Dietary fibre is a key nutrient for optimizing gut health and it has been previously documented that a fibre-deprived diet may have a negative impact on the colonic mucus layer and gut microbiota functionality. Resistant starch (RS) is a form of starch that is not digested in the small intestine and is therefore considered a type of dietary fibre.
A recent review by Dr. Stacey Lockyer from the British Nutrition Foundation in London (United Kingdom) and Dr. Anne Nugent from the University College Dublin in Dublin (Ireland) summarises reported health effects of RS and explores the potential mechanisms of action that underpin them.
RS is defined as “a portion of starch that cannot be digested by amylases in the small intestine and passes to the colon to be fermented by microbiota”. There are currently 5 types of RS that can be found naturally in plant foods or can be produced or modified commercially and incorporated into food products:
- RS1: a physically inaccessible starch and could be found in coarsely ground or whole-kernel grains such as bread, seeds and legumes.
- RS2: a granular starch found in raw vegetables such as raw potato and green banana, that implies that is rarely or never eaten.
- RS3: formed when starchy foods (e.g. potatoes, pasta, rice) are first cooked and then cooled.
- RS4: includes several chemically modified starches and could be found in food products like commercially produced breads.
- RS5: comprises amylose-lipid complexes that are formed during processing and reforming after cooking (e.g. boiled beans with potatoes, mixed with olive oil) or can be created artificially and added to foods.
The review focuses on human evidence regarding the health effects of RS consumption in several areas including gut health, glycaemia and plasma lipids, satiety and bodyweight.
When it reaches the large intestine, RS is fermented by microbes and produces short-chain fatty acids (SCFAs) that may play an important role in several effects on human health. Besides this, RS may have an impact on gut microbiota composition -especially on those bacterial communities involved in amylose breakdown and butyrate and methane production- and biomarkers of bacterial activity, and therefore it is suggested that RS could be used as a prebiotic. It can be hypothesized that high intake of RS is beneficial for host health, but its role in the interplay between diet, gut, and microbiota is likely more complex.
There is also some evidence that RS can counterbalance the negative effects of high red meat intake on colorectal cancer risk by decreasing pro-mutagenic products that arise during protein fermentation or via protecting against deoxyribonucleic acid (DNA) damage through SCFA production.
When compared with digestible carbohydrate consumption, RS consumption may lead to a reduced glycaemic response, which opens the potential for its use in managing diabetes. Nevertheless, RS appears to have little impact on other metabolic markers such as blood pressure and plasma lipids.
Another area of interest is the role of RS in decreasing appetite. Potential mechanisms involved include SCFA production that stimulates the release of gut hormones that promotes feelings of satiety. The authors concluded in their review that RS consumption does not result in significant changes in bodyweight over the long term, as there appears to be inter-individual variation in responses. In this context, it has been previously reported that colonic propionate produced by gut microbes may affect reward-based eating behaviour and therefore could potentially play a role in managing obesity and unhealthy eating.
Finally, emerging evidence suggests RS as a beneficial ingredient in oral rehydration solutions.
In conclusion, as a type of dietary fibre, RS can contribute to daily fibre intakes and associated health benefits. Although several human studies have reported its impact on different health outcomes, further research is needed in order to elucidate whether it can confer significant benefits that are relevant to the general healthy population and to people with chronic diseases.