Bile acids in rainbow trout : Keeping a balance

Abstract

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...