Carbohydrate metabolism during endurance exercise

Oxidation of glucose provides a major source of energy for the working muscles during aerobic exercise. During aerobic exercise at a high rate (e.g. 80-90% of maximal oxygen consumption, typical of an elite marathon runner) the rate of energy expenditure is around 80 -90 kJ per minute. The proportion of this supplied by glucose oxidation varies according to the preceding diet and other factors, but 50% might be a reasonable estimate (i.e. 40-45 kJ/min from glucose). Oxidation of 1 g of glucose releases 17 kJ, so that 42.5/17 or about 2.5 g of glucose must be oxidised each minute.

The amount of glucose available in the blood and extracellular space is around 12 g (see Section 6.1). Therefore, even if it could all be used without adverse consequences, this could support high-intensity aerobic exercise for only a few minutes. The liver glycogen store is around 100 g (see Section 8.2.1). Therefore, this could support exercise for less than 1 hour. The total store of muscle glycogen may be around 300-400 g, giving rather longer. The utilisation of muscle glycogen is more extensive in those muscles being used for the exercise than in others, so not all the whole-body store of muscle glycogen may be used. Remember that these are all 'ball-park' figures. Note that the total store of glycogen store in liver and muscle could support glucose oxidation at a rate of 2.5 g/min for something over 2 h. It is probably not a coincidence that this is about the time taken for an elite runner to finish the marathon. When the glycogen store is depleted, the rate of energy expenditure will drop and performance will suffer. The marathon may be about the longest event that can be undertaken at such a high percentage of maximal oxygen consumption.

Hepatic gluconeogenesis has been ignored here; it probably does not make a large contribution, since hepatic blood flow may be decreased during exercise as the blood is diverted to working muscles. Moreover, much of the gluconeo-genesis that occurs will be from lactate, released by the working muscles from their glycogen stores. Therefore, this is only part of the complete pathway for oxidation of those glycogen stores.

What are the factors responsible for mobilisation of the glycogen stores? In the working muscles, the effects of neural activation of contraction probably predominate, since the glycogen concentration in non-working muscles falls much less, if at all. This has been demonstrated in subjects performing one-legged exercise on a modified exercise bicycle; the glycogen content of the exercised leg falls whilst that of the other leg does not change (Fig. 8.15). As discussed earlier, the stimulation of contraction is intimately linked with the stimulation of glycogen breakdown (Fig. 8.8). An elevation in the concentra

Fig. 8.15 Glycogen concentrations in leg muscle after one-legged exercise (bicycling on a specially adapted bicycle), in the exercised leg (solid points) and the non-exercised leg (open points). Average of two subjects. Based on Bergstrom & Hult-man (1966).

Time (days)

Fig. 8.15 Glycogen concentrations in leg muscle after one-legged exercise (bicycling on a specially adapted bicycle), in the exercised leg (solid points) and the non-exercised leg (open points). Average of two subjects. Based on Bergstrom & Hult-man (1966).

tion of adrenaline may also contribute, potentiating the effect of somatic nerve stimulation; it is not a stimulus on its own, however, as evidenced by the one-legged exercise experiment, in which both legs are exposed to the same adrenaline concentration.

In the liver, the stimulus for glycogen breakdown is not entirely clear. Glucagon would be the obvious signal but, as noted earlier, its concentration is not always elevated during exercise. However, it should be remembered that when the concentration of glucagon is measured in 'peripheral blood' it may not reflect the concentration reaching the liver in the portal vein, so that there may be some increase in glucagon secretion. In addition, the concentration of glucose may rise or fall somewhat - largely depending on the nutritional state of the subject - but the plasma insulin concentration falls gently during sustained exercise (Fig. 8.13), probably representing the effects of increased a-adrenergic stimulation to the pancreas (via sympathetic nerves or plasma adrenaline). Therefore, the glucagon/insulin ratio reaching the liver will undoubtedly rise, favouring glycogen breakdown. In addition, there may be some direct effect of activation of the sympathetic innervation of the liver; this is very difficult to test in humans.

One other aspect of glycogen mobilisation is worthy of mention. Recall that glycogen, a hydrophilic molecule, is stored in hydrated form with about three times its own weight of water. When glycogen is mobilised, that water is released. Therefore, as well as providing the store of carbohydrate, glycogen also contributes to the water necessary for endurance exercise, helping to replace that lost as sweat, etc. If 300 g of glycogen are mobilised in all, this could mean almost one litre of water.

Note that the comment made above, that the plasma glucose concentration may not change much during endurance exercise, does not mean that there are no changes in glucose utilisation. The concentration of glucose in the plasma merely represents the balance between glucose production and glucose utilisation. The turnover of glucose in plasma increases several-fold during endurance exercise (see Romijn et al. 1993). The rate of glucose uptake by skeletal muscle must also increase several-fold. This is brought about by recruitment of GLUT4 transporters to the sarcolemma. But in this case, the recruitment is not driven by insulin. Muscle contraction itself can bring about this translocation. The stimulation of muscle glucose utilisation by the sympathetic nervous system (see Section 7.3.3.3) is also probably involved.

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