Dr. Timothy Noakes’ book, * Lore of Running *, 1985, is the seminal book on this topic. Dr. Noakes is an exercise physiologist. The following is a summary as it pertains to this thread.
Although dehydration is not the critical factor predisposing athletes to heatstroke during exercise, marked dehydration does have detrimental effects: skin blood flow is reduced and body heat storage (and therefore body temperature) is increased. The major factors causing heatstroke are the environmental conditions, the speed at which the athlete runs, and individual susceptibility. The amount recommended by ACSM, based on an original study, was 250 ml of fluid every 15 minutes. But the marathoners in the study drank only about 100 ml/hr. The metabolism of glycogen releases the water stored in the glycogen, an additional important source. Too much fluid can cause hyponatremia, which was reported in 45 cases in three different years during the Comrades Marathon, which is actually about 50 miles (more recently, with the popularity of endurance events, many more have been reported).
The most important factor determining sweat rate is metabolic rate, which is affected by the speed of running and body weight, each increasing the sweat rate. The fluid should contain substances to restore the body’s supplies: electrolytes, sodium, and glucose. Both fitness and heat acclimatization reduce the sodium content of sweat. And this amount is trivial in the fit athlete, no more than 2 g/hr. The average salt intake far exceeds this. Thus you may conclude that sodium replacement is irrelevant, but the issue is more complex. During prolonged exercise the body is forced to deplete its fluid stores as a consequence of the salt losses in sweat. If this did not occur and if the body fluid stores were allowed to remain normal, then a diluted hyponatremia with potentially catastrophic effects would develop in all runners competing in races over four hours. Hence, both sodium and water must be replaced. It is not necessary to replace either magnesium or potassium during exercise. In addition, athletes must ingest carbohydrate during exercise up to 75% VO[sub]2[/sub] lasting more than 4 hours.
To choose the most appropriate type of carb, the factors that determine the rate at which fluid leaves the stomach and can be absorbed into blood from the intestine. Those factors, as determined in a 1974 study) are exercise intensity, fluid temperature, glucose concentration, and ingested volume. Gastric emptying falls with increasing temperature (cold is better) and increasing glucose content. Hypertonic (increased osmolality) solutions are also known to delay gastric emptying. More recent studies indicate many additional factors. Both dehydration and severe environmental conditions impair gastric emptying, but running at an intensity less than 75% VO[sub]2[/sub] increases gastric emptying.
Four factors can limit the amount of carbohydrate which can be used by muscles: gastric emptying, intestinal absorption, muscle glucose uptake, and oxidation. An increased sodium content will expedite the rate at which carbohydrate is absorbed, and vice versa. The rate of intestinal absorption of carbohydrate is more rapid from glucose polymer than from glucose solutions, and probably also the rates of electrolyte and water absorption. The fate of ingested carbohydrate is the same whether it is taken 3 hours before or 120 minutes after the start of exercise. Glucose is oxidized more rapidly than fructose. Fructose must first be converted to glucose by the liver. The intestinal absorption is also lower. Fructose solutions are also more likely to cause GI distress. The problem in determining the optimum carbohydrate solution is that the factors which expedite intestinal absorption (especially increased electrolyte and carbohydrate content) are also the factors which may retard the rate of gastric emptying, except for glucose polymers and starch solution.
Increasing the carbohydrate content of the solution beyond 6% begins to have a marked effect on gastric emptying. The gastric emptying of one drink is about 20 minutes. However, if the solutions are ingested repeatedly, so that the stomach is kept in a more distended state, than higher rates of gastric emptying can be achieved. So, for practical purposes, the rates of gastric emptying for water and for carbohydrate solutions at concentrations up to 10% are the same up to 70% max. But higher concentrations will significantly slow the rate.
An 18% carbohydrate solution ingested at a rate of 100 ml every 10 minutes at a gastric volume of 400 ml would provide the same results from a 7% solution ingested 100 ml every 10 minutes at a gastric volume of 200 ml. Thus, the rate of carbohydrate delivery, rather than water delivery, may be the more important factor. Ingestion of carbohydrate during prolonged exercise delays fatigue and enhances performance. The solution should be hypotonic with a salt content of up to 100 nmol/L.
Ingestion of glycerol or medium – or long – chain fatty acids are of no benefit; whereas, the ingestion of alcohol is contraindicated because it impairs liver glucose production and cannot be directly used as a fuel, having to be first changed to acetaldehyde in the liver, but even then it is an inferior fuel.