Resistant starch is a type of fibre currently experiencing a flurry of interest among scientists, nutritionists and food manufacturers for its benefits to gut health and blood glucose response.

Starches are large carbohydrate molecules found in plants such as potatoes and cereals (like wheat and oats) as well as pulses, like beans and lentils. They are made up of thousands of glucose molecules packed together into granules and are an efficient way for plants to store dense sources of energy in a small space.

Most starches are digestible and are broken down rapidly in the stomach and small intestine to produce glucose, which is absorbed and used for energy release, or stored.

Some types of starch are known as slowly digestible starches and, while they are still digested and absorbed in the small intestine, this process can take up to two hours, leading to a blunted glycaemic response. They have potential benefits in metabolic syndrome, diabetes and cardiovascular disease.

Resistant starch, on the other hand, remains undigested, reaching the large intestine (colon) where it feeds the trillions of microbes contained there.

Key food examples that contain resistant starch include wholegrains, pulses, as well as rice, pasta and potatoes (if cooked and cooled), dried pasta (after cooking), and unripe bananas.

Why is resistant starch less easily digested?

There are various reasons why starch may be resistant to digestion.

1.         It can be physically inaccessible, for instance when it is locked within seeds, whole grains, or the intact whole cells. Fine milling can make this type of starch more accessible to digestion in the small intestine.

2.         The starch molecules have a form that is resistant to the enzymes in the small intestine, such as those found in underripe bananas and raw potatoes. The starch type amylose, which contains longer unbranched chains of starch is abundant in these types of resistant starch and is structure is far harder to hydrolyse

3.         It may become resistant to hydrolysis on cooking and cooling, as the chains of starch coil around one another to form a double helix, as is seen in potatoes, rice, pasta and oats which are cooked and subsequently eaten cold.

4.         It may have been chemically modified for use in some drinks or baked goods like bread and cake

5.         Finally, it may be a combination of starch molecules with free fatty acids, which again forms a structure that evades digestion in the small intestine. These can take the form of amylose-lipid complexes that form a double helix or, for example, industrially produced resistant maltodextrin

Benefits of resistant starch

Many of the benefits of resistant starch are thought to come from its fermentation in the colon to produce SCFA. More research is needed but the current evidence base suggests that resistant starch can have the following effects:

·       Increase faecal bulk and reduce intestinal transit time

·       Have a prebiotic effect by acting as a food substrate for beneficial bacteria, such as Bifidobacteria species

·       When our microbiome digests resistant starch, short chain fatty acids (SCFA) are formed, which can benefit the cells of the gut, having an anti-inflammatory effect, and have immune benefits

·       These acids also reduce the pH of the gut which may lower the risk of colorectal cancer

·       Reduce the glycaemic response i.e. the blood glucose rise following a meal (as long as resistant starch is at least 14% of the total starch content of the meal). It can therefore have potential for the management of diabetes. There is sufficient evidence to permit the use of an EFSA-authorised health claim in Europe and Great Britain with respect to resistant starch and its effects on glycaemic response

·       Have cholesterol reducing benefits, although studies are somewhat equivocal and the effects may vary between different types of resistant starch

Resistant starch has the potential to deliver numerous benefits to health, but we currently need more research to confirm that potential. The dose required to deliver these benefits may well be much higher than current population intake and increasing consumption of the relevant foods may have taste and texture issues for many. However, with the development of functional ingredients by manufacturers, product innovation may make consuming more resistant starch easier and more palatable.