Increasing inclusion of plant-based products and highly lipid commercial diets due to the elevated cost of fish meal highlights the need of the sustainable development of aquaculture through the formulation of functional diets . In this sense, olive (Olea europaea ) juice extract incorporation in aquafeeds has been found to enhance immune properties, gut health and functionality, and somatic growth (Gisbert et al., 2017; Rong et al., 2020) , possibly due to the presence of bioactive compounds such as phenols , mainly hydroxytyrosol, which has anti-obesogenic properties (Lutfi et al., 2017) .
The aim of this study was to evaluate the digestive process in gilthead sea bream ( Sparus aurata) fed a high-fat diet , administered at two rations, ad libitum or restricted, and to test the effects of an olive juice extract rich in hydroxytyrosol as an additive .
Materials and methods
Gilthead sea bream juveniles ( ± 80.8 g) were acclimated to the animal facilities of the Faculty of Biology at the University of Barcelona (Spain). An experimental high-fat diet (24% lipids, 47% protein and 23 MJ/kg of digestible energy ) was formulated and produced by Skretting ARC (Stavanger, Norway). The diet was produced in the absence (HF) or presence (HF-HT) of an olive hydroxytyrosol -rich extract (HIDROX®, 1.66% ) provided by Oliphenol LLC. (Hayward, CA, USA). To establish two different conditions in terms of energy intake, the diets were administered at ad libitum (1.8 % of body weight (BW) in the morning and 1.2% in the afternoon), or at restricted ration (1. 8% of BW only in the morning , named R-HF and R-HF-HT groups). After 8 weeks of growth, samples of pyloric caeca (PC) and proximal intestine (PI) were obtained from nine animals per condition and time, just before feeding in the morning and at 5h post-feeding. Samples were directly frozen in liquid nitrogen and stored at −80 ºC for later analysis. Intestinal pH was measured, and after sample homogenization, t he activity of digestive enzymes (alkaline protease, α-amylase, and lipase) and the trypsin/chymotrypsin ratio were analyzed.
Results and Discussion
No significant differences were found in growth between the HF and HF-HT groups, whereas food restriction significantly diminished it (-22.1%) (Balbuena-Pecino et al., 2023) . Regarding digestive enzyme activities (Figure 1), higher levels were found in PC before feeding, as pancreatic enzymes can be released in this intestinal region prior to food intake to improve the digestive process . Conversely, 5h after feeding, the highest digestive activity was found in PI, where the digestion was taking place. When comparing HF and R-HF groups, in PC, a significant reduction in α -amylase activity was observed in PC, both anticipatory and 5h post-feeding, and lipase anticipatory activity was also down-regulated. However, both groups showed similar enzyme activities in PI . HYDROX addition to the high-fat diet tended to up- regulate the anticipatory activity of α- amylase in PC , and alkaline protease, α- amylase, and lipase activities in PI. In addition, it also increased the digestive activity at 5h post-feeding in the PC for all the enzymes studied. Finally, R-HF-HT fish showed the highest PC lipase anticipatory activity and an intermediate α- amylase activity between R-HF and HF -HT fish in both intestinal segments before feeding , as well as at 5h post-feeding in PC. Additionally , both restricted ration groups presented similar alkaline protease activity in PI before feeding.
Food restriction promoted an optimization of the digestive function in fish, leading to relatively good growth performance despite a reduction in food intake. The observed growth reduction was less than what would be expected with a 40% food restriction . On the other hand, the effect of HT needs further studies as the observed increase in anticipatory activities suggests in general an adjustment of the digestive function to optimize the use of the given diet.
Balbuena-Pecino et al., (2023). Front. Physiol . 13: 966175. doi: 10.3389/fphys.2022.966175
Gisbert et al., (2017). Br. J. Nutr. 117, 351-363. doi: 10.1017/S0007114517000228
Lutfi et al., (2017). PLos One 12 , e0178833. doi:10.1371/journal.pone.0178833
Rong , et al., (2020). J. Ocean . Univ. China 19, 428–438. doi:10.1007/s11802-020-4232-8