Cereal fiber, fruit fiber, and type 2 diabetes: Explaining the paradox

Abstract

While the relationship between dietary fiber and type 2 diabetes mellitus (T2DM) has been much studied, the evidence about its role in the prevention and control of this condition has been conflicting. We critically evaluate prospective cohort studies and randomized controlled trials (RCTs) that examined insoluble/nonviscous/cereal fiber and soluble/viscous/fruit fiber in relation to risk of T2DM. Taken as a whole this evidence indicates that, in the quantities typically eaten, cereal fiber is protective against T2DM while fruit fiber gives little protection. We argue that the protective action of cereal fiber may be explained by the modulating effects of gut microbiota through mechanisms such as: 1) improving glucose tolerance via energy metabolism pathways (colonic fermentation and generation of short-chain fatty acids); 2) reducing inflammation; and 3) altering the immune response. By gaining more knowledge of specific host and gut microbial functional pathways involved in T2DM development and the potential role of cereal fiber, appropriate disease prevention and intervention strategies may be developed.

Introduction

The prevalence of type 2 diabetes mellitus (T2DM) has been rising globally.¹ As the condition is closely related to obesity, which is now a global epidemic, it is predicted that rates will continue their escalating trajectory.² T2DM has a major impact on health and is associated with nephropathy, neuropathy, hypertension, and cardiovascular disease.³ In addition, the disease has created a global economic burden.⁴ These problems are closely associated with the epidemic of T2DM and demonstrate the urgent need for prevention and early intervention.

Obesity is a major risk factor for T2DM. There are still important gaps in our understanding regarding the etiology of the disease. In particular, the relationship between intake of cereal fiber, soluble/viscous, and risk of T2DM has been much investigated but their roles are still far from clear.5., 6.

Dietary fiber is the edible parts of plants that are resistant to digestion and absorption in the human small intestine.⁷ Fiber is commonly classified as either insoluble or soluble based on its solubility in hot water. Insoluble fiber includes cellulose, lignin, and some hemicelluloses. Soluble fiber includes pectins, psyllium, gums, β-glucans, and some hemicelluloses. Soluble fibers can be subdivided into viscous (thick) and nonviscous. Pectin, guar, psyllium, β-glucans, and oat gum are examples of viscous fiber. Fiber can also be classified based on food source, such as fruit fiber and cereal fiber. The actual composition of fruit and cereal fiber is discussed in more detail later in the paper.

Three major mechanisms are thought to be responsible for the benefits of fiber, namely fecal bulking, viscosity, and fermentation.8., 9. Insoluble fiber provides fecal bulking effect; this increases stool mass, decreases intestinal transit time, and improves regularity. The increased stool weight is due to the fiber in combination with the water held inside the fiber matrix. The components of insoluble fiber, such as cellulose and lignin, are mostly not fermentable in the colon. However, soluble fiber is readily fermented in the colon, resulting in bacterial growth and the production of gas and short-chain fatty acids.

Viscous fiber thickens the contents of the intestinal lumen which slows the migration of nutrients into the intestinal walls. This can impede the absorption of cholesterol and bile salts resulting in a lowering of the blood cholesterol level. Another result of this is the lowering of the postprandial blood glucose.

In this paper we critically evaluate the relationship between cereal fiber, soluble/viscous/fruit fiber, and T2DM. Based on our conclusion that cereal fiber is protective against T2DM, we then formulate a hypothesis that proposes that this action is mediated by way of the gut microbiome.