Running in the heat will result in overheating, even if there isn’t a risk of heat stroke. Overheating results from inadequate cooling when the body is unable to keep up with the evaporation from the skin. The body heats up internally and the skin can’t keep up with the cooling process because the blood is being used by the muscles to keep you running. Either the muscles use the blood to keep you moving or the skin uses the blood to cool your body. And when the weather is very hot this tug-of-war can result in overheating and heat exhaustion.
Part of the problem with running in the heat is dehydration. Dehydration is the loss of water from the body through sweat. When we sweat there is water and electrolytes lost which changes the way that our body functions. When working out in the heat for longer than 30 minutes consider a sports drink that replaces water and electrolytes.