Effects of dietary nutrient composition on de novo lipogenesis in gilthead sea bream (Sparus aurata)

Despite more than 20 years of nutritional research and intensive culture, gilthead sea bream appear to utilise diets inherently worse than many others species in aquaculture. Thus, while salmonids at typical slaughter size typically require between 0.9 – 1.1 kg of feed for growing one kg, gilthead sea bream typically require between 1.6 – 2.0 kg, which is similarly reflected in the efficiency with which dietary protein is retained in body growth. While salmonids have
been reported to retain as much as 55% of the dietary protein as growth, gilthead sea bream typically retains less than 30%. So far, there are no indications that differences in nutrient digestibility coefficients can explain these differences, since gilthead sea bream largely digests dietary nutrients similarly or better than salmonids. As dietary nutrients upon digestion can be endogenously converted into other nutrients or metabolites, it can be hard to quantitatively conclude on the fate of them. Using stable isotope tracers (such as 13C labelled starch or protein) allows us to trace specific nutrients and determine to which extent they are endogenously converted into other metabolites. The present thesis comprises three supporting papers which look into the conversion of
dietary starch and protein into body lipids as well as the consequences of this on fatty acid profile of the fish. Results from paper I showed that between 4.2 and 8.4% of digested starch was converted into body lipids de novo, corresponding to a synthesis rate of 18.7 to 123.7 mg/kg biomass/day, when feeding iso-DP and iso-DE diets ranging between 6 and 24% dietary starch, respectively. Additionally, up to 68.8% of the hepatic glycogen pool could be attributed to
dietary starch, while the same was true for up to 38.8% of the whole body glycogen pool. In turn, this implies that almost two thirds of the whole body glycogen and approximately one third of the liver glycogen must have originated from sources other than dietary starch, even when feeding the high starch diet.
Using nine experimental diets differing in dietary DP (33 – 40%) and DE (19.5 – 21.5 MJ/kg), results from paper II showed that between 18.6 and 22.4% of the DP was converted into lipid de novo, corresponding to between 21.6 and 30.3% of the total lipid deposited in the fish during the study. The nutrient retention results combined showed that while protein was spared by a decreasing dietary DP/DE level, the opposite was true for lipid, substantiating that deaminated DP was indeed converted into body lipids. Additionally, a very clear improvement of FCR with increasing DE level combined with an improvement of digestible protein retention with decreasing DP/DE levels suggest that gilthead sea bream are
12 capable of efficiently utilising feeds within a wide range of dietary DP/DE ratios and energy densities. Results from paper III showed that both fatty acid retention dynamics and final fatty acid profile of the fish were clearly influenced by an increment in dietary starch content (using diets otherwise iso-DP and iso-DE). The apparent retention of saturated fatty acids (SAFA) and mono unsaturated fatty acids (MUFA) were positively related to dietary starch level (and
negatively related to dietary lipid level), exceeding 100% in fish fed high starch diets. These findings substantiate that considerable de novo lipogenesis was taking place and apparently subject to nutritional control, while apparent retention of poly unsaturated fatty acids (PUFA) appeared to be un-affected by dietary treatment. Combined, this caused the SAFA and MUFA content of the fish to increase and the PUFA content to decrease when increasing dietary starch level, adversely affecting the overall FA quality of the final product. Considering lipogenesis results, nutrient retention efficiencies and body composition results
obtained in the three trials collectively, gilthead sea bream appear to endeavour to rigorously maintain a certain whole body energy status under a wide variety of dietary DP/DE ratios, energy densities and nutrient compositions, even if substantial amounts of dietary protein is sacrificed to achieve this. This may indicate that this species has evolved to maximise energy storage in the from of lipid for seasonal, migratory or maturation purposes at the expense of increasing body size through more efficient use of protein for growth. De novo lipogenesis
appear to play a key role in maintaining this energy homeostasis