Graduate Student Literature Review: Gut integrity under heat stress in dairy cows-Biological mechanisms and the protective role of methionine.

Heat stress represents a major challenge for dairy production, impairing animal performance, health, and welfare. Among the affected systems, the gastrointestinal tract has emerged as a central site in the pathophysiology of this condition, as multiple mechanisms contribute to the loss of intestinal integrity, including redistribution of splanchnic blood flow to the periphery, tissue hypoxia, oxidative stress, mast cell activation, and alterations in hydration status and osmotic homeostasis. These processes compromise epithelial barrier function, favoring the translocation of luminal components, which triggers systemic inflammation, oxidative stress, and the redirection of nutrients from productive functions toward maintenance and defense. In this context, methionine has been proposed as a functional amino acid with the potential to support intestinal health. Through its role in one-carbon metabolism, methionine contributes to the synthesis of key compounds such as glutathione and taurine, associated with antioxidant defense and inflammatory modulation; polyamines, related to epithelial proliferation and renewal; and phosphatidylcholines, essential for membrane stability. In addition, methionine may support mucosal barrier homeostasis by regulating goblet cells and mucin expression and may also influence the intestinal microbiota. Although these mechanisms are well established in other species, important knowledge gaps remain regarding the effects of heat stress on intestinal barrier function in ruminants, as well as the role of methionine and optimal supplementation strategies in lactating dairy cows. Therefore, this review aims to discuss the mechanisms by which heat stress impairs intestinal barrier function and to evaluate the potential of methionine as a nutritional strategy to mitigate its effects in dairy cows.