Resistant starch intake facilitates weight loss in humans by reshaping the gut microbiota

The global obesity epidemic necessitates new weight-reduction strategies. Obesity significantly increases the risk of comorbidities like diabetes and cardiovascular disease. Weight loss mitigates these risks, highlighting the importance of effective weight management. The gut microbiota is increasingly recognized as a key regulator of host physiology, influencing inflammation, fat storage, and glucose metabolism. While fecal microbiota transplantation (FMT) from healthy donors has shown inconsistent results, combining dietary interventions with FMT may improve outcomes. Manipulating the gut microbiome through dietary interventions, such as prebiotics (including resistant starch, RS), offers a promising anti-obesity approach. RS, a fermentable fiber resistant to digestion in the small intestine, reaches the colon where it's fermented by gut microbiota. Rodent studies show RS reduces body fat, but human data have been inconsistent, possibly due to variations in dietary compliance and fat intake. A robust trial in obese individuals is needed to clarify RS's effects and optimal dosage, along with multi-omics approaches and gnotobiotic animal models to establish causality between microbiome changes and metabolic benefits. This study uses a crossover, randomized clinical trial to investigate the effects of RS supplementation on obesity and metabolic phenotypes in individuals with excess body weight, incorporating metagenomics and metabolomics analyses, and investigating the effects of RS-modified gut microbiota in mice.

Prior research indicates that modulating gut microbiota through dietary fiber can positively impact metabolic disorders. Studies have shown the gut microbiota's role in regulating inflammation, fat storage, and glucose metabolism. While fecal microbiota transplantation (FMT) has shown mixed results in treating obesity, combining FMT with dietary interventions has shown promise. Prebiotics, including various fermentable fibers, increase beneficial gut bacteria (*Bifidobacterium* and *Lactobacillus* species), improving gut barrier function and reducing inflammation. However, many prebiotic studies rely on correlations and haven't established causal links between microbiota modulation and metabolic benefits. Resistant starch (RS), a type of fermentable fiber, has shown promise in animal studies, reducing body fat. Human studies, however, have yielded inconsistent results, likely due to differences in RS intake, dietary composition (particularly fat intake), and study design. The role of the RS-associated gut microbiota in its therapeutic effects remains unclear, necessitating further research.

This study employed a randomized, double-blinded, placebo-controlled crossover design (ChiCTR-TTRCC-13003333). Thirty-seven participants (BMI ≥ 24 kg/m² and/or increased waist circumference) with no chronic disorders or relevant medications were enrolled. The 20-week study included two 8-week intervention periods (high-amylose maize RS, 40 g/day, or control starch, energy-matched), separated by a 4-week washout period. Participants received an isoenergetic and balanced background diet throughout the study. Anthropometric measurements, biochemical analyses (including insulin sensitivity via hyperinsulinemic-euglycemic clamp and meal tolerance tests), and stool sample collection were performed at multiple time points. Shotgun metagenomic sequencing and metabolomics profiling were conducted on stool and serum samples. Furthermore, fecal microbiota transplantation (FMT) experiments were performed in antibiotic-treated mice using samples from human participants. Mice were fed a Western diet and monitored for body weight, fat mass, glucose metabolism, inflammation markers, and intestinal permeability. Separate experiments investigated the effects of *B. adolescentis* supplementation (live vs. heat-killed) in conventionally raised mice, also monitoring for relevant metabolic and inflammatory markers. Statistical analyses included linear mixed models, Wilcoxon tests, ANOVA, and correlation analyses.

The primary outcome, body weight, significantly decreased after the 8-week RS intervention (−2.81 kg, 95% CI −3.55 to −2.07 kg, P<0.001), while no significant change was observed with the control starch. Fat mass and waist circumference also significantly reduced. Visceral and subcutaneous fat areas, measured by MRI, were lower after RS consumption. Glucose tolerance and insulin sensitivity improved significantly after RS intervention, with increased glucose infusion rate (GIR) during the hyperinsulinemic-euglycemic clamp. Levels of pro-inflammatory cytokines (TNFα and IL-1β) were significantly lower after RS consumption. Faecal lipid excretion (NEFA, TG, and TC) was significantly higher after RS, suggesting decreased lipid absorption. Metagenomic analysis revealed significant differences in gut microbiota composition between RS and control groups. *Bifidobacterium adolescentis*, *Bifidobacterium longum*, and *Ruminococcus bromii* were significantly increased after RS intervention. *B. adolescentis* showed the strongest correlation with various metabolic phenotypes (BMI, GIR, etc.). FMT experiments in mice showed that RS-altered gut microbiota reduced body weight, fat mass, and improved glucose tolerance. Mice receiving RS microbiota also exhibited reduced inflammation and increased intestinal ANGPTL4 expression, consistent with reduced lipid absorption. Supplementation with live *B. adolescentis* in mice significantly alleviated weight gain and adiposity, increased intestinal ANGPTL4 expression, and reduced luminal lipase activity, further supporting the role of *B. adolescentis* in RS's effects. In germ-free mice, RS alone had no significant effect, but RS combined with *B. adolescentis* replicated the beneficial effects observed in conventional mice, highlighting the crucial role of the gut microbiota in mediating RS's effects.

This study provides strong evidence that RS supplementation, in conjunction with a balanced diet, facilitates weight loss and improves insulin sensitivity in overweight or obese individuals. The observed benefits are strongly linked to RS-induced alterations in the gut microbiota, specifically the increase in *B. adolescentis*. The mechanistic pathways involved include alterations in bile acid metabolism (increased secondary bile acids), reduced inflammation through gut barrier restoration, and inhibition of lipid absorption via ANGPTL4 modulation. The findings underscore the importance of considering dietary context (particularly fat intake) in future RS studies. The success of the study design, with its balanced background diet and high adherence rate, enabled the identification of clear associations between RS, microbiota changes, and metabolic benefits. The study's findings suggest that manipulating gut microbiota through dietary interventions like RS may be a viable strategy for managing obesity and related metabolic disorders. The strong correlation between *B. adolescentis* and the beneficial effects of RS suggests this bacterium could be a potential therapeutic target.

This study demonstrates that resistant starch (RS) supplementation, combined with a balanced diet, effectively promotes weight loss and improves insulin sensitivity in overweight or obese individuals. These benefits are strongly linked to alterations in the gut microbiota, particularly the enrichment of *Bifidobacterium adolescentis*. The observed mechanisms involve modulation of bile acid metabolism, reduced inflammation, and decreased lipid absorption. Future research should focus on larger-scale studies to confirm these findings and explore the long-term effects of RS on weight maintenance and metabolic health. The role of *B. adolescentis* as a potential therapeutic agent warrants further investigation.

The study's relatively small sample size and stringent inclusion criteria may limit the generalizability of the findings to other populations. The reliance on database-dependent taxonomic analysis might have overlooked strain-level functional diversity and discarded difficult-to-classify sequences. Further validation in larger, more diverse cohorts is required to strengthen the conclusions. Long-term studies are needed to determine whether the positive effects of RS are sustainable for long-term weight management and whether this intervention strategy can apply to other specific populations.