Nutritional strategies to minimize fatigue during prolonged exercise: Fluid, electrolyte and energy replacement

Jun 1, 1997Journal of sports sciences

How drinking fluids, replacing salts, and eating energy help reduce tiredness during long exercise

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Abstract

Athletes should consume approximately 100 ml of a dilute carbohydrate solution every 10 minutes during moderate-intensity exercise lasting over 90 minutes.

Ingesting carbohydrate (other than fructose) during moderate-intensity exercise is important for athletes.
Only about 20 g of ingested carbohydrate is utilized in the first hour of exercise.
Increasing carbohydrate concentration to around 10 g per 100 ml can help match peak plasma glucose oxidation rates.
Excessive carbohydrate intake may delay fat mobilization by reducing plasma insulin concentration.
Sufficient muscle glycogen levels at the start of exercise are crucial, regardless of carbohydrate intake during exercise.
Carbohydrate ingestion prevents hypoglycemia but does not mitigate fatigue from low muscle glycogen levels.

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While the presence of palatable (20 mmol l-1) concentrations of NaCl in drinks containing carbohydrate consumed during intense exercise would not be expected to promote absorption or significantly help maintain fluid balance, there is no doubt that athletes should ingest some from of carbohydrate (other than fructose) during moderate-intensity exercise lasting > 90 min. As only approximately 20 g of ingested carbohydrate is oxidized in the first hour of exercise, athletes should probably consume 100 ml every 10 min of a dilute (3-5 g 100 ml-1) carbohydrate solution and thereafter increase the carbohydrate concentration to approximately 10 g 100 ml-1 to match the peak (approximately 1 g min-1) rates of plasma glucose oxidation. Drinking more than those amounts of carbohydrate may increase muscle glycogen oxidation by attenuating the fall in plasma insulin concentration and thereby delaying fat mobilization, especially at relatively low (55% of peak oxygen consumption) intensity exercise. As carbohydrate ingestion does not slow the rate of glycogen utilization in working muscle, it is also advisable for endurance athletes to start exercise with an adequate supply of muscle glycogen, irrespective of whether or not they ingest carbohydrate during exercise. While carbohydrate ingestion 'spares' conversion of liver glycogen to plasma glucose and prevents hypoglycemia, it does not delay the fatigue associated with a low (approximately 20 mmol kg-1) glycogen content in working muscle. Conversely, increases in glycogen content of working muscle at the start of exercise have no effect on the rates of plasma glucose oxidation. Higher initial rates of glycogen utilization by active muscles in 'carbohydrate-loaded' subjects decrease the indirect oxidation (via lactate) of non-working muscle glycogen, rather than the conversion of liver glycogen to plasma glucose. Hence, athletes should ingest carbohydrate during endurance exercise even if they have 'carbohydrate-loaded' before exercise.

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Nutritional strategies to minimize fatigue during prolonged exercise: Fluid, electrolyte and energy replacement

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