DIETARY FIBER DECREASES THE METABOLIZABLE ENERGY CONTENT AND NUTRIENTDIGESTIBILITY OF MIXED DIETS FED TO HUMANS

Food labeling regulations implemented by the U.S. Food and Drug Admini stration emphasize nutrient composition and energy content of foods. D ietary energy and digestibility of complex foods can be affected by th e content and type of dietary fiber. The metabolizable energy (ME) con tent and apparent digestibility of dietary fiber in human diets are di fficult to assess. Fiber can affect the digestibility of fat and prote in and, subsequently, the ME content of the diet. This study was condu cted to measure the ME content of nine diets with different fat and fi ber concentrations. Diets varied in level of fat (18, 34 or 47% of ene rgy) and level of total dietary fiber (3, 4 or 7% of diet dry matter) and were consumed for 2 wk. Subjects (n = 17) consumed three diets (14 d for each diet) containing different levels of fiber and one level o f fat. Food consumption was measured and all urine and feces were coll ected during a 5-d period. Combustible energy, protein, fat, total die tary fiber (TDF) and neutral detergent fiber (NDF) were measured in co mposite samples of food and feces, and urine was analyzed for combusti ble energy and nitrogen. Metabolizable energy and apparent digestibili ty coefficients were calculated. Overall, increasing fiber intake decr eased fat and protein digestibility. As a consequence of these interac tions, the ME content of the diets decreased as fiber intake increased , and TDF and NDF had similar effects on the ME value. A published emp irical formula accurately predicted the ME content of diets using eith er TDF or NDF.