Could resistant starch be the redeemer of dietary carbohydrate?

Abstract

anaging postprandial glycaemia is perhaps the Holy Grail against the background of a Western diet typically consisting of over 50% carbohydrate. When we consider carbohydrate we of course think only of the starch component, and quantitatively starch is the most important, but it is becoming increasingly evident that not all starches are created equal and so cannot simply be categorised as a single ‘nutrient’. In simple terms, dietary starch is a glucan, or a chain of D-glucose monomers linked by glycosidic bonds which when digested by intestinal amylases liberate glucose which can be rapidly absorbed. In vitro, starches can be classified as either rapidly digestible (RDS), slowly digestible (SDS) or resistant (RS) and translational work has shown that this classification is useful in predicting the glycaemic excur sion following ingestion. 1 The relative proportions of RDS and SDS are key in determining the glycaemic index (GI) of a starchy food and GI may be a useful tool for diabetes self-management. 2 So where does RS fit in? RS are starches which are resist ant to small bowel enzymatic digestion and so do not result in a blood glucose response when eaten and thus are considered to be ‘unavailable carbohydrates’, fulfilling the criteria for inclusion as a dietary fibre. 3 Starches can be resistant due to encapsulation such as within wholegrains and seeds (Type I), due to the native starch granule or the inherent amylose:amylopectin as with green banana or corn (Type II), due to retrogradation or food processing in cooked and cooled starches (Type III), or following crosslinking or other modification (Type IV). Each of these starch subtypes would be classified as a non-viscous dietary fibre and would exert health benefits as demonstrated with other dietary fibres in the diet in terms of glycaemic control. 4 Due to the effects of ripening and food processing, it has traditionally been difficult to estimate the true intakes of RS within the UK and becomes an added issue for food labelling. The energy value of RS is attributed to the micro bial fermentation of the starch and subsequent energy derived from the absorption of the microbial by-products.