Dietary Fiber for Prevention of Type 2 Diabetes

Diets high in fiber have long been touted for their health benefits, in particular for the prevention of gastrointestinal cancers. A new study published in the Journal of Nutrition examines the impacts of a diet high in cereal fiber (termed the high cereal fiber diet, HCF) on improving insulin sensitivity and reducing the risk for type 2 diabetes.

Impact of Dietary Fiber Consumption on Insulin Resistance  and the Prevention of Type 2 Diabetes

The designation of fiber, as it relates to the human diet, primarily refers to a group of highly complex non-digestible carbohydrate substances composed of a variety of different sugar molecules. By non-digestible it is meant that the enzymes that humans produce to breakdown the food we consume [carbohydrates(sugars), fats, and proteins] cannot degrade dietary fiber in the stomach and small intestines. However, the bacteria present in the colon (large intestine) can degrade fibers and this process itself creates health benefits such as the stimulation of healthy bacteria. Other components of what is referred to as dietary fiber include the lignans and the beta-glucans. Lignans are plant-derived polyphenols that constitute a family of molecules also known as phytoestrogens. Lignans possess potent anti-oxidant properties and as such are beneficial at reducing the level of inflammation within the vasculature. Beta-glucans are soluble highly fermentable fibers that are composed of polymers of glucose. Consumption of beta-glucans (for example bran-derived) is correlated to reduction in circulating levels of saturated fats in the blood contributing to reductions in the risk for coronary vascular disease.

Most sources of dietary fiber are whole-grain cereals, fruits, vegetables, and legumes. These foods contain a variety of different forms of dietary fiber. Whole-grain foods can possess up to 12%-15% of their weight in dietary fiber while some bran-derived foods may have as much as 25% of their weight as dietary fiber.

Different dietary fibers are most commonly classified according to their solubility in water but are also commonly classified based upon their fermentation rate by the gut bacteria (termed microbiota). Most natural-fiber sources are fruit, certain vegetables, and some food products that are derived from barley and oats. Barley and oats are rich in both insoluble dietary fibers and beta-glucans.  Of significance to type 2 diabetes, insoluble cereal fibers do not directly influence post-feeding glucose levels in the blood and, therefore, have no direct influence on the glycemic index or glycemic load of carbohydrate-containing foods.

The results of the published study showed that dietary intake of greater than 30 grams/day of insoluble cereal fibers or 30-40 grams/day of whole grain foods that were enriched in cereal fibers reduced the risk of type 2 diabetes by up to 30%. Of significance to the results of this study, the consumption of soluble fibers, such as those found in fruits and vegetables presented no significant protective effects on the future development of type 2 diabetes. However, I should point out that there are clear and significant health benefits to a diet rich in fruits and vegetable, but that the correlation to reduced risk for diabetes is not one of those health benefits.

Going into the details of the biochemical mechanisms of how dietary insoluble fiber contributes to reductions in insulin resistance and the development of type 2 diabetes is beyond the scope of this posting. However, I will briefly point out that some of the primary mechanisms involve gut bacteria and altered metabolism that results in reduced fat mass accumulation and enhanced insulin sensitivity. Within the gut the production, by gut bacteria, of a class of molecules termed short-chain fatty acids (SCFA) from insoluble fiber is a significant contributor to the overall health benefits of dietary cereal fiber. SCFA are absorbed from the intestines and within the body these molecules exert numerous beneficial effects on energy metabolism, fat deposition, and insulin sensitivity. Diets high in insoluble cereal fiber also improve insulin sensitivity even in the absence (independently) of weight loss through a mechanism that is associated with reduced absorption of dietary protein. However, the data showing increased insulin resistance and development of type 2 diabetes with high protein consumption is greatest and most significant in overweight and obese individuals, who are also assumed to be most sedentary.

I feel compelled to state that diets high in protein are highly beneficial as they reduce appetite, enhance weight loss, and result in reductions in the levels of fats in the blood. In addition, there is a significant difference in the risks for developing type 2 diabetes with the consumption of animal proteins versus plant proteins. For example, reducing just 1% of total energy consumed from animal protein with plant protein has been shown to result in an 18% reduction if type 2 diabetes risk.

What is the TAKE HOME MESSAGE?? The many health benefits of consumption of a more plant-based diet cannot be overlooked if you are trying to stay healthy and fit. Ensure that your diet is enriched in foods that contain high concentrations of insoluble dietary fibers and you may eliminate your chances of developing one of the most deadly diseases of the modern era: type 2 diabetes.

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