Resistant starch (RS) is a fermentable dietary fibre that cannot be digested by human amylases (digestive enzymes) in the small intestine and moves into the colon, where it undergoes fermentation by gut microbiota. RS is naturally occurring in foods like legumes, cereal, rice, and wholegrain bread.
Researchers in China conducted a randomised clinical trial (RCT) in individuals with excess body weight and found that modulation of gut microbiota by RS may offer solutions for metabolic disorders.
“RS can facilitate weight loss at least partially through Bifidobacterium adolescentis, and gut microbiota is essential for the action of RS,” wrote the researchers in Nature Metabolism.
“We found that RS supplementation, coupled with isoenergetic (same total amount of calories for all participants) and balanced diets, significantly reduced body weight and improved insulin sensitivity in humans,” said the researchers.
Although studies in rodents have demonstrated that RS could lead to a decrease in total body fat, human data did not show the same results. The potential of RS to be used for obesity treatment in humans therefore remains unclear.
Thus, the researchers conducted an RCT to investigate the impact of RS on gut microbiota and its role in weight loss among obese individuals.
The study was conducted in Shanghai, China from 3 July 2013 to 14 October 2016. Thirty-seven participants between the ages of 25 to 41 were recruited for a placebo-controlled, double-blinded, crossover design trial.
The participants had a body mass index ≥24 kg m−2, and/or waist circumferences ≥85 cm for men and ≥80 cm for women. None had any chronic disorders or ongoing treatments that will impact the study.
The 20-week study duration was divided into two eight-week blocks, with one each for the RS supplement (2.8 kcal g−1, 91.2 g, containing 40 g RS) and the placebo (control starch [CS], 3.55 kcal g−1, 72 g, amylopectin, containing 0 g RS, with equal energy supply).
There was a four-week washout period between the eight-week blocks.
Study participants were randomly allocated into two groups: RS-Washout-CS or CS-Washout-RS.
Starch was provided as powder in pre-packaged sachets to be mixed with 300 ml water. Each participant consumed one sachet twice a day, 10–15 min before meals. Throughout the trial, the participants ate isoenergetic and balanced background diets three meals per day. The food was prepared according to the Chinese and American guidelines for prevention and management of adults with overweight and obesity. Except for differences in dietary fibre intake due to the supplement or placebo use, the total energy consumed and percentage of macronutrients were similar during the RS or CS study periods.
At the end of each eight-week block, blood, urine and faecal samples were collected from participants. The researchers also gathered data for their weight, visceral fat areas (VFA), subcutaneous fat areas (SFA), body fat, waist circumference, lipid profiles, insulin sensitivity, and gut microbiome.
Additionally, the researchers analysed the influence of RS-modified gut microbiota transferred from selected human donors to antibiotic-treated mice. This was to explore the effects of RS on body fat and glucose metabolism, as well as the advantages conferred by gut microbiota through RS.
The results showed that body weight and other obesity-related symptoms were significantly reduced for participants who took the RS supplement. There was also an improvement in insulin resistance. In contrast, no significant change was observed in the placebo group. Regardless of whether it was in the RS-Washout-CS group or the CS-Washout-RS group, significant reductions were observed in body weight and other obesity-related outcomes after the RS intervention.
“Moreover, fat mass and waist circumference reduced significantly after the RS intervention compared with the CS intervention. During the RS intervention period, the body weight, waist circumference and fat mass of participants significantly decreased from week 2 onwards. Both visceral fat areas (VFA) and subcutaneous fat areas (SFA), measured by abdominal magnetic resonance imaging (MRI) were lower following the RS consumption compared with those following CS consumption,” said the researchers.
To identify the gut microbiota signature associated with the RS intervention, the researchers also compared the changes in the abundance of each species after the RS and CS treatments. They found that Bifidobacterium adolescentis may play a crucial role in alleviating obesity as seen by its strong correlation with lower BMI, waist circumference and VFA in this study.
As for the study on mice, it was found that the benefits of RS are associated with changes in gut microbiota composition.
“Supplementation with Bifidobacterium adolescentis, a species that is markedly associated with the alleviation of obesity in the study participants, protects male mice from diet-induced obesity,” said the researchers.
Through this study, the researchers found that body weight was regained during the washout period. Weight regain is one of the biggest challenges of weight loss treatment, which occurs after stopping medication. Thus, long-term adherence to an RS-rich dietary pattern to maintain the composition of the microbiome may be crucial for weight maintenance.
While the researchers acknowledged limitations such as the relatively small sample size used in this trial, and that more attention should be paid to the individual dynamics and functional responses of microbiota in RS supplementation, they were optimistic about the potential of RS for weight loss.
“As RS occurs naturally in foods and can also be added to daily diets, our findings provide a pragmatic lifestyle to treat obesity and its related metabolic disorders. Manipulating the gut microbial composition through diet may represent a strategy for modifying host energy balance to promote health,” concluded the researchers.
Source: Nature Metabolism
DOI: https://doi.org/10.1038/s42255-024-00988-y
“Resistant starch intake facilitates weight loss in humans by reshaping the gut microbiota”
Authors: Huating Li, Lei Zhang et al.
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