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Short answer: It’s good and we should eat more of it. But what is it and where can we find more?

Resistant starch is just like it sounds – starch that resists digestion in the small intestine. Like many things in science, it was discovered by accident by John Cummings and colleagues at the MRC Dunn Clinical Nutrition Centre in Cambridge in the 1980s. At the time, they were trying to find a more accurate method to measure dietary fibre, which is defined as non-starch polysaccharide of the plant cell wall that arrive in the large intestine (bowel) undigested.

As we described in the May 2021 issue of GI News, this part of the gastrointestinal tract houses a vast array of ‘bugs’ that ultimately impact health. Collectively known as our microbiome, these friendly microorganisms need to be fed, i.e., provided with substrates (e.g., carbohydrates) for fermentation in order to survive and thrive. Our large bowel is sort of a fermentation vat producing short-chain fatty acids and other molecules that are usually beneficial to health.

When Cummings and colleagues separated out the various components reaching the large bowel, they were surprised to find that a relatively large amount of starch was present. Indeed, there was as much starch in weight terms escaping the digestive enzymes in the small intestine as there was dietary fibre (non-starch polysaccharides). Much of this resistant starch could be fermented by the large bowel’s resident microflora just like fibre. This was a stunning finding! Up ‘til then, all starch was assumed to be digested and absorbed in the small intestine. This chance finding opened up a new field of nutrition – the study of resistant starch.

In one experiment involving 12 healthy volunteers, Cummings together with Emily Beatty and Susan Kingman, fed four sources of starch – potato, banana, wheat and maize – for 15 days each. They compared the results with a starch-free diet containing the same weight of dietary fibre. They found resistant starch increased the weight of stools (a good outcome) but tended to prolong transit time (not so good). It was not as effective as dietary fibre from wheat bran at relieving constipation. They also found that some forms of resistant starch granules were not biodegradable at all by some participants.

When the scientists went looking for it, resistant starch started to crop up in all manner of foods. Nearly all the starch in bananas was resistant and over 50% of starch in banana flour. In potatoes that had been cooked and cooled, about 10% of the total starch turned out to be resistant while 90% was rapidly digested and absorbed in the small intestine (they were still high GI). They also found a lot of resistant starch in processed foods. For example, cornflakes, wheat flakes and bread had 5-15% of starch in that form.

We now distinguish at least five different kinds of resistant starch, depending on their structural and functional characteristics. The first type that is naturally-occurring physically inaccessible starch such as that found in partly milled grains. Resistant starch type 2 is starch that is protected from digestion due to its crystalline structure (e.g., raw starch granules). The third type is found in cooked and cooled potato and raw bananas and produces more acetic acid (the same acid as we find in vinegar) in faeces. Resistant starch type 4 is chemically modified starch used by food industry to improve gelling or freeze-thaw resistance. Finally, there’s retrograded starch that is found in processed wheat and maize products. This form produces more propionic acid than acetic acid during fermentation.

Food authorities such as the European Food Standard Authority have approved health claims for resistant starch, e.g., ‘Replacing digestible starch with resistant starch induces a lower blood glucose rise after a meal’. However, they do not allow claims related to digestive health because further research is needed. But four decades after Cummings’ initial discovery, we have learned that starch can be rapidly digested, slowly digested or resistant. Resistant starch may turn out to be as important as dietary fibre to our health.

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Professor Jennie Brand-Miller holds a Personal Chair in Human Nutrition in the Charles Perkins Centre and the School of Life and Environmental Sciences, at the University of Sydney. She is recognised around the world for her work on carbohydrates and the glycemic index (or GI) of foods, with over 300 scientific publications. Her books about the glycemic index have been bestsellers and made the GI a household word.