Effect of dietary NSP level and bile acid supplementation on nutrient digestibility and the bile acid metabolism in rainbow trout (Oncorhynchus mykiss)

The effects of dietary bile acid supplementation (BAS) on apparent digestibility coefficients (ADC) and the bile acid metabolism in rainbow trout (Oncorhynchus mykiss) were investigated for diets differing in non-starch polysaccharide (NSP) level. Four experimental diets were formulated according to an 2 by 2 factorial design. A low-NSP and High-NSP diet (0 vs. 160 g kg−1 inclusion of an NSP-rich ingredient mixture) were first formulated. The aim of this was to create differences in the amount of faecal waste produced, and consequently as previously shown in rainbow, the amount of faecal bile acid loss (FBAL). Both NSP-level diets were tested with 0% and 0.3% BAS (0 vs. 3 g kg−1 inclusion of a bile acid mixture). The bile acid mixture contained taurocholic acid (T-CA) and glycocholic acid (G-CA) in a 2:1 ratio. Glycocholic acid does not naturally occur in trout and was used as marker to see if supplemented bile acids are efficiently absorbed and enter enterohepatic circulation (EHC). Fish were fed to apparent satiation for 42 days. Dietary NSP level did not enhance FBAL. BAS was beneficial for fat ADC, but not for the ADC of protein and carbohydrates. BAS improved fat ADC regardless of NSP level, which suggests that bile acids are a limiting factor for fat digestion in rainbow trout. Total bile acid synthesis was a 4-fold lower in fish fed the 0.3% BAS diets compared to the 0% BAS diets. Lower bile acid synthesis with BAS occurred alongside enlarged body and liver + gallbladder bile acid pools and a higher content of bile acids in the chyme. These higher bile acid levels most likely caused a negative feedback inhibition of bile acid synthesis. Negative synthesis rates of G-CA were found when feeding the 0.3% BAS diets, which suggests that this bile acid is catabolised or converted into other bile acid types in rainbow trout. The enlarged pool of T-CA, but especially of the body foreign G-CA, with BAS in both the body and liver + gallbladder shows that dietary bile acids are effectively taking part in EHC. The relative absorption of T-CA and G-CA over the different intestinal compartments was comparable, which shows that EHC does not differentiate between T-CA and G-CA. While relative absorption of bile acids was highest between the distal intestine and the faeces, the absolute decrease in chyme bile acid content was highest between the pyloric region and the mid intestine. The latter shows that significant bile acid absorption occurs more proximal in the intestine of rainbow trout than previously thought.