The Essential (and Targetable) Role of Inflammation in the Poor Muscle Growth and Metabolism of Heat-Stressed Livestock

Heat stress is a global barrier for meat animal production. Sustained exposure of livestock to hot environmental conditions makes optimal welfare difficult to achieve and produces considerable deficits in growth and efficiency. In this review, we discuss how livestock are particularly susceptible to heat stress during the period preceding harvest (i.e., finishing), when body weights and adiposity are at their greatest. Prenatal heat stress during critical windows for fetal development also limits postnatal growth and efficiency by inducing adaptive programming in muscle and other tissues. Skeletal muscle is the epicenter of the physiological changes that occur in response to heat stress due to its substantial nutrient consumption and metabolic heat production. The high metabolic plasticity and less essential role in acute survival make muscle an effective target for nutrient reappropriation to support vital neural, cardiac, and endocrine tissues during stress. Sustained heat stress induces myoblast stem cell dysfunction, which limits the capacity for protein synthesis and, in turn, hypertrophic muscle growth. In addition, heat stress markedly reduces muscle glucose oxidation and increases lipid oxidation, glycogen deposition, and glycolytic lactate production, all of which are associated with metabolic inefficiency. Recent studies show that the effects of h eat stress on muscle are facilitated in part by the induction of systemic inflammation. Cytokines like TNFα and IL-6 induce dysregulation of cellular processes for myoblast- facilitated muscle growth and intramuscular fat deposition, which is consistent with reduced carcass quality and yield observed in heat-stressed animals. Climate predictions include more frequent and extreme heat events, making practical options to manage the impact of heat stress a necessity for sustainable meat production. This review describes the associations between heat stress, systemic inflammation, and the impact on muscle growth, composition, and metabolism. It also presents recent evidence for the potential benefit o f nutraceutical moderators of inflammation on heat stress outcomes.