EVALUATION OF INTESTINAL ABSORPTION OF BETA-CAROTENE

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Many nutrition-cancer studies have demonstrated that beta-carotene (BC) because of its anti-oxidant activity and independent from its provitamin A property is important in the prevention of certain cancers. Since BC intake in supplemental or even excess doses apparently does not cause vitamin A (VA) toxicity, the use of BC has been considered for chemoprevention trials by NCI to reduce the incidence of certain cancers among high risk populations. In the last 2 years, our ongoing NCI supported project has shown that the absorption of intact BC and also intestinal conversion of BC to VA are quite variable in different subjects. Extensive BC absorption tests as determined by measurement of serum BC response following ingestion of graded doses of BC VA are with various amounts of food in young and old subjects have shown that several factors influence intestinal absorption of BC. These factors include the dose of BC, the total amount of calories, the modality of food delivery, the rate of fat absorption and the age and body composition of the subjects. However, in depth quantification of intestinal absorption in humans has been hampered because of the lack of suitable and safe isotopic BC compounds. Recently, a method for the measurement of the net absorption of dietary constituents or supplements in a single day has been described. This method (total gut washout) consists of cleansing the gastrointestinal tract with a special solution before and after a test meal. Subsequently, the macro- and micronutrients in rectal effluent are measured. This method avoids some of the difficulties inherent with isotopic studies. Using this method the present study aims to evaluate the net alimentary tract disappearance of BC and macronutrients,when BC (15mg) or placebo is given with a regular American diet. Initially, young (18-25) and old (65-75) men will be screened with a breath hydrogen test to select 16 subjects with normal intestinal flora. Then in each subject, the jejunal flora and the net alimentary tract endogenous losses of macronutrients, retinoids and carotenoids will be measured. Subsequently, in a random manner, each subject will participate in 8 experiments with 4 week intervals between tests. These experiments will evaluate the serum concentration time curves of BC and triglycerides when BC or placebo are given with various diets (500-1000 Kcal, 25-40% fat) and will also measure the total intestinal absorption of BC and fecal loss of BC. The effect of factors such as intestinal flora and intestinal transit time on BC absorption will also be studied. This study will provide us for the first time with quantitative data on BC absorption and will help us in establishing dietary guidelines for BC supplementation and for chemopreventive trials.