A phytobiotic-based additive protects against the deleterious effects of fasting and refeeding challenges

Phytobiotics are increasingly valued in functional feeds for their growth-promoting, immunostimulant, and antioxidant properties. The present study evaluates the immunomodulatory benefits of a commercially available phytobiotic-based additive (APEX) in enhancing fish performance and skin mucosa properties during fasting periods, a common challenge in aquaculture. Gilthead seabream juveniles were acclimated and fed either a commercial diet or a functional diet containing 0.3 % phytobiotic additive. Samples were collected at pre-fasting and following a 15-day fasting period and a 7-day gradual refeeding phase. The functional diet mitigated weight loss during fasting and enhanced body weight gain during refeeding, indicating improved performance. Analyses of exuded mucus, mucus metabolites, antibacterial capacity, and histological parameters provided insights into mucus properties and skin barrier status throughout the fasting and refeeding challenge. Food deprivation may affect mucus production, thereby compromising the protective condition, which was significantly reversed upon refeeding. Mucus defensive capacity, assessed via in vitro pathogenic bacteria co-cultures, showed that fasting reduced antibacterial activity against Vibrio anguillarum, Pseudomonas anguilliseptica, and Tenacibaculum maritimum in fish fed the commercial diet, with no recovery upon refeeding. Conversely, the functional diet preserved the antibacterial capacity of skin mucus during fasting and refeeding, meanwhile enhanced the skin barrier performance in fish fed the functional diet at pre-fasting and refeeding stages. In conclusion, short-term fasting detrimentally affects skin health and overall fish welfare, compromising the skin barrier. The use of a phytobiotic-based functional feed additive protects and prolongs the defensive properties of skin mucus during fasting, benefiting the recovery process.