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October 2005 -
Presentation Summaries
Fiber and Gastrointestinal Health
Daniel D. Gallaher, PhD
University of Minnesota
Dietary fiber, which consists of various indigestible polymeric carbohydrates derived from plant sources, has a number of important physiological effects. The best established effects are improved laxation, reduction in serum cholesterol and improved glucose tolerance. There is somewhat less evidence for other effects, such as reduced risk for colon cancer, increased satiety and stimulation of the immune system, and these are still under investigation.
Two attributes of dietary fiber seem to be responsible for most of its health benefits - susceptibility to fermentation and viscosity. Common types of dietary fiber may be categorized in this way:
Fibers that are resistant to fermentation provide the best laxation or "bulk", as it is commonly known. Those that produce the greatest increase in fecal output are, in descending order: wheat, fruits and vegetables, gums and mucilages, cellulose, oats, corn, legumes and pectin. A quick look at sales of OTC laxatives and antacids confirms the fact that many Americans lack adequate dietary fiber: in 2003 sales of antacids were around $900 million and laxatives close to $800 million, double the amount spent on pain remedies other than headache and triple the amount spent on acne products, by comparison.
Fermentable fibers, in contrast, increase the bacterial mass and have only a modest laxation effect. They are nonetheless important, however, because of their effects in the colon. Most fiber-rich foods contain some fermentable fiber. For example, fruits and vegetables contain pectin; legumes contain pectin and oligosaccharides; oats and barley contain beta-glucans.
Effects of Fermentable Fibers
Within the large intestine, fermentable fibers produce increases in short chain fatty acids (SCFA), which leads to acidification of contents, and changes in bacterial enzyme activities. The short chain fatty acids produced are mainly acetate, propionate and butyrate. By acidifying the colonic environment, SCFA play an important role in both colonic and systemic physiology. Butyrate, which is particularly increased by fermentation of prebiotics such as oligofructose and inulin, has been found to inhibit growth of transformed cell lines. This may have implications for prevention of colon cancer, however, there is as yet very little evidence from human studies. Both colon cancer and inflammatory bowel disease have been associated with decreased concentrations of butyrate, and some studies have shown that administration of butyrate may be useful for prevention or treatment of experimental colitis. In experimental animals, prebiotics have been shown to reduce formation of colon tumors and/or aberrant crypts, a precancerous marker of risk.
Another mechanism potentially affecting cancer risk is the reduction in concentration of potentially cancer-promoting secondary bile salts, which occurs as a result of the lowered pH of the colonic environment. The bacterial enzyme 7-alpha dehydroxylase converts primary bile acids to secondary bile acids, but its activity declines when the pH is below 6.5. In an experiment with rats, we studied 7-alpha dehydroxylase activity and excretion of lithocholic acid (a secondary bile acid) in relation to intake of a high concentrate of dried plums, a low concentrate of dried plums, another diet matched the carbohydrate of the dried plums or a basal diet. The lowest 7-alpha dehydroxylase activity and excretion of lithocholic acid occurred in animals consuming the high dried plum concentrate, and conversely, the highest 7-alpha dehydroxylase activity and excretion of lithocholic acid occurred in subjects consuming the basal diet.
All three SCFA serve as fuel sources for different cells, with butyrate being the preferred fuel of colonocytes. Propionate is absorbed from the colonic lumen and transported to the liver. It has been suggested that propionate may reduce cholesterol production, but there is little experimental support for this concept.
Effects of Fiber Viscosity
In contrast to fermentation, viscosity is clearly associated with reductions in serum cholesterol and may be responsible for improved glucose tolerance. In a laboratory study, hamsters were fed different dietary fibers with comparable intestinal contents viscosity but widely varying fermentability. All the fibers had approximately the same cholesterol lowering ability, regardless of their fermentability, suggesting that fermentability was not a factor in the cholesterol lowering effect. Another rodent study found that the cholesterol lowering ability of different dietary fibers is directly proportional to the intestinal contents viscosity.
Interestingly, the mechanism for cholesterol lowering may be different for different viscous fibers, with some appearing to enhance bile acid excretion and others reducing cholesterol absorption. In our dried plum feeding study at the University of California at Davis, we did not see an increase in bile acid excretion, although we did see reductions in total and LDL cholesterol with increased fiber intake. However, in a study comparing cholesterol absorption in animals fed a variety of high and low viscosity fibers, dietary cholesterol absorption did not differ appreciably with viscosity, but bile acid excretion did increase in the groups fed the high viscosity fiber diets. Thus, the evidence regarding mechanism of action is mixed and may vary among fiber types.
Do Fruits and Vegetables Lower Cholesterol?
Many epidemiological studies have associated diets high in fruits and vegetables with reduced cardiovascular risk, but neither the specific foods nor the exact components responsible for this effect are known precisely. Since fruits and vegetables are rich in dietary fiber, particularly in pectins and mucilages that have been shown to lower cholesterol when fed as purified ingredients, it is logical to ask whether whole fruits and vegetables containing these fibers can also lower serum cholesterol. Table 2 summarizes studies of cholesterol lowering with various fruits and vegetables done over the past two and a half decades. In general, it appears that any of these particular foods can have a beneficial effect on blood lipids if eaten regularly for a month or more.
TC = total cholesterol
LDL-C = low density lipoprotein cholesterol
TG = triglycerides
Effects on Glucose Tolerance
Numerous studies have shown improvements in glucose or insulin tolerance with viscous or gel-forming fibers. Additionally, most fruits and vegetables have a low glycemic index. Viscosity is thought to be responsible for the improvement in glucose tolerance, through delayed gastric emptying and/or a decreased rate of diffusion within the small intestine, thereby slowing digestion or glucose absorption.
A study on the effect of viscosity on stomach emptying in humans compared high and low viscosity guar administered either orally or duodenally. Only the orally administered, high viscosity guar had a significant effect. In two other studies, the results were mixed. It is likely that delayed gastric emptying is the mechanism behind improvements in glucose tolerance, but further studies are needed.
Possible Effects of Dietary Fiber on Body Fat
Fruits and vegetables high in viscous fiber may have a beneficial effect on body weight, not only by displacing more calorically dense foods in the diet, but also through the effects of dietary fiber on body fat. Leptin is a 146 amino acid peptide hormone product of the ob gene, primarily produced in adipose tissue. It has been shown in a number of studies that plasma concentrations of leptin are correlated with total fat mass. In rats fed diets containing fibers of varying viscosity, plasma leptin concentrations were consistently reduced in the animals fed high viscosity fiber diets. It was further shown that the animals on the high viscosity fiber diets had lower fat pad weights as a percent of total body weight. These results, though preliminary, suggest another positive health effect of foods that are high in viscous dietary fiber.
Summary
Dietary fiber clearly plays is an integral part of the ideal digestive health diet. A mixture of dietary fibers is most desirable, including types that are resistant to fermentation to promote laxation and types that are viscous to reduce blood lipid concentrations and improve glucose tolerance. As well, the viscous fibers may promote satiety, lower blood pressure and reduce adiposity. High fiber foods for the ideal digestive health diet include fruits and vegetables, legumes, whole grains such as wheat for nonfermentable fiber and whole grains such as oats and barley for viscous fibers.
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