*Geek Box: Classifications of Resistant Starch
Starch is the most common component of plant foods in the human diet, and is broadly classified into three types according to the rate of digestion: rapidly digestible starch [i.e., digested and absorbed within ~20mins]; slowly digestible starch [i.e., digested and absorbed within ~120mins]; and resistant starch [i.e., undigested and not absorbed].
Starch is formed from two compounds, amylose and amylopectin. While cooking starches increases the digestibility of the starches present in the food, a portion of starch is resistant to breakdown by human digestive enzymes and passes undigested to the colon. Due to this characteristic, these are known as “resistant starches” [RS], and these RS also exhibit fermentability by bacteria in the colon that results in the generation of SCFA.
However, RS may be further classified into five subtypes according to differences in the characteristics of each subtype and how those characteristics relate to the resistance to human digestive enzymes. The classifications are denoted as RS1, RS2, RS3, RS4, and RS5.
RS1 is starch that is physically inaccessible to digestion by enzymes [e.g., whole cereal grains protected by the cell of the grain cell]. RS2 is a starch that is inaccessible to digestive enzymes due to its very compact structure [e.g., green bananas, or high-amylose corn]. RS3 is a starch that is physically modified by the subsequent cooling of starchy foods cooked with moisture [e.g., potatoes or rice]. RS4 are starches that have been chemically modified [e.g., potato starch added to baked products]. And finally, RS5 are a complex of amylose and lipid, i.e., starch and fat, which are created when any food with a high amylose content is mixed with fats.
Each of these RS1-5 types are structurally diverse, and exhibit different properties which may relate to differential effects on health markers.