Influence of nutrient preload on encapsulated macronutrient selection in European sea bass

The sea bass is a teleost that is able to regulate its energy intake by selecting from pure macronutrient sources, although the regulatory mechanisms involved in this selection are unknown. Nutrient preloads are known to reduce food intake and modify macronutrient selection patterns in mammals, but no information is available on its effects in fish. The aim of the present work was to determine the effect of orally administered macronutrient preloads of protein (P), fat (F) or carbohydrate (CH) on the subsequent macronutrient selection, using for the purpose feed consisting of CH, P or F packaged separately in gelatin capsules. The macronutrient preloads left the total food intake unaltered, but caused differential changes in the pattern of macronutrient selection. The CH preload increased the selection of CH (39%) and decreased that of P (20%), independently of the fish's previous nutritional preferences. The F preload induced an F increase (32%) and a P decrease (18%) in P-preferring fish, but not in F-preferring fish in which the macronutrient selection pattern remained unaffected. The P preload stimulated F selection by 42% in P-preferring fish, but left the macronutrient selection pattern unchanged in F-preferring fish. In conclusion, oral macronutrient preloads affected the pattern of macronutrient selection in fish, acting by post-ingestive mechanisms. The effect was influenced by the fish's previous nutritional preference and/or status, which could depend on its metabolic capacity.