Time-related changes in nutrient digestibility and faecal bile acid loss of rainbow trout (Oncorhynchus mykiss) as affected by dietary fat level and non-starch polysaccharide level

Time-related changes in apparent digestibility coefficients (ADC) and faecal bile acid loss as affected by dietary fat level and non-starch polysaccharide (NSP) level were studied in rainbow trout (Oncorhynchus mykiss). Low-Fat versus High-Fat and Low-NSP versus High-NSP diets were formulated. Fish were fed for 6 weeks to apparent satiation and faeces were sampled in week 2, 4 and 6. Most nutrient ADC increased with time and increases were consistently larger for the High-Fat and High-NSP diets. Nevertheless, time of adaptation was equal between diets/nutrients and steady-state digestion was reached at week 3. Although faecal bile acid loss was affected by diet, the observed time-related decrease in faecal bile acid loss was mainly driven by a decrease in feed intake and faeces production. Fat ADC improved over time regardless of NSP level, which does not support the hypothesis that enhanced faecal bile acid loss (related to satiation feeding of high-NSP diets) causes a depletion of the total body bile acid pool size that is critical for proper fat digestion. The high-fat diets consistently resulted in the lowest Fat ADC, which might have been related to a lower availability of bile acids relative to the level of dietary fat.