High Fiber Diet Reduces Diabetes Risk
Impact of Dietary Fiber Consumption on Insulin Resistance and the Prevention of Type 2 Diabetes
Large prospective cohort studies consistently show associations of a high dietary fiber intake (>25 g/d in women and >38 g/d in men) with a 20–30% reduced risk of developing type 2 diabetes (T2D), after correction for confounders. It is less well recognized that these effects appear to be mainly driven by high intakes of whole grains and insoluble cereal fibers, which typically are nonviscous and do not relevantly influence postprandial glucose responses [i.e., glycemic index (GI)] or are strongly fermented by the gut microbiota in the colon. In contrast, a dietary focus on soluble, viscous, gel-forming, more readily fermentable fiber intakes derived from fruit and certain vegetables yields mixed results and generally does not appear to reduce T2D risk. Although disentangling types of fiber-rich foods and separating these from possible effects related to the GI is an obvious challenge, the common conclusion that key metabolic effects of highfiber intake are explained by mechanisms that should mainly apply to the soluble, viscous type can be challenged. More recently, studies in humans and animal models focused on gaining mechanistic insights into why especially high-cerealfiber (HCF) diets appear to improve insulin resistance (IR) and diabetes risk. Although effects of HCF diets on weight loss are only moderate and comparable to other types of dietary fibers, possible novel mechanisms have emerged, which include the prevention of the absorption of dietary protein and modulation of the amino acid metabolic signature. Here we provide an update of our previous review from 2008, with a focus on mechanistic insights of how HCF diets may improve IR and the risk of developing T2D.
Prospective cohort studies clearly indicate that diets high in insoluble cereal DF and whole grains might significantly reduce diabetes risk. In contrast, there is no compelling evidence that soluble DFs from fruit and vegetables play a key role in this context. Many of the proposed protective mechanisms of DF consumption are either shared by soluble and insoluble DFs or they are more likely to be relevant with soluble, viscous DF consumption. Interference of insoluble cereal fibers with the absorption or digestion of dietary protein is a concept that deserves further investigation and may contribute to explaining both the opposite directions of observed diabetes risk in individuals who consume HCF compared with high-protein diets and the increasing number of observations that mainly a high intake of animal protein, but not plant-derived protein (which also provides additional fiber), appears to increase IR and the incidence of T2D in highrisk individuals.