The digestion and selective absorption of dietary fatty acids in Arctic charr, Salvelinus alpinus

Abstract

To compare the rates of digestion and absorption of individual fatty acids, Arctic charr, Salvelinus alpinus (L.), were fed isoenergetic diets containing 40 g kg−1 coconut oil, and various combinations of 10 g kg−1 of polyunsaturated fatty acids (PUFA) (18:2n-6 or 18:3n-3) and monounsaturated fatty acids (MONO) (20:1n-9 or 22:1n-9) in the form of free fatty acids (FFA) or triacylglycerol (TAG). The average lipid digestibility for all diets measured by use of the chromic oxide method in the pyloric caeca area, midgut and hindgut were 72%, 83% and 88%, respectively, showing that lipid digestion and absorption occur mainly in the pyloric caeca area, but also extend throughout the intestinal tract. Analyses of digesta present in the intestinal segments suggest the predominance of non-specific lipolytic activity producing primarily FFA and glycerol from dietary TAG. Comparisons of the fatty acid composition of the lipid classes in the digesta showed that the utilization of dietary TAG was dependent both on the rate of release of the individual fatty acids from TAG, and their subsequent rate of absorption. When supplied as either FFA or TAG, the levels of PUFA (18:2n-6 or 18:3n-3) in the digesta were very low, indicating almost complete utilization. Both MONO used (20:1n-9 or 22:1n-9) were absorbed less efficiently than PUFA, but the rate of release from TAG seemed to be rate limiting only for 22:1n-9, which accumulated in the digesta. The rates of absorption of 20:1n-9 and 22:1n-9 when fed as FFA were the same. Comparisons of the levels of fatty acids in the dietary coconut oil TAG with those of the digesta lipids showed that 12:0 was a good substrate for intestinal lipase and was quickly absorbed. The lipolysis of 14:0 and 16:0 was intermediate while the longer-chain 18:0 appeared very resistant to digestion and was a major component of TAG, diacylglycerols and monoacylglycerols present in particularly the hindgut digesta. The absorption of 18:0 also appeared to be very low. The results suggest that PUFA are released very rapidly from dietary TAG by intestinal lipases in Arctic charr, and are specifically absorbed compared with long-chain saturated and monounsaturated fatty acids. The rate of lipolysis may be the rate-limiting step in the digestion of very long chain monounsaturated fatty acids such as 22:1n-9, while both the rate of lipolysis and absorption may be rate limiting for long-chain saturated fatty acids such as 18:0.