Nutrition and diet

Resistant starch

Also called: RS, fermentable starch

Resistant starch is starch that resists digestion in the small intestine and reaches the colon, where bacteria ferment it into short-chain fatty acids (especially butyrate). It behaves more like fibre than starch. Cooking and then cooling carbohydrates (potato, rice, pasta) increases their resistant starch content. Slightly green bananas, oats, beans and lentils are also rich sources.

How it works

Normal starch (in cooked rice, potato, bread) is broken down to glucose by amylase enzymes in the small intestine and absorbed quickly. Resistant starch escapes this process. It travels intact to the colon. There, gut bacteria, especially Ruminococcus bromii, Bifidobacterium and Faecalibacterium prausnitzii, ferment it. The fermentation produces butyrate, which feeds colon cells, strengthens the gut barrier, and reduces inflammation.

Four types of resistant starch

  • RS1: physically inaccessible (whole grains, seeds, beans).
  • RS2: granular (raw potato starch, slightly green bananas, plantains).
  • RS3: retrograded (cooked-then-cooled starches: cold potato, cold rice, cold pasta).
  • RS4: chemically modified (industrial; common in processed foods).

Foods rich in resistant starch

  • Cooked-and-cooled potato (4 g per 100 g, much more than freshly cooked).
  • Cooked-and-cooled rice (1.6 g per 100 g cooked, doubled if cooled overnight).
  • Cooked-and-cooled pasta.
  • Slightly green bananas (resistant starch is high when green and drops sharply as they ripen).
  • Plantains, especially less ripe.
  • Black beans, kidney beans, chickpeas (1 to 4 g per 100 g cooked).
  • Oats (especially overnight oats).
  • Hi-Maize (a high-amylose corn starch sold as a supplement).
  • Raw potato starch (1 tablespoon = 8 g RS, used as a supplement).

The cooked-and-cooled trick

When you cook starch, the molecules unfold (gelatinise) and become easy to digest. When you cool the same food, some of the starch molecules recrystallise (retrograde) into a more digestion-resistant form. So cold leftover potato has more resistant starch than freshly cooked. Reheating partially undoes this but cold-and-then-warm leftovers still have more resistant starch than freshly cooked. Practical: cook potato, rice or pasta the night before, refrigerate, eat cold or briefly reheated.

What it does for your gut

  • Feeds Bifidobacterium and butyrate producers.
  • Increases stool bulk.
  • Lowers post-meal blood glucose response.
  • Modestly improves insulin sensitivity over weeks.
  • May lower the risk of some non-colorectal cancers in high-risk groups; a randomised trial in Lynch syndrome found no effect on colorectal cancer itself.

Common questions

How much resistant starch should I aim for?
15 to 20 g per day is the typical research target for measurable gut benefit. European intake is typically about 3 to 6 g/day. The gap is closeable through cooked-cooled carbohydrates and beans.
Does reheating destroy resistant starch?
Partially. Some retrograded starch returns to digestible form on reheating. But cold-then-reheated still has more resistant starch than freshly cooked. So overnight oats, cold leftover rice, and reheated leftover potato all count.
Will it cause gas?
Yes initially. Like all fibre, gut bacteria need a few weeks to scale up to ferment new amounts. Start with 1-2 servings a day and build up.
Is raw potato starch a useful supplement?
Useful and cheap if you're trying to deliberately feed butyrate producers. 1-2 tablespoons in water or smoothies. Higher doses (around 50 to 60 g/day) are not better, as excess passes through unfermented and can cause gas. Cook-and-cool food sources are better long-term.

Sources

The Book of Suna is general information, not medical advice. It is not a substitute for talking to a qualified healthcare professional about your own situation.