Moderate sustained swimming and dietary protein-to-starch ratio modulate anticipatory digestive enzyme activities in gilthead sea bream (Sparus aurata)

In gilthead sea bream, both moderate sustained swimming and dietary protein-sparing through highly digestible carbohydrates contribute to improving nutrient utilization, metabolic efficiency, and aquaculture sustainability. However, their combined effects on anticipatory digestive enzyme activities remain poorly understood. Gilthead sea bream fingerlings (3-5 g) were fed for 6 weeks under a factorial design combining two isoenergetic and isolipidic diets differing in protein and gelatinized starch content—HP (56% protein, 8.6% starch) and HC (48% protein, 24.9% starch)—under voluntary or moderate sustained swimming conditions. Anticipatory total protease, α-amylase and lipase activities were measured 24 h post-feeding in pyloric caeca and proximal intestine. Trypsin-like and chymotrypsin-like activities were also characterized. In pyloric caeca, protease activity increased in response to moderate sustained swimming (+12.1%), whereas the HC fish had reduced protease activity (-13.8%) compared to the HP fish. α-Amylase activity significantly increased (2.3-fold) in HP-fed fish subjected to moderate swimming compared with all experimental groups. A significant diet x swimming interaction was observed for lipase activity: sustained swimming increased lipase activity (+39.4%) in HP-fed fish, whereas it decreased in HC-fed fish (-27.6%) relative to their respective voluntary-swimming controls. In proximal intestine, protease activity was reduced both in fish subjected to moderate sustained swimming and those fed the HC diet. Likewise, α-amylase activity was decreased in fish fed the HC diet, irrespective of swimming condition, whereas lipase activity decreased in response to moderate sustained swimming only in HC-fed fish. Moderate sustained swimming and dietary protein-to-starch ratio modulated anticipatory digestive enzyme activity, with the most consistent effects observed in pyloric caeca. These findings highlight the functional specialization of this intestinal region and indicate that exercise-induced responses are diet-dependent. Notably, the combination of moderate sustained swimming and the HP diet promoted the highest anticipatory enzyme activities, which were associated with the improved growth performance observed under these conditions.