Cereal fiber, fruit fiber, and type 2 diabetes: Explaining the paradox
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.
Section snippets
Findings from cohort studies
Several large prospective cohort studies have shown that persons with a relatively high intake of cereal fiber have a significantly lower risk of developing T2DM than do persons with a low intake.10., 11., 12., 13., 14., 15., 16. These findings are based on multivariate analyses where the associations were adjusted for several possible confounding factors such as age, BMI, sex, smoking, alcohol intake, family history of T2DM, and physical activity. By contrast, a much weaker protective
Gut microbiome, cereal fiber, and type 2 diabetes
The findings from cohort studies indicate that cereal fiber is protective against T2DM whereas soluble/viscous fiber provides little or no protection. The mechanisms responsible for these associations appear to be largely unrelated to the impact of fiber on glycemia. We propose that these observations may be best explained by the action of gut microbiota, the population of microorganisms that include more than 1000 bacterial species and 100-fold more genes than are found in the human genome.37.
Cereal fiber, gut microbiome, and type 2 diabetes: supporting evidence for the proposed hypothesis
Animal studies and investigations of healthy humans have suggested that some benefits of cereal fiber are mediated by way of bacterial fermentation of undigestible constituents, and that this may play an etiological role in T2DM.25., 80. Research has also revealed that the composition and metabolism of the gut microbiota can be modulated through fiber,81., 82. and that these carbohydrates can improve metabolic markers in experimental models.67., 83. For example, when the plasma concentrations
Conclusions
Cohort studies consistently reveal a strong protective association between consumption of cereal fiber and risk of T2DM. By contrast, only a weak protective association is seen for sources of soluble/viscous fiber, such as fruit. The dominant source of cereal fiber for the subjects in these studies is whole wheat which is rich in insoluble (nonviscous) fiber. RCTs, by contrast, point in the opposite direction: they demonstrate that foods containing insoluble/nonviscous fiber have little effect
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Conflict of interest statement: There are no conflicts of interest.