Predicted Apparent Digestion of Energy-Yielding Nutrients Differs between the Upper and Lower Digestive Tracts in Rats and Humans1–3

The apparent digestibility of energy-yielding nutrients (carbohydrate, protein, and fat) was predicted in the human upper digestive tract and large bowel separately for 4 diverse diets containing either a single dietary fiber source [wheat bran and pectin (PE) diets] or mixed fiber sources [low-fiber (LF) and high-fiber (HF) diets). A human balance study was undertaken to determine fecal energy and nutrient excretion and a rat model was used to predict human ileal energy and nutrient excretion. Total tract energy digestibility ranged from 92 (HF diet) to 96% (PE diet and LF diet), while at the ileal level it ranged from 79 to 86% for the HF diet to the LF diet. The predicted upper-tract digestion of starch, sugars, and fat was high, with ileal digestibilities exceeding 90% for all diets. Nonstarch polysaccharides were poorly digested in the upper tract for all diets except in the PE diet. The daily quantity of protein excreted at the ileal level was between 2 (HF diet) and 5 (PE diet) times higher than that at the fecal level. The large differences between fecal and ileal nutrient loss highlight that fecal digestibility data alone provide incomplete information on nutrient loss. There is a need to be able to routinely determine the uptake of energy in the upper and lower digestive tracts separately.