The ability to endure exercise depends partly on the amount of glycogen in the active muscles before the exercise begins, even though most of the energy for exercise is derived from the metabolism of fat. In some people, a diet rich in carbohydrate (more than 80 per cent of energy intake) prior to prolonged exercise can load up the muscles with up to twice the amount of glycogen found with a more balanced diet and this can be further increased by a more complex regimen of exercise and diet. For ordinary bursts of exercise, however, the normal content of glycogen in muscles suffices. There is a prevalent belief that eating much protein will increase the ability to exercise. This is not so if the diet already contains adequate protein. During prolonged exercise a small amount of energy comes from protein, more comes from carbohydrate, but most comes from fatty acids brought to the muscles by the blood. Eventually, almost all the energy comes from these fatty acids. Eating extra protein, even just before exercise, in no way alters the nature of the metabolism of the exercising muscles. The belief that extra meat will improve performance may, of course, result in a sense of well-being after such a meal and that may enable an extra effort to be made. A vegetarian consuming an adequate diet may do equally well.
Any increase in metabolism increases the need for the vitamins B1, B2 and niacin. This is generally met by the vitamin content of a good mixed diet and of the extra food eaten to satisfy the increased energy output. However, if there is loss of body weight because food intake is insufficient foods particularly rich in these three vitamins should be eaten.
An increase in water intake is essential for prolonged exercise, especially in warm conditions. If possible, small amounts of water should be taken during the exercise, but failing this enough water must be taken before the exercise begins. In the absence of sufficient water there is a fall in blood volume as much body water is lost via the skin and lungs, resulting in a poor delivery of oxygen and nutrients to the active muscles and a poor removal of metabolic products. The kidneys, also, will function less well. The loss of water during prolonged exercise can be considerable, up to two litres when the temperature is moderate, with the loss rising to as much as four litres at high temperatures. During recovery, it may take an hour or more after drinking water for the adverse effects of water loss to wear off and the sense of thirst is not a good guide to how much water is needed. It is unlikely that too much water will be taken under these conditions. For exercise of up to about one hour, plain cool water is probably as good as anything for rehydration. After one hour, some athletes find water mildly sweetened with glucose better than plain water, but the extra energy provided by the glucose is only small. Adding more than about 2.5 g of glucose to 100 ml of water will probably delay absorption of the water; if a strong glucose solution is used water will actually be lost from the blood to dilute the glucose in the stomach before water absorption can take place.
It is very unlikely that sweating during prolonged exercise will cause a significant loss of salt if a normal diet has been eaten. If there is any doubt about the body’s salt balance the best way to deal with this is to add some extra salt to the usual food. The use of salt tablets during prolonged exercise can be harmful and will almost certainly delay the absorption of any water taken to combat dehydration. The various mixtures of salts in ‘sports drinks’ are probably of little significance in combating the fatigue accompanying prolonged exercise.
In strenuously trained athletes, particularly runners, the blood haemoglobin is often below the average but as this is usually due to an increased plasma volume the total oxygen carrying power of the blood is not diminished. These athletes sometimes have free haemoglobin in their plasma as a result of red blood cell breakage in the pounding feet and violently contracting muscles. This will produce small amounts of free haemoglobin in the urine. There may also be small amounts of occult blood in the faeces. Sometimes very prolonged exercise, such as running a marathon, may result in appreciable blood loss in the faeces together with abdominal pain. Provided the diet is good there is rarely need for iron supplementation.