Blackspot seabream (Pagellus bogaraveo) lipogenic and glycolytic pathways appear to be more related to dietary protein level than dietary starch type

The lipogenic potential of carbohydrate type on blackspot seabream was determined using two isolipidic (10%) and isonitrogenous (45%) diets containing either crude (CS) or gelatinized corn starch (GS). A third diet containing a lower protein level (35%) and gelatinized starch (GS) was also tested to determine if any lipogenic effect detected was due to carbohydrate and not from excess dietary protein. The three diets were formulated to have similar energy (21 kJ g− 1 DM) content. Triplicate groups of fish (24 g initial body weight) were fed each dietary treatment by hand to apparent satiety for 90 days. Both starch type and dietary protein level significantly affected growth. Fish fed the CS diet increased voluntary feed intake (VFI) and digestible protein intake resulting in both higher final body weight (FBW) and daily growth index (DGI). Although the lower protein level (35%) increased VFI, this increase was not enough to reach the same digestible protein intake observed in fish fed the high protein level (45%), resulting in lower FBW and DGI.Despite the differences observed in growth, nutrient gain and hence whole body composition were not affected by starch type (CS vs GS) or dietary protein level (35 vs 45%). Similarly, glycaemia was not affected by starch type or dietary protein level in any of the treatments, with the highest values being observed at 4 h post feeding. On the other hand, both starch type and dietary protein level significantly affected insulin levels 2 h after feeding. Plasma lipid levels showed no significant differences at 2 and 24 h post feeding, regardless of dietary treatment. A reduction of 10% dietary protein level (45 to 35%) markedly depressed (> 40%) hepatic activities of lipogenic enzymes (glucose-6-phosphate dehydrogenase, G6PD; malic enzyme, ME; fatty acid synthetase, FAS), and significantly increased PK activities at each postprandial time (2, 4 and 24 h). Starch type did not cause any significant effect on hexokinase (HK), glucokinase (GK), pyruvate kinase (PK) (glycolysis) or glucose-6-phosphatase (G6Pase) (gluconeogenesis) postprandial hepatic activities. Blackspot seabream lipogenic and glycolytic pathway regulation appear to be more related to dietary protein level than to dietary starch type.