Man is very inefficient when it comes to converting the energy stored in food into mechanical work. Only 20 – 25% of the available energy stored in carbohydrate or fat is actually converted into a form which muscles can use to contract and generate force. The rest is released as heat, which is why we get warm when we train. As the rate of energy utilization rises, so does the rate of heat production. In order to stop hyperthermia (excessive rise in body temperature) the body must take action. The aim is to keep the body temperature around 37 – 38°C.
There are number of mechanisms which the body calls upon to lose heat. Obviously the surrounding environment plays a role. For example if it is hot, the body gets hot quicker, and it’s harder to lose heat. One method whereby the body can lose heat is by convection, i.e. heat dissipating from the body, but you are not able to adjust the amount lost by this method very much, even during training, when we need to get rid of more heat.
Another method of cooling is sweating. The evaporation of fluid from the skin is very effective. For every one liter of sweat that evaporates, some 600 kcal / 2500 kJ of heat energy may be released. During prolonged exercise it is possible to lose as much as two liters of sweat per hour. But, as I’m sure you.ll have noticed, not all sweat evaporates, as some drops off the skin and is wasted; a disadvantage to the heavy sweater.
As sweating is an effective cooling mechanism, care must be taken to ensure dehydration doesn’t impair the process. Bear in mind the body not only loses water in sweat, but also electrolytes, although electrolyte replacement has no real advantage (discussed later). Losses of fluid corresponding to as little as two percent of body weight can seriously impair the capacity to perform muscular work. In temperate climates, most athletes lose one to five percent of body weight in prolonged exercise, even when taking regular fluid throughout. In extreme conditions, losses of eight to ten percent have been reported. So in severe dehydration and electrolyte loss, a reduction in blood plasma volume can occur, which could result in circulatory failure.
The body needs to balance the loss and intake of fluids in order to maintain its capacity to regulate body temperature. So with the production of heat, performance falls off, and more effort is required to maintain the same exercise intensity, even if the individual does not feel particularly hot (because cooling methods are in operation). The result is heat exhaustion, and in extreme cases this can be fatal.