Resistant starch (RS) is a type of starch that is not broken down by human digestive enzymes, and it is therefore considered a dietary fiber.^[6] Starch occurs naturally in plants as a storage form of glucose. Upon consumption, some starch is quickly broken down into glucose molecules that can be absorbed into the bloodstream, but RS resists digestion and absorption in the small intestine and travels to the large intestine where it can act as a food source for microbes of the gut microbiota — particularly via bacterial fermentation.^[7] Bacterial fermentation of RS in the large intestine leads to the production of compounds capable of influencing human health — most notably short-chain fatty acid (SCFAs), like butyrate.^[8] While intriguing, the research on RS in humans has been mixed and inconsistent, and it’s not entirely clear how RS influences the gut microbiome or general health.

RS could have beneficial effects on gut health in several ways. Research suggests that RS supplementation may increase fecal weight and levels of butyrate (a SCFA), and reduce fecal pH — features that could potentially promote the health of the colon.^[8] However, these effects seem to vary depending on the type of RS used and on other factors that can influence the baseline microbiome (e.g., sex, dietary habits, health and weight status, geographical location).^[9][10] RS may stimulate the growth of potentially beneficial bacteria in the gut, but again, these effects tend to vary greatly and no consistent pattern has emerged.^[11]

Beyond the gut, RS supplementation might reduce fasting blood glucose levels,^[1] improve insulin sensitivity (HOMA-IR),^[1] and reduce markers of inflammation (TNF-alpha, interleukin-6).^[3][12] However, these effects are inconsistently found and tend to be small in magnitude, meaning they may not be particularly impactful.

RS is generally considered safe and tends to be well-tolerated. Side effects are usually gastrointestinal in nature, including flatulence, bloating, diarrhea, and abdominal discomfort (particularly at higher doses of RS).^[13][8] These occur largely because of the gas produced during bacterial fermentation, and starting at a lower dose and increasing over time can help improve tolerance.

One major limitation of RS is how variable its effect can be between individuals. Many of the potential benefits of RS are due to its interactions with the gut microbiome, but the microbes in the gut can vary greatly between different people, which could lead to inconsistent and unpredictable effects. It is not yet clear who might benefit the most from RS supplementation.^[13][9]

Most research suggests that RS works by acting as a source of fermentable fiber to feed microbes of the gut microbiome. When RS undergoes bacterial fermentation, it leads to the production of SCFAs like butyrate, propionate, and acetate.^[3] These SCFAs are capable of influencing the health of our body in a multitude of ways, including the promotion of a healthy gut environment and assistance with regulation of appetite, inflammation, and metabolism.^[14] The process of RS fermentation also encourages the growth of more bacteria capable of producing SCFAs,^[15] which might explain why the effects of RS tend to be more pronounced when taken for longer durations.^[1]