In 1975, Trowell suggested that the etiology of diabetes might be related to a dietary fiber deficiency. This is supported by several key pieces of evidence. Vegetarians who consume a high-fiber lacto-ovo vegetarian diet appear to have a lower risk of mortality from diabetes-related causes compared to nonvegetarians. Consumption of whole grain cereals is associated with a lower risk of diabetes. Importantly, the same dietary pattern appears to lower the risk of obesity, itself an independent risk factor in the etiology of type 2 diabetes. Obesity is emerging as a problem of epidemic proportions in affluent and developing countries. Consumption of whole grain cereal products lowers the risk of diabetes. A report showed that in 91 249 women questioned about dietary habits in 1991, greater cereal fiber intake was significantly related to lowered risk of type 2 diabetes. In this study, glycemic index (but not glycemic load) was also a significant risk factor, and this interacted with a low-fiber diet to increase risk. These results provide epidemiological evidence of a role of fiber in the etiology of diabetes.
Potential Mechanisms Indicating a Role in the Etiology of Diabetes
It can be hypothesized that a reduction in the general and postprandial glycemic and insulinemic response may delay the development of insulin resistance and thus the development of diabetes (NIDDM) although there is very little direct evidence to support this hypothesis. However, diets high in both carbohydrate and dietary fiber have been reported to improve insulin sensitivity. Much of the research in this area has studied the effect of dietary fiber on the management rather than the prevention or etiology of diabetes.
There is good evidence that diminished glucose absorption lowers the insulin response to a meal.
The action of fiber in this regard may be through slowing the digestion of starch and other nutrients. It seems that soluble fiber may play a role because large amounts of soluble dietary fiber have been shown to reduce postprandial glucose concentration and insulinemic responses after a single meal in both normal and diabetic subjects. However, the effect appears to be dependent on viscosity rather than on solubility per se. The very viscous gum, guar gum, gum tragacanth, and oat gum are all very effective whereas psyllium and some pectins are less viscous and less effective. One suggested mechanism for reducing the glycemic response is an impairment in the convective movement of glucose and water in the intestinal lumen due to the formation of a viscous gel: Glucose is trapped in the gel matrix, such that there is less movement toward the absorptive brush border of the surface of the intestinal wall and the glucose needs to be squeezed out by the intestinal motor activity of the intestine. However, other factors may also be important. There may be some impairment in digestive activity in the lumen, an alteration in hormonal secretion by cells in the gut mucosa, and a reduced gut motility that delays transit time. In the case of whole grains, there is scope for the fiber to interfere with the physical accessibility of starch to small intestinal a-amylase. Clearly, there is also potential for foods of low glycemic index to be high in RS and this does seem to be the case. A specific instance is a novel barley cultivar that exhibits both characteristics. It should be noted that there are reports of a second meal effect (i.e., the dietary fiber ingested at one meal can affect the glucose rise after the subsequent meal). The mechanism for this is unknown.
A Role for Fiber in the Etiology of Other Diseases?
Although much of the earlier observational studies in native African populations were wide ranging, most attention has subsequently focused on CHD and cancer. Probably this is a reflection of the socioeconomic importance of these conditions in economically developed societies. However, fiber has a role in the prevention and management of other conditions, but much of the relevant information has come from interventions, not from case-control or cross-sectional studies.
Unquestionably, fiber is of direct benefit in relieving the symptoms of constipation and diverticular disease but there is little information about its role in the etiology of these conditions. Numerous interventions have shown that foods high in insoluble NSPs (e.g., certain cereal brans) and some soluble NSP preparations (e.g., psyllium) are very effective at controlling constipation and diverticular disease and enhancing laxation. The actual effect can vary with source. Wheat bran increases undigested residue, and fiber from fruits and vegetables and soluble polysaccharides tend to be fermented extensively and are more likely to increase microbial cell mass. Some NSP (and OS) preparations retain water in the colon. The physical form of the fiber is also important: Coarsely ground wheat bran is a very effective source of fiber to increase fecal bulk, whereas finely ground wheat bran has little or no effect and may even be constipating. RS appears to be a mild laxative and seems to complement the laxative effects of NSPs. The effective dose appears to be approximately 20-30 g of total fiber/day consumed either in food or as a supplement. In addition, animal studies show that NSPs and RS appear to prevent colonic atrophy seen in low-fiber diets. The mechanism of action appears to be greater fecal bulking and fermentation and the generation of SCFAs, which is necessary to prevent atrophy.
Diarrhea Colonic SCFA absorption stimulates fluid and electrolyte uptake in the colon and thus can assist in reducing diarrhea. Complex carbohydrates may also play a role in modifying the colonic microflora thus reducing the number of pathogens. An etiological role for fiber is unknown, but there is good evidence that RS can act to minimize the fluid losses that occur in serious conditions such as cholera.
Inflammatory bowel diseases (colitis and Crohn's disease) Clearly, inflammatory conditions have an immune component. In the case of Crohn's disease, there appears to be no established therapeutic or etiological role for fiber. The situation is slightly different for distal ulcerative colitis, in which fiber intake seems unrelated to incidence. However, rectal infusion of SCFAs (especially butyrate) has been reported to lead to remission, so it appears that either the generation of these acids or their delivery to the distal colon may be the issue.
See also: Cancer: Epidemiology and Associations Between Diet and Cancer. Cereal Grains. Colon: Disorders; Nutritional Management of Disorders. Coronary Heart Disease: Prevention. Diabetes Mellitus: Etiology and Epidemiology. Diarrheal Diseases. Dietary Fiber: Physiological Effects and Effects on Absorption. Food Safety: Bacterial Contamination. Obesity: Prevention. Vegetarian Diets.
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