Factors Controlling the Size of Carbohydrate Stores
Although Exercises 3.2 might suggest that the body can only store a fixed amount of carbohydrate, this amount can in fact be quite different between individuals and may vary greatly in the same person at different times. Let us consider the factors that explain this observation.
One factor is the level of training. The concentration of glycogen in the leg muscles of untrained subjects who eat normal diets is about 14 g/kg of wet muscle (Blom et al, 1987a; Hultman, 1967), whereas values of up to 36 g/kg are usually found in trained athletes who have not exercised for 24 to 48 hours (Costill et al, 1981; Noakes et al, 1988b; W.M. Sherman et al, 1983; see Exercises 3.3). Average muscle glycogen levels in athletes during training are lower, about 21 g/kg.
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A small part of this difference is a result of a dietary change, because as people become more fit they naturally tend to eat diets higher in carbohydrate, which will increase muscle glycogen stores. However, untrained subjects eating high-carbohydrate diets increase their muscle glycogen stores to about 18 g/kg (Hultman, 1967; Jardine et al, 1988) or about half the values measured in trained athletes. Similarly, muscle glycogen levels in trained subjects eating low-carbo-hydrate diets are about twice the values measured in untrained subjects eating the same diet.
To convert from g/kg to mmol/kg wet muscle, multiply by 5.56. Notice that although liver glycogen stores are intact after a 42-km race, muscle glycogen levels are very low. CHO = carbohydrate. ‘‘Calculated at a liver glycogen utilization rate of 40 g/hr during exercise at 75 to 85% V02max (Ahlborg & Felig, 1982) for an athlete running 42.2 km in 2.5 hours and not ingesting carbohydrate.
Additional evidence that training increases the muscle’s capacity to store glycogen is provided by the finding that the muscle glycogen content of the legs is greater than that of the arms, almost certainly because the legs are used more and are therefore “fitter” (Hultman, 1967).