A study performed by the US Army and Air Force, describes how heat and dehydration degrade physical performance in their study subjects.

Heat sources can come from external sources like the Sun, air and ground. In addition, Metabolic heat is produced internally during physical activity. When the body begins to store heat, the core temperature rises. In response, the body will begin to regulate and dissipate heat by sweating and pumping additional blood to the skin. As sweating increases, dehydration will occur unless water loss is replaced. Additionally, maintaining high skin blood flow will strain the cardiovascular system, forcing the heart to work harder.

Figure-1 illustrates the effects of Heat and Dehydration on aerobic exercise performed by study subjects. In a hot climate, physical output was cut in half (50%) of normal output when the subjects reached 4% of Body Weight Loss (BWL) in water. For 150lb person, that equates to losing just 3 quarts of water. Without rehydrating, physical output steadily declines.

Figure-2 illustrates how body core temperatures respond to heat at different activity levels. A high intensity activity like competitive running can demand a Metabolic rate of 1,000 watts. Study subjects could maintain a steady core temperature at this activity level until the ambient temperature reached 70Fƒ. At temperatures greater than 70Fƒ, core temperatures could no longer be maintained and started increasing.

To help avoid heat related stress, the study recommends hydration with Electrolytes, though it discouraged salt tablets and sports drinks containing High Fructose as they were prone to cause gastrointestinal discomfort.

Department of the Army and Air Force. 2003. Heat Stress Control and Heat Casualty Management. TB MED 507/AFPAM 48-152(I)