Dietary lipid level, hepatic lipogenesis and flesh quality in turbot (Psetta maxima)

Abstract

A study was undertaken with marketable size turbot to evaluate the effects of dietary fat levels on chemical composition, lipogenesis and flesh quality. Four experimental diets containing graded levels of fish oil in order to obtain 10%, 15%, 20% and 25% of crude fat were fed to triplicate groups of turbot (initial body weight of 660 g) for 12 weeks in full strength seawater at temperature of 17°C. Nutrient digestibility was not influenced by dietary fat levels. The best growth performance was observed in fish fed 10% and 15% dietary fat. High dietary lipid levels led to higher fat deposition in whole fish, although lipid level in muscle remained low (1.1% in dorsal muscle and 1.7% in ventral muscle irrespective of diet). Significant subcutaneous fat accumulation was detected in turbot. No protein sparing effect by lipid was observed in turbot fed high dietary fat. Hepatic lipogenic enzymes (glucose-6-phosphate dehydrogenase, G6PD; malic enzyme, ME and acetyl CoA carboxylase) did not show any clear change in activity in response to dietary fat content. With regards to quality parameters, there were no differences in gutted and fillet yields among treatments. Sensory analyses of dorsal fillets indicated only a difference in exudation (corresponding to loss of water) and whiteness within treatments in accordance with instrumental colour analyses and on ventral fillets, only a difference of sweet flavour was observed. No differences in hardness were detected by either instrumental texture analysis or sensory analysis. In conclusion, although high dietary lipid levels affected growth and whole body composition of turbot adversely they induced very few alterations in flesh quality.

Dietary lipid level affects growth performance and nutrient utilisation of Senegalese sole (Solea senegalensis) juveniles

“…Therefore, the observed differences in growth performance among treatments seem to be mainly due to the dietary lipid levels. This is in agreement with previous studies in turbot that reported a decline in growth rate with diets containing increasing dietary fat levels either in juvenile (17) or commercial-sized fish (18) . A growth depression was also observed when halibut juveniles were fed a diet containing 30 % lipid and a low level of carbohydrates (15) .…”

“…Nevertheless, contrarily to the viscera, muscle total lipids were not affected by increasing dietary lipid levels. These results are in agreement with earlier observations in flatfish species such as halibut (13,16) , turbot (18) and Senegalese sole (4) , and suggest that other tissues such as skin and subdermal adipose tissue or other carcass fraction (fins, brain, bones) might be important fat-storage sites. In the present study, Senegalese sole subdermal fat only represented about 0·2 % of wholebody weight and 4 % of total body lipids, showing that lipids are probably being accumulated under the skin like in turbot (18) and between the dorsal and ventral fins and around the belly flap like in halibut (31) .…”

“…Both the viscerosomatic index and viscera total lipids of Senegalese sole increased with dietary lipid levels but were generally lower than values previously found in other teleosts including flatfish species (4,18,29,30) . Lipid content of the viscera represents 0·1 -0·2 % of body weight and 4 % of the wholebody lipids.…”

“…High dietary lipids enhanced growth and protein utilisation in plaice (Pleuronectes platessa) (11) , whereas no beneficial effects were reported in halibut (Hippoglossus hippoglossus) (12 -17) and Senegalese sole (4) . Moreover, turbot (Psetta maxima and Scophtalmus maximus) was negatively affected by dietary lipids (17,18) .The quantity, quality and body fat stores vary widely among fish species and mainly depend on dietary factors. In salmonids, fat deposition occurs mainly in the viscera and to a lesser extent in the muscle (19) , whereas in flatfish species, such as turbot, the marginal body parts and subdermal fat constitute the most important lipid deposition sites (18,20) .…”

“…Moreover, turbot (Psetta maxima and Scophtalmus maximus) was negatively affected by dietary lipids (17,18) .…”