Bile acids in rainbow trout : Keeping a balance

Abstract

In mammals, effects of dietary factors on the bile acid metabolism and the relationship with fat digestibility have been extensively studied. Such information is largely lacking in fish, and this despite several indications that dietary changes associated with the transition from fishmeal-based to plant-based diets can affect the bile acid metabolism and fat digestion in fish. Fat is an important energy source for fish, especially for carnivorous species. Given the importance of bile acids for proper fat digestion, understanding how dietary factors affect the bile acid metabolism is of value for future aquaculture production. This thesis aimed to increase insights on how dietary factors can affect the bile acid metabolism of rainbow trout and on how disruptions of the bile acid metabolism reflect on fat digestibility. From the results of the different experiments carried out in this thesis, the following conclusions were made:

• Several nutritional factors can alter faecal bile acid content in rainbow trout (e.g., protein source, non-starch polysaccharide level, bile acid supplementation and feeding level).Contrasting findings, both within this thesis and between literature, show that many more factors, which are currently not known, affect faecal bile acid content.

• Regardless of faecal bile acid content, factors that lower dry matter ADC and/or increase feed intake (i.e., factors that increase faecal waste production) can enhance faecal bile acid loss in rainbow trout.

• Hampered fat digestion in rainbow trout with enhanced faecal bile acid loss seems related to a lack of emulsification capacity in the small intestine as shown by the positive effect of bile acid supplementation on fat ADC, while absent for ADC of protein and carbohydrates.

• Diets that supply lower levels of cholesterol, minimal taurine and that meet amino acid requirements (plant-based diet) do not hamper fat digestion and are most likely not limiting for the bile acid metabolism of rainbow trout. Plant-based diets were not limiting for fat digestion, which most likely was related to these diets resulting in a lower feed intake compared to fishmeal-based diets.

• The total body bile acid pool size of rainbow trout (μmol fish-1) increases with body weight. This increase seems to be linear, however, this thesis shows that the rate of increase can be altered by diet composition (e.g., NSP level and type of non-protein energy source).

• Bile acid synthesis in rainbow trout can be quantitatively altered depending on diet composition (e.g., bile acid supplementation and type of non-protein energy source).

• The efficiency of enterohepatic circulation of rainbow trout’s body own taurocholic acid does not differ from that of body foreign glycocholic acid.

• Although relative absorption of bile acids in the intestine was highest in the distal intestine, more than half of the bile acids is absorbed in the proximal intestine of rainbow trout.

• Rainbow trout has a better intestinal conservation of cholic acid compared to chenodeoxycholic acid.

This thesis was the first to quantify disruptions of the bile acid metabolism in fish. Several contradictory results in the observed effects of the investigated dietary factors on the bile acid metabolism and fat digestion show that there are many more unknow factors which were not considered and thus need to be clarified by future research. Factors that enhance faecal bile acid loss seem to hamper fat digestion in rainbow trout. Further research is now needed to clarify the reason for the inverse relationship between faecal bile acid loss and fat digestion, and validate the proposed concept of total bile acid pool size depletion by enhanced faecal bile acid loss due to the limited capacity for bile acid synthesis