Diets labelled with 13C-starch and 15N-protein reveal daily rhythms of nutrient use in gilthead sea bream (Sparus aurata)

All functions in animals rely on daily rhythms, and mealtime can act as a rhythm-marker of nutrients assimilation and use. The effects of meal timing and food composition on carbohydrate use and protein retention of gilthead sea bream were studied. Three groups were fed twice a day (10 am and at 5 pm) for two months with two alternating diets: a commercial diet (Cd) and a high-carbohydrate, low-protein diet (Ed). The Ed/Cd group received the Ed diet in the morning and the Cd diet in the afternoon, and the Cd/Ed group received these diets in the reverse order. The Cd/Cd group only received the commercial diet (control group). After 56 days, two force-feeding experiments (PF1 and PF2) measured for all three groups the fate of a single meal labelled with 15N-protein and 13C-starch through the retention of both isotopes in the main organs and tissue reserves. In PF1 fish were fed at 10 am (morning mealtime), and in PF2 at 5 pm (afternoon mealtime). Fish were sampled at the next two mealtimes (PF1: 7 and 24 h post-feeding, PF2: 17 and 24 h post-feeding). Nutrients recovery differed according to, first, the dietary regime, and second, the last meal received (Cd or Ed). Daily lower protein intake was compensated with higher protein retention combined with more use of carbohydrates for energy. Nevertheless, carbohydrates from the morning meal were used more efficiently. So, the use of carbohydrate for energy production and protein for growth can be improved by adjusting diet composition and mealtime.