Review: Additives delivery through drinking water to support gut health in swine and poultry — a systematic literature review

In swine and poultry, early-life stressors such as weaning and heat stress can impair gut health and growth performance. While additives are commonly added to feed, reduced feed intake under stress may limit their effectiveness. In contrast, water intake remains relatively constant, making drinking water a promising alternative delivery route. This systematic review assessed the impact of water-delivered additives-including probiotics, plant extracts and essential oils, amino acids, and vitamins-on gut health and performance in piglets and broilers. A structured search was performed from February 2023 to April 2025 using Google Scholar, with the keywords: "probiotics OR prebiotics OR plant extract OR essential oil OR amino acids OR vitamins" AND "drinking water" AND "piglets OR broilers" AND "gut health OR intestinal morphology OR immune response OR microbiota OR oxidative stress". From 309 articles identified, 59 were retained. Results were interpreted across the four pillars of gut health: epithelial integrity and digestion, immune fitness, oxidative status, and microbiota balance. Water-supplemented probiotics improved intestinal morphology, reduced diarrhoea, modulated immune responses, and enhanced microbial balance. Plant extracts and essential oils exhibited anti-inflammatory, antioxidant, and antimicrobial effects, contributing to improved gut integrity and resilience. Amino acids such as methionine and cysteine enhanced villus structure and oxidative balance, while vitamin C improved immune function and microbiota composition under heat stress. Overall, water-based supplementation appears to be a promising strategy to support gut health and performance during challenging phases. However, direct comparisons with feed-based supplementation are scarce, and further well-designed studies are needed.