Abstract
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.
Highlights
In order to formulate diets that fulfil the nutritional and energetic requirements of fish, it is imperative to have accurate data on the digestibility of their constituting nutrients and energy (Glencross et al, 2007)
Actual non-starch polysaccharide (NSP)-rich ingredients were chosen over purified NSP sources since the NSP purification process alters physico-chemical characteristics which in turn might alter the potential of NSP to interact with bile acids (Kritchevsky, 1978)
Faecal bile acid loss expressed per unit of body weight (Figure 1c,f; Table 4) significantly decreased over time (p < .001; week 2: 15 vs. week 4: 10 vs. week 6: 5 μmol/kg BW/d averaged over diets)
Summary
In order to formulate diets that fulfil the nutritional and energetic requirements of fish, it is imperative to have accurate data on the digestibility of their constituting nutrients and energy (Glencross et al, 2007). In order to have more accurate estimates of digestibility, most studies deal with the issue of diet adaptation by allowing for a certain period of time before the collection of faecal samples. The available studies show that the minimal adaptation time is highly dependent on fish species. The study by Amirkolaie and Schrama (2015) showed that harder-to-digest carbohydrate types result in a longer adaptation period. This effect of carbohydrate type on diet adaptation time is of value in the context of the ongoing replacement of fish meal by plant ingredients in aquafeeds, since plant ingredients often containing significant amounts of non-starch polysaccharides (NSP) (Sinha et al, 2011)
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