Fermentable carbohydrate reaching the colon after ingestion of oats in humans

A simulated digestion technique was used to investigate the characteristics of starch hydrolysis in uncooked, cooked and extrusion-cooked oat products in vitro. The rate and extent of starch hydrolysis were both significantly lower in uncooked rolled oats than in the same material after brief boiling or extrusion cooking. A similar degree of maldigestion in vivo would lead to as much as an estimated 68% of oat starch entering the colon. Breath-hydrogen measurements were used to compare the fermentable carbohydrate content of uncooked and briefly boiled rolled oats in human volunteers and to estimate the relative contributions of soluble dietary fiber and undigested starch to the fermented component. Isolated oat gum (beta-glucan) was readily fermented in vivo and was apparently the main fermentable component of cooked rolled oats. Uncooked rolled oats gave a higher excess hydrogen production than cooked oats, but the results were variable and the differences not statistically significant. In an additional experiment, lactulose was used as a fermentable reference material to calculate the apparent fermentable carbohydrate content of rolled oats and oat gum. Uncooked rolled oats were estimated to contain a statistically significant quantity of undigested starch, amounting at most to 1.01 +/- 0.40 g (mean +/- SEM) of undigested starch per 50 g of fresh weight. We concluded that starch hydrolysis in oats is limited to some extent by the physical state of the food matrix, but this effect may be greatly overestimated by simulated digestion procedures in vitro.