the overall microbial structure.

High dosage (>0.5 g/kg body weight/day): May cause microbial metabolic disorders. Animal experiments have shown that excessive isoleucine (1 g/kg/day) can lead to an imbalance in the ratio of Bacteroidetes/Firmicutes and promote the proliferation of conditional pathogens such as Enterococcus faecalis, possibly due to excessive SCFAs inhibiting the growth of other microbiota.

2. Regulatory Differences in Pathological States

In inflammatory bowel disease (IBD) models, isoleucine supplementation (0.4 g/kg/day) can reverse microbial dysbiosis, reducing pro-inflammatory bacteria (e.g., Desulfovibrio) and increasing anti-inflammatory bacteria (e.g., Faecalibacterium prausnitzii), with better effects than single probiotic use.

Studies on obese mice have shown that isoleucine (0.3 g/kg/day) can improve insulin resistance by regulating microbiota: on the one hand, it reduces gram-negative bacteria (e.g., E. coli) producing endotoxins; on the other hand, it promotes the proliferation of Akkermansia, restores intestinal barrier function, and reduces metabolic endotoxemia.

IV. Synergistic Effects of Isoleucine with Other Nutrients

1. Interaction with Dietary Fiber

When isoleucine is used in combination with soluble dietary fiber (e.g., pectin), it can enhance the metabolic efficiency of microbiota for both. Dietary fiber provides a carbon source for microbiota, promoting the proliferation of isoleucine-decomposing bacteria (e.g., Bacteroides thetaiotaomicron), thereby producing more SCFAs and synergistically improving the intestinal environment.

2. Synergistic Effect with Probiotics

Isoleucine can serve as a growth promoter for probiotics (e.g., Bifidobacterium, Lactobacillus). In vitro experiments have shown that 0.1% isoleucine can increase the viable count of Bifidobacterium by 2-3 orders of magnitude, possibly due to isoleucine providing essential amino acids and reducing medium osmotic pressure. In clinical studies, isoleucine combined with probiotics can more effectively improve the diversity of gut microbiota in the elderly and reduce the incidence of constipation.

V. Research Limitations and Future Directions

Most current studies are based on animal models, with limited human clinical data, especially regarding the influence of different ages, genders, and baseline intestinal conditions on the regulatory effect of isoleucine.

The synergistic or antagonistic effects of isoleucine with other BCAAs (leucine, valine) need further exploration, as an imbalance in their ratio may affect the microbial regulation effect.

In the future, combined with metagenomics and metabolomics technologies, it is necessary to analyze the precise molecular pathways of isoleucine regulating microbiota, providing a theoretical basis for targeted regulation of gut microecology.

Conclusion

L-isoleucine exerts a positive regulatory effect on gut microbiota balance through multiple mechanisms, including serving as a microbial metabolic substrate, improving the intestinal barrier, and regulating immune function. Its effect is dose-dependent: physiological dosage can promote the proliferation of beneficial bacteria and metabolic optimization, while excess may cause microbial imbalance. In clinical applications, isoleucine may be used as an auxiliary means to improve gut microecology, but intervention strategies should be formulated based on individual microbial characteristics and nutritional status.