The picture is rather complicated because glucose, sodium, and chloride are absorbed from the intestine in equal amounts, and each interacts to accelerate the absorption of the others (Schedl & Clifton, 1963). Thus an increased sodium chloride content will expedite the rate at which the carbohydrate is absorbed from that solution and vice versa; the sodium concentration that optimizes carbohydrate absorption is about 90 to 120 mmol/L, while the optimum osmolality to achieve the same effect is believed to be in the range of 200 to 250 mmol/kg (Leiper & Maughan, 1986).
The rate of intestinal absorption of carbohydrate is more rapid from glucose polymer than from glucose solutions (B.J. M Jones et al, 1983, 1987), but the influence of glucose polymers on the rates of electrolyte and water absorption are not known (Leiper & Maughan, 1988). Presumably glucose polymers would expedite both.
Finally, those factors in the ingested solution that influence the rate at which the active muscles take up and oxidize the ingested carbohydrate during exercise are not particularly well known.
The fate of the ingested carbohydrate is the same whether it is taken 3 hours before (Jandrain et al, 1984) or 15 or 120 minutes after the start of exercise (Krzentowski et al, 1984; see Exercises 4.5).
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The ingested carbohydrate is burned by the muscles in place of blood glucose derived from the liver. The rate of combustion of the ingested carbohydrate increases with increasing exercise intensity (Pimay et al, 1982) and the amount ingested (Pallikarakis et al, 1986). When ingested as a 25% solution at a rate of 100 g/hr during 4-3/4 hours of exercise at 45 % V02max, ingested glucose was combusted at a rate of 70 g/hr, thereby providing between 85 and 90% of the total carbohydrate expenditure during the latter phase of exercise (Pallikarakais et al, 1986).
Massicotte et al. (1986) have shown that ingested glucose is oxidized more rapidly than ingested fructose. They found that 75% of a glucose load ingested during 3 hours of exercise was metabolized, but only 56% of an equivalent.
Carbohydrate ingested during or prior to exercise (exogenous carbohydrate) is utilized by the muscles as an energy fuel in place of (blood) glucose derived from the liver (endogenous carbohydrate).