Resistant starch was formally classified in 1992 when researchers identified that a significant fraction of starch in certain foods resisted amylase digestion and reached the colon intact. It is now classified into four types: RS1 in physically inaccessible starch within intact plant cell walls, RS2 in raw potato and unripe banana, RS3 retrograded starch formed when cooked starchy foods are cooled, and RS4 chemically modified starch. RS2 and RS3 from whole plant foods are the most clinically relevant.
The Metabolic Effects
- Insulin sensitivity: a meta-analysis of 10 RCTs found that resistant starch supplementation significantly improved insulin sensitivity in adults with metabolic syndrome and type 2 diabetes; the mechanism is butyrate-mediated improvement in muscle glucose uptake
- Second meal effect: resistant starch from one meal reduces the glucose response at the subsequent meal, hours later; this is mediated by altered gut hormone responses including GLP-1 and PYY
- Colorectal cancer protection: resistant starch is the primary substrate for butyrate-producing bacteria; butyrate is both the main fuel for colonocytes and a direct inhibitor of colorectal cancer cell proliferation
- Appetite regulation: fermentation products reduce appetite-stimulating hormones and increase satiety hormones over 24 hours after consumption
The Best Whole Food Sources
Legumes are the richest whole food source of resistant starch, with cooked and cooled lentils and beans containing 4 to 7 grams per 100g. Cooked and cooled white rice contains 3 to 4 grams per 100g, significantly more than freshly cooked rice. Oats, particularly uncooked rolled oats in overnight porridge, contain 3 to 4 grams per 100g. Slightly unripe bananas provide 5 to 6 grams per 100g.