Oxidation of Carbohydrate Ingested During Prolonged Endurance Exercise
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Classic studies conducted in the 1920s and 1930s established that the consumption of a high carbohydrate (CHO) diet before exercise and the ingestion of glucose during exercise delayed the onset of fatigue, in part by preventing the development of hypoglycaemia. For the next 30 to 40 years, however, interest in CHO ingestion during exercise waned. Indeed, it was not until the reintroduction of the muscle biopsy technique into exercise physiology in the 1960s that a series of studies on CHO utilisation during exercise appeared. Investigations by Scandinavian physiologists showed that muscle glycogen depletion during prolonged exercise coincided with the development of fatigue. Despite this finding, attempts to delay fatigue during prolonged exercise focused principally on techniques that would increase muscle glycogen storage before exercise. The possibility that CHO ingestion during exercise might also delay the development of muscle glycogen depletion and hence, at least potentially, fatigue, was not extensively investigated. This, in part, can be explained by the popular belief that water replacement to prevent dehydration and hyperthermia was of greater importance than CHO replacement during prolonged exercise. This position was strengthened by studies in the early 1970s which showed that the ingestion of CHO solutions delayed gastric emptying compared with water, and might therefore exacerbate dehydration. As a result, athletes were actively discouraged from ingesting even mildly concentrated (>5 g/100ml) CHO solutions during exercise. Only in the early 1980s, when commercial interest in the sale of CHO products to athletes was aroused, did exercise physiologists again begin to study the effects of CHO ingestion during exercise. These studies soon established that CHO ingestion during prolonged exercise could delay fatigue; this finding added urgency to the search for the optimum CHO type for ingestion during exercise.
Whereas in the earlier studies, estimates of CHO oxidation were made using respiratory gas exchange measurements, investigations since the early 1970s have employed stable 13C and radioactive 14C isotope techniques to determine the amount of ingested CHO that is oxidised during exercise. Most of the early interest was in glucose ingestion during exercise. These studies showed that significant quantities of ingested glucose can be oxidised during exercise. Peak rates of glucose oxidation occur ≈75 to 90 minutes after ingestion and are unaffected by the time of glucose ingestion during exercise. Rates of oxidation also appear not to be influenced to a major extent by the use of different feeding schedules. Irrespective of whether subjects ingest glucose as single or multiple feedings, around 20g of glucose is oxidised in the first hour of exercise.
Since repetitive feedings would be expected to accelerate the rate of delivery of glucose from the stomach to the duodenum, the similar rates of ingested glucose oxidation after single and multiple feedings suggest that exogenous CHO oxidation may not be limited by the rate of gastric emptying. Instead, studies in which rates of both gastric emptying and of ingested CHO oxidation were measured have shown that ingested glucose oxidation in the first 60 to 90 minutes of exercise must be limited by factors distal to the stomach; that is, either by the rate of glucose absorption into the bloodstream, or by the rate of exogenous glucose oxidation by the active muscles.
Other data, however, indicate that it is the rate of absorption of glucose into the bloodstream, rather than its oxidation by muscle, which limits the rate of exogenous CHO oxidation. Thus, in glycogen-depleted or fasted subjects exogenous CHO oxidation is not increased. Further, the rate of exogenous CHO oxidation plateaus when the exercise intensity increases above 50% of maximal oxygen consumption V̇2max, suggesting that glucose delivery to the blood may be limiting.
Comparisons between glucose and fructose ingested by fed subjects during exercise show that the rate of oxidation of fructose is less than that of glucose. Oxidation rates of ingested maltose, sucrose and glucose polymer solutions are very similar to those reported for glucose provided the CHO is ingested in sufficiently large volumes.
The practical implications of these findings for optimal performance in endurance events are that athletes should be encouraged to maximise CHO delivery to the working muscles. This is best achieved by ingesting a pre-exercise bolus feeding (200 to 400ml) of a mildly concentrated (5 to 7 g/100ml) long-chain glucose polymer solution, followed by repetitive, multiple feedings (100 to 150ml) of the same solution every 10 to 15 minutes for the first 2 hours of prolonged exercise. Thereafter, athletes should ingest a single (200 to 300ml) bolus of a more concentrated (15 to 20 g/100ml) long-chain glucose polymer solution followed by 100 to 150ml of the same drink every 10 to 15 minutes until the completion of exercise. This drinking pattern will ensure that both fluid and CHO delivery are maintained at rates necessary to sustain performance during the later stages of prolonged, exhaustive exercise.
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