Bile acids in rainbow trout : Keeping a balance

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 synthesi

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.</p

The effect of dietary protein source (fishmeal vs. plant protein) and non-starch polysaccharide level on fat digestibility and faecal bile acid loss in rainbow trout (Oncorhynchus mykiss)

This study investigated in rainbow trout (Oncorhynchus mykiss) if diet composition and feeding level affect faecal bile acid loss, and whether this reflects on the apparent digestibility coefficient (ADC) of fat. Six diets were formulated with either fishmeal or plant protein as main protein source. This created a contrast in the supply of bile acids, the bile acid precursor cholesterol, taurine and the taurine precursors (methionine + cysteine) involved in bile acid conjugation. For both protein sources, three diets were formulated with increasing inclusion of a non-starch polysaccharide (NSP)-rich ingredient mixture (0.0, 82.0 and 164.2 g/kg diet). This aimed at enhancing faecal bile acid loss. Fish were fed both restrictively (1.2% BW/day) and to satiation. A similar fat ADC was found when substituting fishmeal with a plant protein mixture, suggesting that the lower content of bile acids, cholesterol, taurine, methionine and cysteine in the plant-based diets did not limit fat digestion. Faecal bile acid loss increased alongside dietary NSP level, however, only during satiation feeding and most strongly for fish fed the fishmeal-based diets. Enhanced faecal bile acid loss was not caused by NSP-bile acid binding/entrapment, but by an increase in faeces production. During satiation feeding, fat ADC negatively correlated with faecal bile acid loss. From this it is concluded that bile acid availability/synthesis can become limiting for fat digestion in rainbow trout under conditions that enhance faecal bile acid loss (i.e. dietary NSP level and feeding level).</p

Effect of type of dietary non-protein energy source (starch vs. fat) on the body bile acid pool size and composition, faecal bile acid loss and bile acid synthesis in rainbow trout (Oncorhynchus mykiss)

Effects of the type of dietary non-protein energy source on the size and composition of the total body bile acid pool, on faecal bile acid loss and on bile acid synthesis were investigated in rainbow trout. Two diets were formulated (similar DP:DE ratio) that differed in the inclusion of either maize starch (Starch) or rapeseed oil (Fat) as main non-protein source. Fish were fed to satiation for 44 days. Type of non-protein energy source did not substantially affect the body bile acid pool composition. However, feeding the Starch diet resulted in a larger total body bile acid pool size compared with the Fat diet, and this despite enhanced faecal bile acid loss when feeding the Starch diet that was related to more faeces being produced. Bile acid synthesis in fish fed the Starch diet was more than two times higher compared with fish fed the Fat diet. The difference in body bile acid pool size between diets suggests upregulation of bile acid synthesis in fish fed the Starch diet beyond the level needed to compensate for the higher faecal bile acid loss and/or downregulation of bile acid synthesis in fish fed the Fat diet. The underlying mechanisms for this difference in synthesis need further investigation.</p

Faecal waste characteristics of yellowtail kingfish (Seriola lalandi) fed with pelleted and natural feed

Yellowtail kingfish (Seriola lalandi) is a fast-growing fish species. One of the challenges of farming yellowtail kingfish in recirculating aquaculture systems is their poor faeces integrity, also referred to as ‘diarrhoea-like’ faeces. Whether diarrhoea-like faeces occur under conditions when feeding on its natural feed items or are diet induced, is unclear. This study assessed the effect of feed type (pelleted vs. natural feed) on the faecal characteristics and faecal waste production of yellowtail kingfish. Three dietary treatments were studied over a 35-d experimental period: a pelleted diet based on marine ingredients (Marine; open formula); an experimental pelleted feed based on the ingredient composition of a commercial kingfish feed (Commercial Dummy, CD; closed formula); and a diet composed of four, individually fed raw (unprocessed) natural ingredients and commercial dummy pellets at a ratio of 1:1 on dry matter basis (Natural and Commercial Dummy, NCD). The NCD treatment was intended to clarify whether diarrhoea-like faeces are naturally occurring in yellowtail kingfish. Each dietary treatment was tested in four tanks, which were stocked with 27 yellowtail kingfish (mean initial weight 39 g). Fish were fed to apparent satiation twice daily for 1 h. For each tank nutrient digestibility and faecal characteristics were measured. The inclusion of natural ingredients reduced the faecal waste production (p < 0.001). Furthermore, the faeces integrity of yellowtail kingfish fed with natural ingredients was not poor (not diarrhoea-like). At the natural treatment, fish excreted faecal pellets and short strings, which were not observed at the other treatments. Faecal waste collected from fish receiving only pelleted feed was classified as diarrhoea-like. The highest faeces removal efficiency by settling was observed at the NCD treatment compared to the other treatments (p < 0.001). Consequently, the lowest amount of non-removed faeces per feed intake (p < 0.001) was observed at the NCD treatment (62.9 g OM/kg OM FI), followed by the Marine (101.1 g OM/kg OM FI) and Commercial Dummy treatment (111.7 g OM/kg OM FI). In conclusion, this study shows the potential of dietary interventions to alter the amount and integrity of faecal waste. This offers possibilities to reduce the total suspended solid load for yellowtail kingfish farming in recirculating aquaculture systems

Effect of dietary protein source and ingredient grinding size on fish performance, faecal waste production and characteristics of yellowtail kingfish (Seriola lalandi) fed restrictively and to apparent satiation

Recently, yellowtail kingfish (Seriola lalandi) is being cultured in recirculating aquaculture systems (RAS). Yellowtail kingfish have a poor faecal integrity, which makes the removal of faeces by traditional RAS technology difficult. Reducing the faecal waste load in RAS can be achieved by reducing the amount of faeces produced (e.g., increasing digestibility) and/or increasing the removal of faeces. This study assessed the effect of partial fish meal replacement by plant ingredients and the effect of ingredient grinding size on the amount of faecal waste produced and faecal characteristics, like faecal removal efficiency and particle size distribution (PSD), in yellowtail kingfish. This was investigated during two 35-d experiments, where fish were fed restrictively (experiment R) or to apparent satiation (experiment S). For each experiment, individual batches of four experimental diets were produced according to a 2 × 2 factorial design (protein source × ingredient grinding size). The formulas used were identical for both experiments. FM100 diets contained only fish meal as protein source, whilst at FM30-P70 diets approximately 70% of the fish meal were replaced by plant protein ingredients. The effect of ingredient grinding size was tested by including 40% of either a fine or coarse grinding mixture. Tanks were stocked with 20 fish and 27 fish for experiment R and experiment S, respectively. For each tank, fish performance, faecal waste production, faecal removal efficiency and faecal PSD were measured. During both experiments, ingredient grinding size did not affect the faecal removal efficiency or PSD, whilst fish fed the fine FM30-P70 diets restrictively showed a lower faecal waste production. The inclusion of plant ingredients resulted in a lower absolute growth and higher FCR. Furthermore, fish fed the FM30-P70 diets showed a higher faecal waste production, a smaller PSD and a lower faecal removal efficiency. This ultimately resulted in a higher amount of non-removed faeces by 58.3% and 37.1% compared to FM100 diets for the experiment R and experiment S, respectively. In conclusion, the replacement of fish meal with plant ingredients in yellowtail kingfish diets is challenging due to the adverse effects on fish performance, faecal waste production and faecal characteristics. However, feeding yellowtail kingfish to apparent satiation partly reduced these adverse effects of plant ingredient inclusion in terms of faecal waste production and faecal characteristics. Reducing the ingredient grinding size of yellowtail kingfish diets tended to lower the faecal waste production, whilst not negatively affecting the fish performance or faecal characteristics