Beneficial effects of sustained activity on the use of dietary protein and carbohydrate traced with stable isotopes 15N and 13C in gilthead sea bream (Sparus aurata)
To determine the effects of sustained swimming on the use and fate of dietary nutrients in gilthead sea bream, a group of fish were forced to undertake moderate and sustained swimming (1.5 BL s−1) for 3 weeks and compared with a control group undertaking voluntary activity. The exercise group showed a significant increase in specific growth rate (C: 1.13 ± 0.05; E: 1.32 ± 0.06 % day−1, P < 0.05) with no significant change in food intake (C: 3.56 ± 0.20; E: 3.84 ± 0.03 % of body weight). The addition of 13C-starch and 15N-protein to a single meal of 1 % ration allowed analysis of the fate of both nutrients in several tissues and in their components, 6 and 24 h after force-feeding. In exercised fish improved redistribution of dietary components increased the use of carbohydrates and lipid as fuels. Gilthead sea bream have a considerable capacity for carbohydrate absorption irrespective of swimming conditions, but in trained fish 13C rose in all liver fractions with no changes in store contents. This implies higher nutrient turnover with exercise. Higher retention of dietary protein (higher 15N uptake into white muscle during the entire post-prandial period) was found under sustained exercise, highlighting the protein-sparing effect. The combined effects of a carbohydrate-rich, low-protein diet plus sustained swimming enhanced amino acid retention and also prevented excessive lipid deposition in gilthead sea bream.