Nutritional interventions to alleviate the negative consequences of heat stress

Abstract

Energy metabolism is a highly coordinated process, and preferred fuel(s) differ among tissues. The hierarchy of substrate use can be affected by physiological status and environmental factors including high ambient temperature. Unabated heat eventually overwhelms homeothermic mechanisms resulting in heat stress, which compromises animal health, farm animal production, and human performance. Various aspects of heat stress physiology have been extensively studied, yet a clear understanding of the metabolic changes occurring at the cellular, tissue, and whole-body levels in response to an environmental heat load remains ill-defined. For reasons not yet clarified, circulating nonesterified fatty acid levels are reduced during heat stress, even in the presence of elevated stress hormones (epinephrine, glucagon, and cortisol), and heat-stressed animals often have a blunted lipolytic response to catabolic signals. Either directly because of or in coordination with this, animals experiencing environmental hyperthermia exhibit a shift toward carbohydrate use. These metabolic alterations occur coincident with increased circulating basal and stimulated plasma insulin concentrations. Limited data indicate that proper insulin action is necessary to effectively mount a response to heat stress and minimize heat-induced damage. Consistent with this idea, nutritional interventions targeting increased insulin action may improve tolerance and productivity during heat stress. Further research is warranted to uncover the effects of heat on parameters associated with energy metabolism so that more appropriate and effective treatment methodologies can be designed.

Figure 1

(A) Effects of heat stress and pair-feeding on basal insulin concentrations in growing Holstein bull calves. The vertical line separates period 1 (thermoneutral conditions and ad libitum feed intake) from period 2 (either heat-stress conditions and ad libitum feed intake or thermoneutral conditions and pair-fed). (B) Effects of heat stress (ad libitum feed intake) and pair-feeding (thermoneutral conditions) on the insulin response to a glucose tolerance test (GTT) in growing Holstein bull calves. Conversion factor for insulin is 1 μg/L = 172.5 pmol/L. Adapted from Reference with permission.