Physiological and molecular responses to dietary phospholipids vary between fry and early juvenile stages of rainbow trout ( Oncorhynchus mykiss)

Abstract

The present study investigates the effects of dietary phospholipids (PL) on intestinal lipid absorption in fry and early juvenile stages of rainbow trout ( Oncorhynchus mykiss) fed fresh or oxidized lipids. Six semi-purified casein-based diets were formulated with 12% fresh or oxidized fish oil and 6% PL supplied as soybean lecithin or egg lecithin or 6% soybean oil for PL-free diets. Each diet was distributed to 3 replicate tanks of 700 swim-up fry (initial mean body weight: 66 mg) or 200 early juvenile fish (initial mean body weight: 455 mg) over a 3-week or 17-week growth trial at a water temperature of 17 °C. Feeding oxidized compared to fresh lipids significantly reduced body weight of both fry and juvenile trout (0.15 ± 0.02 vs. 0.28 ± 0.07 g and 10 ± 2 vs. 22 ± 2 g, respectively). At fry stages, within groups fed fresh lipids, trout fry fed the PL-supplemented diets had a significantly higher final body weight than fry fed the PL-free diet (0.31 ± 0.06 vs. 0.21 ± 0.02 g). Histological examination revealed a higher occurrence of intestinal steatosis (accumulation of large lipid droplets in the enterocytes of the anterior intestine) in fish fed the PL-free diet compared to fish fed PL-supplemented diets at fry stages (13/16 vs. 1/32), whereas no significant differences were recorded at juvenile stages (1/18 vs. 0/18). The levels of circulating total cholesterol, triacylglycerol (TAG) and PL in the juvenile fish were not significantly modified by dietary PL supplementation whereas the plasma TAG/PL ratio was lower in juvenile fish fed oxidized compared to fresh lipids (0.31 ± 0.06 vs. 0.45 ± 0.08). At the fry stages, within groups fed fresh lipids, expression of the gene coding for apolipoprotein B (apoB) was 2.3-fold reduced in the intestine of fish fed the PL-free diet compared to the PL-supplemented diets whereas no significant difference was recorded at juvenile stages. The expression of other apolipoproteins or key-enzymes involved in intestinal lipoprotein synthesis was not significantly modified by dietary PL supplementation. In conclusion, the results indicate that the beneficial effect of dietary PL on growth and intestinal lipid absorption is restricted to the fry stages and support previous opinions that PL are needed for intestinal TAG absorption. The limiting step seems to be the synthesis of apoB by the enterocytes in rainbow trout fry fed fresh lipids but this requires further investigation in fish fed oxidized lipids.