From “Effect of Glucose Ingestion on Energy Substrate Utilization During Prolonged Muscular Exercise” by F. Pimay, M. Lacroix, F. Mosora, A. Luyckx, and P. Lefebvre, 1977, European Journal of Applied Physiology, 36, p. 250. Copyright 1977 by Springer-Verlag Heidelberg. Adapted by permission. fructose load was utilized as fuel during the same period of exercise. Furthermore, whereas glucose was being used as a fuel within 20 minutes of ingestion, ingested fructose first contributed to energy metabolism 40 minutes after ingestion.
The authors concluded that ingested fructose is less available for muscle metabolism than is glucose, because the fructose must first be converted to glucose in the liver before it can be oxidized by muscle, and the activity of this metabolic pathway in the liver is low. The intestinal absorption of fructose is also slower than that of glucose. Thus fructose ingestion is not more beneficial than glucose ingestion during prolonged exercise. Two additional studies (Guezennec et al, 1989; Massicotte et al, 1989) have confirmed these results. Fructose solutions also are more likely to cause gastrointestinal distress during exercise than are equivalent glucose or sucrose solutions (R. Murray et al, 1989).
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Studies have also found that the more carbohydrate delivered to the intestine, the faster the rate at which the active muscles can utilize that carbohydrate. Moodley et al. (1991) found that glucose polymer solutions were more rapidly oxidized than were equivalent glucose or sucrose solutions, whereas Hawley et al, (1991) showed that starch was the most rapidly oxidized of all the carbohydrates. Others have not found this effect, possibly because they studied glucose
Polymers at low concentrations (Massicotte et al, 1989) or only when solutions were ingested before exercise (Guezennec et al, 1989).
In summary, the problem in determining the optimum carbohydrate solution for ingestion during exercise is that the very factors which expedite the intestinal absorption of the carbohydrate present in a solution, especially increased electrolyte and carbohydrate content, are also the factors which may retard the rate at which the solution empties from the stomach (Exercises 3.1). The only exceptions are glucose polymer and starch solutions, which empty either at the same rate or more rapidly from the stomach than do equivalent glucose solutions (Sole & Noakes, 1989) and are also more rapidly absorbed from the intestine. Either or both of these facts could explain why the rate of muscle glucose oxidation is greatest from glucose polymer and starch solutions (Hawley et al, 1991; Mood-ley et al, 1991).
Before deciding how all these factors influence the choice of the drink for ingestion during exercise, we will first consider the evidence that shows that the drinking pattern adopted during exercise, rather than the electrolyte and carbohydrate content of the ingested fluid, is the primary factor determining the rate of carbohydrate utilization by muscle.