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During physical activity the muscle cells convert energy obtained from the combustion of the fuels stored as glycogen in muscle, as fat in adipose tissue, and as circulating fuels (glucose and free fatty acids) into ATP (adenosine triphosphate – the fuel used for mechanical work). Additional energy can be obtained from the oral intake of nutrients, which adds to the circulating fuels in the blood.

Glycogen stored in the liver is broken down to glucose, which then enters the bloodstream from where it is extracted by the muscles and the brain. At rest this occurs at about 10g/hour and during exercise speeds up to about 60g/hour. Hypoglycaemia (low blood sugar levels) can develop in athletes who do not ingest adequate amounts of carbohydrate before and during prolonged exercise (considered to be 90 minutes or longer). Hypoglycaemia is an important cause of central nervous system fatigue (brain fatigue).

From food to fuel.