Diets high in carbohydrates, specifically the high-fiber, slow-digesting, fermentable kind, have been shown in several studies to improve blood glucose and cholesterol levels, decrease insulin resistance (a good thing), and help control weight. (Here's one study.)
One way they're thought to do this is by increasing the number of receptors for insulin, and improving how insulin binds to its receptor. (More good things.)
This study describes an additional mechanism. It involves a specific kind of carbohydrate: resistant starch. (I've written about resistant starch, what it is and what foods contain it, on these posts.) Simply, resistant starch is starch (carbohydrate) that resists digestion in the small intestine and ends up in the colon where bacteria feed on it, producing an array of chemicals including some called short chain fatty acids (SCFAs).
In this study, those SCFAs were found to increase the amounts of two hormones that are produced and released from the cells of our gut: 1
- Glucagon-like peptide-1 (GLP-1)
- Peptide YY (PYY)
- RS stimulates GLP-1 and PYY secretion in a substantial day-long manner, independent of meal effect or changes in dietary glycemia.
- Fermentation and the liberation of SCFAs in the lower gut are associated with increased proglucagon and PYY gene expression.
- Glucose tolerance, an indicator of increased active forms of GLP-1 and PYY, was improved in RS-fed diabetic mice.
This helps to explain the "second meal effect," which I talked about here:
"The foods we eat hours and up to a day after a meal that produces SCFAs, even if those subsequent meals contain a lot of easily digestible (high GI) carbohydrate, will be digested more slowly, and the glucose that enters the bloodstream afterwards will be cleared faster."Good sources for resistant starch? Cooled cooked potato, cooled cooked pasta (cooling rearranges starch structure making it more difficult to digest), starchy beans e.g. kidney beans, and unripe bananas - the green ones.