An introduction to resistant starch

Resistant starch is one of the most recently “discovered” types of dietary fibre. It continues gaining scientific attention due to the fact that certain species of gut bacteria feed on resistant starch to produce short-chain fatty acids, which are beneficial to health.

What is resistant starch?

Resistant starch (RS) is a polysaccharide that can’t be digested in the small intestine, and thus gets fermented by bacteria in the large intestine. This fermentation process produces gases and short-chain fatty acids (SCFA) (1).

Sources of resistant starch

RS comes from plant foods, including cereals, tubers and legumes.

Types of resistant starch

RS1: resistant to digestion because it’s physically protected by cell walls in grains, seeds and tubers
RS2: resistant to digestion due to the structure of the starch granules, e.g. raw potatoes, green bananas, some legumes, etc.
RS3: retrograded starch from cooked and cooled foods such as potatoes, bread, etc.
RS4: chemically modified (i.e. non-naturally occurring) starches, e.g. Hi-maize(R), normally added to manufactured food products (1, 2)
RS5: forming complexes with lipids (3)

Factors that influence resistant starch content

The amount of RS present in a food will change with:

Mechanical processing (milling, chewing)
Food processing
Cooking
The presence of water
Chemical structure of the starch
Additives
The “unique snowflake factor” (i.e. individual digestibility) (1)
Ripeness, e.g. in the case of green bananas (2)

Energy in resistant starch

The contribution of RS to energy intake has been estimated to be 8 kilojoules (2 kilocalories) per gram (1). This is roughly half the energy produced by carbohydrates, including ordinary starch.

Recommended and actual intake

It has been estimated that we should be consuming about 20 grams per day for bowel health (2).

The actual intake of RS seems to be inversely proportional to the degree of industrialisation of the country. Intake in Australia, which is higher than other industrialised countries, is 5-7 grams per day (1). Other estimations set this range at 3.4-9.4 grams per day (2). In comparison, countries like India and China have an average intake of 10-18 grams per day (1).

Health benefits of resistant starch

Gut health

As mentioned above, one of the by-products of RS fermentation is SCFA. SCFA include butyrate, acetate and propionate. SCFA are the preferred fuel for colon cells improving the health of the colon (1, 2).
RS may help with constipation by increasing stool volume, softening stools and increasing transit time (1, 2).
RS can also lower the pH of the gut (making the environment more acidic) and decrease the production of potentially harmful metabolites (2).
RS has been shown to reduce the severity of symptoms in patients with inflammatory bowel disease and to help those in remission to remain disease-free (4). This might be, in part, due to the fact that it protects the mucous layer in the colon from degradation (2).

Metabolic health

RS seems to have a beneficial effect on insulin and glucose levels, at least in the short-term. This could be due to the fact that RS lowers the glycaemic index of a food (1).
RS can reduce triglycerides in healthy individuals and body weight in individuals with type 2 diabetes (4).

General health

RS can enhance absorption of some nutrients, e.g. calcium (1).

Considerations

Different types of RS have different effects, and even within types, the source can produced different results. For example, RS2 from raw potato starch seems to be the one that can increase butyrate in humans (1). However, many of the studies looking at metabolic health have used RS2 derived from high amylose maize starch (2, 4).
The potential benefits of RS seem to be enhanced when combined with other dietary fibres. In addition, it seems that some types of RS might protect probiotics in their way down the gastrointestinal tract (1).

References

P. NA. Health properties of resistant starch. Nutr Bull [Internet]. 2005;30(1):27–54. Available from: https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1467-3010.2005.00481.x
Landon S, Colyer CGB, Salman H. The Resistant Starch Report: An Australian update on health benefits, measurement and dietary intakes [Internet]. 2012. Available from: http://foodaust.com.au/wp-content/uploads/2012/04/Hi_Maize-supplement_web.pdf
Bendiks ZA, Knudsen KEB, Keenan MJ, Marco ML. Conserved and variable responses of the gut microbiome to resistant starch type 2. Nutr Res. 2020 May;77:12–28.
Montroy J, Berjawi R, Lalu MM, Podolsky E, Peixoto C, Sahin L, et al. The effects of resistant starches on inflammatory bowel disease in preclinical and clinical settings: a systematic review and meta-analysis. BMC Gastroenterol. 2020 Nov;20(1):372.

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