Interactions between body fluid homeostasis and thermoregulation in humans

Humans are unique in their ability to control body temperature with a large amount of skin blood flow and sweat rate while exercising in an upright position. However, cutaneous vasodilation in the body reduces total peripheral resistance and blood pooling in cutaneous veins decreases venous return to the heart and cardiac filling pressure. In addition, hypovolemia by sweating accelerates the reduction in cardiac filling pressure. These may threaten the maintenance of blood pressure if they are not compensated for. To prevent this, cutaneous vasodilation and sweat rate are suppressed by baroreflexes or hyperosmolality with dehydration. These mechanisms suppress heat dissipation, accelerate the increase in body temperature, and sometimes cause heat stroke. As a countermeasure to prevent this, we have recommended glucose electrolyte solutions but recently found that aerobic training with carbohydrate + whey protein supplementation markedly improves heat dissipation mechanisms by plasma volume expansion. In this article, we will discuss the importance of improving body fluid homeostasis for thermoregulation under heat stress in humans and the strategy to attain this.