It follows that the only way in which dehydration can be prevented during exercise is to replace both the sodium and the water lost through sweat as these losses develop (Carter & Gisolfi, 1989; Ryan et al, 1989). Runners are not at risk of developing deficiencies of either magnesium or potassium during exercise, thus neither need to be replaced.
More important is the fuel content of the fluid. post 3 explained that athletes must ingest carbohydrate during prolonged exercise at up to 75 % V02max lasting more than 4 hours. To choose the most appropriate type of carbohydrate, we need to look more closely at factors that determine the rate at which fluid leaves the stomach (gastric emptying) and can be absorbed into blood from the intestine (gastrointestinal absorption).
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Exercises 3.1 lists some of the factors that influence the rate of gastric emptying. Costill and Saltin (1974) first studied these factors in detail, and the results of their experiments are depicted in Exercises 4.4, a to d.
These authors found that up to an exercise intensity of 60% V02max (Exercises 4.4a), the rate of gastric emptying is about 750 ml/hr with a 2.5% glucose solution, but thereafter the rate falls steeply and is only about 320 ml/hr at an exercise intensity of 90% V02max.
Gastric emptying falls with increasing temperature (Exercises 4.4b) of the ingested fluid and is only 350 ml/hr at a temperature of 40 °C, whereas emptying is almost 900 ml/hr at 5 °C.
The rate of gastric emptying falls with increasing glucose content (Exercises 4.4c) of the ingested fluid and is about 100 ml/hr at a glucose concentration of 10 g per 100 ml (10%). Also shown on this graph are the gastric emptying rates of