In their first significant study, these authors (Coyle et al, 1986a) compared the blood glucose concentrations and the rates of carbohydrate oxidation and muscle glycogen utilization in cyclists who ingested either a placebo (no carbohydrate) or a glucose polymer solution (approximately 400 g in 4 hours) during prolonged exercise.
They found that glucose polymer ingestion improved endurance performance by preventing hypoglycemia and maintaining a high rate of carbohydrate oxidation (Exercises 4.8). Interestingly, carbohydrate ingestion did not reduce the rate of muscle glycogen utilization, as also found in runners during marathon and short ultramarathon races (Noakes et al, 1988b). Their subsequent work (Coggan & Coyle, 1988) has shown that glucose infused at a rate in excess of 1 g/min, beginning at the point of exhaustion, can prolong performance.
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Carbohydrate ingestion at the point of exhaustion was less effective probably because the rate of delivery from the intestine was unable to match the high rate of oxidation (Coggan & Coyle, 1987). Exercise performance was even enhanced if the carbohydrate was ingested late in exercise but prior to the onset of exhaustion (Coggan & Coyle, 1989). The authors suggested there might be no benefit of ingesting carbohydrate throughout prolonged exercise. Ingesting a high carbohydrate load 30 or more minutes before the time at which exhaustion is expected to occur might have the same effect. It is still too early, however, to accept this possibility unconditionally.
These studies suggest that the rate of carbohydrate oxidation of about 1 gm/ min is required to support prolonged exercise at the point of exhaustion. Researchers have found (Moodley et al, 1991) that at an exercise intensity of 70% of V02max, about 50% of the carbohydrate delivered to the intestine from glucose polymer solutions is oxidized by the muscle; therefore, the rate of carbohydrate delivery to the intestine from the stomach would need to be about 120 g/ hr. As shown in Exercises 4.7, the only realistic way to achieve this would be to ingest 600 ml/hr of an 18% carbohydrate solution and to maintain a gastric volume of 400 ml.
Whether such a concentrated carbohydrate drink should be ingested for the entire duration of exercise is not known. Perhaps a less concentrated drink could be ingested initially and the more concentrated solution saved for the final third or final quarter of an endurance race when the rate of exogenous carbohydrate oxidation becomes the vital factor determining performance.
The ingested solution should probably be hypotonic (200 to 250 mmol/kg) with a sodium chloride content of up to 100 mmol/L in order to optimize intestinal absorption of carbohydrate and water. However, these proposals are based on the results of studies performed at rest. They need to be confirmed by studies performed during exercise, because intestinal water and electrolyte absorption may be altered during exercise (Barclay & Tumberg, 1988).