PHYSIOLOGICAL EFFECTS OF VEGETABLE AND CEREAL FIBER

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Much epidemiological evidence supports that dietary fiber is protective against colon carcinogenesis. Yet, recent studies find that vegetable fiber is associated with decreased colon cancer risk while cereal fiber is not. Studies on the physiological effects of dietary fiber have yielded mixed results and potential mechanisms of fiber's role in colon cancer prevention remain obscure. Furthermore, there is evidence that the metabolic effects of diet in general and fiber in particular differ between the sexes. Colon cancer incidence is higher in young women than young men, more women are breath methane excretors than men and constipation and slow transit time are more common in women. Thus, the overall objective of the proposed research is to examine differences in the fate of two dietary fiber sources in healthy men and women consuming controlled diets. Both a vegetable and cereal fiber will be fed at 2 levels, 10 grams and 30 grams of dietary fiber daily, levels representing current fiber intakes and recommended fiber intakes for the American public. Both breath methane excretors and non-methane excretors will be chosen to participate since breath methane excretion has been linked to colon cancer. Data will first be collected while subjects consume their habitual diets and then subjects will consume 5 test diets in random order: 1) fiber-free; 2) +10 g/d dietary fiber (DF) as cereal fiber; 3) +30 g/d DF as cereal fiber; 4) +10 g/d DF as vegetable fiber; 5) +30 g/d DF as vegetable fiber. On all diets, indicators of gastrointestinal function, including stool weight, whole gut transit time, and breath hydrogen excretion will be measured for 1 week after subjects have consumed the defined diets for 2 weeks. Breath methane and fecal pH will be measured since both have been shown to be higher in colon cancer patients. Digestibility of neutral detergent fiber and its carbohydrate components will be quantitiated as a measure of fiber breakdown, as will fecal volatile fatty acids, fecal bile acid and neutral sterol excretion. Finally, calcium, magnesium, and iron balances and blood indicators of mineral status will be measured to determine if 30 grams of dietary fiber daily will upset mineral balance.