Procyanidins and monomeric flavan-3-ols inhibit digestive enzymes in vitro, particularly pancreatic α-amylase. The inhibition of digestive enzymes reduces the absorption of nutrients and increases the proportion of the digesta that is fermented in the large intestine, thus indirectly modulating the composition and activity of intestinal microbiota. Accordingly, grape seed extract containing procyanidins reduced the weight gain of mice after an 8 week administration.