Bioprocessed plant protein diets enriched with hydrolyzed microalgae-grape marc extracts improve growth and welfare in Sparus aurata juveniles

This study evaluated the effects of an enzymatically hydrolyzed plant-based diet supplemented with a nutraceutical compound on growth performance, metabolism, welfare, and hepatic and intestinal health in Sparus aurata . Juvenile fish (13–14 g) were fed ad libitum for 84 days with three experimental diets: a control diet (CT; 20% fishmeal), a plant-protein-based diet in which hydrolyzed plant proteins replaced 60% of fishmeal (PP), and the same plant-based diet supplemented with 2% of a microalgae- and grape marc-derived nutraceutical (GG; LB-Green Grape ). Fish fed the PP diet exhibited reduced final weight and length, likely due to differences in feed intake and feed efficiency. These growth impairments were associated with elevated plasma Gh levels, reduced Igf1 levels, and a decreased Igf1/Gh ratio, together with evidence of hormonal uncoupling at the molecular level. In addition, the PP group showed increased expression of hsd11b2 and nr3c1 in the head kidney, suggesting a compensatory response to nutritional impairment. At the hepatic level, no signs of tissue damage were observed; however, hepatocyte area increased significantly in PP-fed fish. In addition, the intestinal length index was reduced in these fish compared to the CT group, possibly due to the enzymatic pre-treatment of plant proteins. In contrast, supplementation with LB-Green Grape partially restored growth performance and normalized key endocrine and molecular alterations, including the Igf1/Gh ratio and hormonal coupling. The nutraceutical also attenuated the upregulation of hsd11b2 and nr3c1 , while reducing star expression, indicating modulation of steroidogenic activity in the interrenal tissue. Both hepatocyte area and intestinal length remained comparable to those of the CT group. Notably, only the GG group exhibited a significant increase in goblet cell density in the anterior intestine, suggesting enhanced mucosal defense capacity. Additionally, nutraceutical supplementation reduced feed production costs relative to the unsupplemented plant-based diet, although both diets remained more expensive than the control (CT). Overall, most physiological, metabolic, and molecular parameters remained stable across diets, supporting the effectiveness of enzymatically treated plant-based formulations. Supplementation with LB-Green Grape further enhanced growth performance and physiological resilience without compromising health, contributing to more sustainable aquaculture feeds.