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). K E Y W O R D Sbile acids, cholesterol, fat digestibility, non-starch polysaccharides, Oncorhynchus mykiss, taurine | 1171 STAESSEN ET Al.

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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.

G e n e r a l I n t r o d u c t i o n | 13 esterified to glycerol. Fatty acids consist of a carboxylic acid group on one end of a carbon chain, and a methyl group on the other end. Fatty acids can differ from each other in two ways, being the carbon chain length and the degree of saturation.The utilisation of triacylglycerides entails several processes, including digestion, absorption, reconstitution, transport to different tissues, incorporation into cells and producing energy through βoxidation (Olsen and Ringø, 1997). The focus of this thesis is fat digestion. Efficient fat digestion requires the presence of emulsifiers, the latter coming from the feed (e.g., proteins and phospholipids), but mainly from endogenously secreted bile acids (see section 1.3) and phospholipids (Bakke et al., 2010). Fat digestion also requires lipase for hydrolysis. There are three main types of lipase that are secreted by the pancreas, being phospholipase, pancreatic lipase (i.e., co-lipase-dependent lipase) and the bile salt-activated lipase (i.e., carboxyl ester lipase) (Tocher, 2010). Both pancreatic lipase and bile salt-activated lipase can hydrolyse triacylglycerides. However, the bile salt-activated lipase, which requires the presence of bile acids, has a broader substrate specificity (e.g., wax esters, mono-, di-and triacylglycerides, phospholipids, fat-soluble vitamin esters and cholesteryl) than the pancreatic lipase (Iijima et al., 1998;Kurtovic et al., 2009). The bile salt-activated lipase might be the dominant type of lipase in marine fish, however, the importance of bile salt-activated lipase in freshwater fish is not clear (Kurtovic et al., 2009;Romano et al., 2020). Whether monoacylglycerides are the main products of lipolysis in fish, in accordance with mammals, is not known (Bakke et al., 2010). Dietary fat is digested mainly in the proximal part of the intestine, or the pyloric caeca if present, although this can vary depending on fatty acid chain length and degree of saturation. (Diaz et al., 1997;Olsen and Ringø, 1997;Røsjø et al., 2000). Most fish studies reporting fat digestibility found values that lie within the range of 85-99% (Bakke et al., 2010).The absorption of fat in fish is still not well understood, although it is thought to be broadly similar as in mammals (Tocher, 2010). Fatty acids that are released by mixed micelles (see section 1.3) are thought to be absorbed by diffusion or facilitated transport (Bakke et al., 2010;Concha et al., 2002).In the enterocytes, the absorbed free fatty acids are re-esterified to form triacylglycerides (Oxley et al., 2007). Transport of fat in the body requires macromolecular protein-containing complexes which are called lipoproteins. Different classes are distinguished by order of density and decreasing size: chylomicrons, VLDL (very low-density lipoproteins), IDL (intermediate density-lipoproteins), LDL (low density-lipoproteins) and HDL (high-density lipoproteins). Fat from the intestine is initially transported to the liver and other adipose tissues in the form of c...

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