Utilization and metabolism of palmityl and oleoyl fatty acids and alcohols in caecal enterocytes of Atlantic salmon (Salmo salar L.)

The substitution of fish oil with wax ester-rich calanoid copepod-derived oil in diets for carnivorous fish, such as Atlantic salmon, has previously indicated lower lipid digestibility. This suggests that the fatty alcohols (FAlc) present in wax esters may be a poorer substrate for intestinal enzymes than the fatty acids (FA) in triacylglycerol (TAG), the major lipid in fish oil. The hypothesis tested was that the possible lower utilization of dietary FAlc by salmon enterocytes is at the level of uptake and that subsequent intracellular metabolism was identical to that of FA. A dual-labelled FAlc-FA metabolism assay was employed to determine simultaneous FAlc and FA uptake and relative utilization in enterocytes isolated from pyloric caeca of Atlantic salmon fed either a diet supplemented with fish oil or wax ester-rich Calanus oil. The diets were fed for 10 weeks before caecal enterocytes from each dietary group were isolated and incubated with equimolar mixtures of either [1-¹⁴C]16:0 FA and [9,10(n)-³H]16:0 FAlc, or [1-¹⁴C]18:1n-9 FA and [9,10(n)-³H] 18:1n-9 FAlc. Uptake was measured after 2 h with relative utilization of labelled FAlc and FA calculated as a percentage of uptakes. Differences in uptake were observed, with FA showing higher uptake than FAlc, and 18:1 chains a higher uptake than 16:0. A proportion of unesterified FAlc was possibly recovered in the cells, but the majority of FAlc was recovered in lipid classes such as TAG and phospholipids indicating substantial conversion of FAlc to FA followed by esterification. However, incorporation of FA and FAlc into esterified lipids was higher when derived from FA than from FAlc. Twenty-five to fifty percentage of the absorbed 16:0 FA was recovered in TAG fraction of the enterocytes compared with 15-75% of 18:1 FA. Twenty to thirty percentage of the absorbed 16:0 FA was recovered in the phosphatidylcholine fraction of the enterocytes compared with only 5-15% of the 18:1 FA. Less than 15% of the fatty chains taken up by the cells were used for energy production, with significantly higher oxidation of 18:1 in enterocytes from fish fed the fish oil diet compared with the Calanus oil diet. However, overall, dietary copepod oil had little effect on FAlc and FA metabolism. Metabolic modification by elongation and/or desaturation was generally low at 1-5% of the uptake. We conclude that our hypothesis was generally proved in that the uptake of FAlc by salmon enterocytes was lower than the uptake of FA and that subsequent intracellular metabolism of FAlc was similar to that of FA. However, unesterified FAlc was possibly recovered in the cells suggesting that the conversion to FA may not be concomitant with uptake.