Benefits
Supports healthy fasting insulin levels
In a meta-analysis of 13 studies (428 adults with overweight or obesity), resistant starch lowered fasting insulin, more in trials of people with diabetes. A small 4-week study in 10 healthy adults found 30 g/day raised insulin sensitivity on a clamp test. A pooled analysis of 22 type 2 starch trials found no effect on insulin resistance.
Supports healthy fasting blood sugar
Pooled trials found small drops in fasting blood sugar, mainly in people with type 2 diabetes. One analysis tied the effect to 30 to 40 g/day in people with diabetes and obesity, and another found type 2 starch lowered fasting glucose in diabetes and prediabetes. A 12-week trial of 45 g/day in prediabetes found no change in insulin sensitivity.
Butyrate production in the colon
In 20 healthy young adults, raw potato starch raised average stool butyrate from 8 to 12 mmol/kg, but 6 people had no rise; in them the bacteria that break down resistant starch did not increase. In another trial, a daily potato side dish shifted gut bacteria in 50 adults without changing stool butyrate.
Post-meal blood sugar response
A review of 36 trials in people with type 2 diabetes or prediabetes, 31 of them pooled, found that resistant starch from intact plant foods (type 1) and high-amylose starch (type 2) lowered the blood sugar rise after meals, both in single-meal tests and with regular use over weeks.
Body weight and waist size in adults with overweight
In a double-blind crossover trial in 37 adults with overweight or obesity on a provided diet, 40 g/day of high-amylose maize resistant starch for 8 weeks lowered weight 2.8 kg more than control starch, with less fat mass and waist. Other studies give mixed results: a pooled analysis of 22 type 2 trials found weight loss only in type 2 diabetes, and a 12-week trial saw no change in organ, liver or muscle fat.
Mechanism of action
Colonic fermentation to short-chain fatty acids
Resistant starch is not broken down by digestive enzymes in the small intestine, so it reaches the colon, where bacteria ferment it into short-chain fatty acids such as acetate, propionate and butyrate. In a 4-week study, blood acetate and propionate were higher after meals on resistant starch than on placebo.
Depends on specific gut bacteria
Breaking down resistant starch depends on specific bacteria such as Bifidobacterium adolescentis and Ruminococcus bromii. In one study these rose from about 2 to 9 percent of gut bacteria in people whose butyrate rose or was already high, but stayed near 1.5 percent in people whose butyrate did not rise.
Bile acid changes
In an 8-week weight-loss trial, the gut bacteria changes after resistant starch went with an altered bile acid profile, and the same group's mouse experiments suggested Bifidobacterium adolescentis plays a part. This route has not been confirmed in people.
Clinical trials
Systematic review and random-effects meta-analysis of 13 controlled studies in adults with a BMI of 25 or more, searched to November 2018. (Wang et al. 2019, Nutr Diabetes)
428 adults with overweight or obesity, with or without diabetes.
Resistant starch lowered fasting insulin (SMD -0.72; -1.26 in trials of people with diabetes and -0.64 without) and fasting glucose (SMD -0.26), raised a measure of insulin sensitivity (HOMA-S, SMD 1.19) and lowered LDL cholesterol (SMD -0.35) and HbA1c (SMD -0.43). The authors cautioned that the result should be read carefully because individuals and their gut bacteria differ.
Systematic review and meta-analysis of randomized trials giving at least 8 g/day of type 2 resistant starch. (Snelson et al. 2019, Nutrients)
22 randomized trials, 670 participants: healthy people and people with overweight or obesity, metabolic syndrome, prediabetes or type 2 diabetes.
Type 2 resistant starch lowered triglycerides by 0.10 mmol/L in healthy people (269 participants) and body weight by 1.29 kg in people with type 2 diabetes (90 participants), but the authors noted both results were heavily influenced by a few positive studies that contradicted most trials. It had no effect on any other outcome, including fasting glucose, HbA1c, insulin resistance and satiety. Trials lasted 1 to 12 weeks with 10 to 60 participants, and most had an unclear risk of bias. The authors concluded short-term use is of limited cardiometabolic benefit.
Systematic review and meta-analysis of 14 parallel or crossover studies in people with type 2 diabetes and/or simple obesity, searched to August 2018. (Gao et al. 2019, Lipids Health Dis)
People with type 2 diabetes, with or without obesity, and people with simple obesity.
In type 2 diabetes with obesity, fasting glucose, fasting insulin and HOMA-IR were lower than control, but BMI and HOMA-%S and %B did not differ, and there was no significant effect in simple obesity. Results by dose were inconsistent: 30 to 40 g/day lowered fasting glucose more, while 10 g/day lowered fasting insulin more. The authors concluded resistant starch can improve insulin resistance in type 2 diabetes, especially with obesity.
4 weeks of 30 g/day resistant starch compared with placebo, with insulin sensitivity measured by euglycemic-hyperinsulinemic clamp and muscle and fat tissue metabolism by arteriovenous sampling. (Robertson et al. 2005, Am J Clin Nutr)
10 healthy adults.
Insulin sensitivity on the clamp was higher after resistant starch than after placebo (9.7 versus 8.5 units, p = 0.03) and 33% higher during a meal test (p = 0.05). Forearm muscle glucose clearance was higher and fat tissue released fewer fatty acids. The supplement was well tolerated, and the authors called for studies in people with insulin resistance.
Randomized, double-blind, placebo-controlled crossover trial: 40 g/day high-amylose maize resistant starch or an energy-matched control starch for 8 weeks each, 4-week washout, with an identical provided background diet throughout. (Li et al. 2024, Nat Metab)
37 adults with overweight or obesity.
Body weight fell 2.81 kg more on resistant starch than on control starch, with lower fat mass and waist size, and insulin resistance improved. The trial was small and used a controlled diet, so results may differ in everyday eating.