What is Resistant Starch?
Resistant starch (RS) is starch and its degradation products that escape digestion in the small intestine and instead reach the colon where they are fermented by gut bacteria, functioning as a type of dietary fiber. 1
Definition and Classification
Resistant starch is formally recognized as a component of dietary fiber by major regulatory authorities including the European Food Safety Agency (EFSA), European Community, and French food safety authorities (AFSSA). 1
RS consists of several distinct subtypes based on their physical and chemical structure: 1
- RS Type 1: Physically enclosed starch trapped within intact cell structures (e.g., whole grains, legumes) 1
- RS Type 2: Some types of raw starch granules with specific crystalline structures 1
- RS Type 3: Retrograded amylose formed when cooked starchy foods are cooled 1, 2, 3
- RS Type 4: Chemically and/or physically modified starches 1
Formation and Food Sources
The most clinically relevant form for practical dietary manipulation is RS Type 3, which forms when gelatinized starches are cooked and then cooled. 2, 3 This process, called retrogradation, creates resistant starch structures that can be maximized by refrigerating cooked rice, potatoes, or bread for 24 hours before consumption. 2, 3
RS occurs naturally in many foods, with average global consumption estimated at 3-10 grams per day. 4 Common dietary sources include legumes, whole grains, green bananas, and cooled cooked starches. 1, 5
Physiological Properties and Health Effects
RS functions as a prebiotic substrate that is fermented by colonic bacteria into short-chain fatty acids (SCFAs), primarily acetate, propionate, and butyrate. 6, 7 This fermentation process provides approximately 2 kcal/g of energy compared to 4 kcal/g for regular digestible starch. 2, 3
Key metabolic effects include:
- Reduced postprandial glucose and insulin responses compared to digestible starch 2, 3, 8
- Enhanced glycemic control, particularly beneficial in individuals with metabolic diseases 8
- Improved bowel function through increased stool bulk and colonic fermentation 1
- Potential reduction in colorectal cancer risk, possibly mediated through butyrate production 7
- Modulation of gut microbiota composition, promoting beneficial bacterial species like Bifidobacterium adolescentis and Ruminococcus bromii 1
Clinical Context and Limitations
The American Diabetes Association emphasizes that total carbohydrate amount in meals remains more important than the source or type for overall glycemic control. 2, 3 RS formation through cooling should be considered an adjunctive strategy to total carbohydrate counting and portion control, not a replacement. 2, 3
Important caveats:
- Individual response varies based on gut microbiome composition and enzymatic capacity 1
- RS is less effective than non-starch polysaccharides for stool bulking, though it may offer greater protection against colorectal cancer 7
- Some RS types (particularly certain oligosaccharides) are FODMAPs and may trigger gastrointestinal symptoms in susceptible individuals, including athletes 1
- Food processing significantly impacts RS content: milling, fermentation, and heating generally reduce RS, while cooling, extended cooking time, and storage can increase it 8