Carbohydrate and fluid needs of the soccer player.
Soccer is a game that demands a combination of repeated maximal sprinting wit with 10 to 11km of moderate running, sometimes performed under extremely warm conditions. Over the course of a match, there is partial to near complete depletion of glycogen reserves in the leg muscles (depending on the extent of initial reserves and the level of competition), with a resultant decrease in physical performance. Blood glucose levels also fall, sometimes to values likely to cause a deterioration of both tactical thinking and cooperation between players (3.0 to 3.8 mmol/L), while in tropical climates, fluid losses can amount to 4 to 5kg of bodyweight. The effectiveness of glucose solutions in correcting these problems is limited for 2 main reasons: concentrations greater than 2.5% slow the rate of gastric emptying and thus fluid absorption, while provoking a secretion of insulin with a resultant hypoglycaemia. Fructose solutions are less liable to increase insulin secretion, but they have an equal propensity for slowing gastric emptying; moreover, the ingested fructose is largely metabolised in the liver, without boosting blood glucose. However, glucose polymer preparations have a low osmotic pressure per unit content of glucose equivalent, so that substantial amounts of carbohydrate can be administered in this fashion before gastric emptying is inhibited. If polymers are given before and during a soccer game, they sustain blood glucose, sparing muscle glycogen stores and increasing game performance. If the concentration of polymer is too high, one possible complication is a movement of water from the plasma into the gut; nevertheless, with an appropriate choice of concentration (for example, 7% polycose, 360 mOsm/L, plasma volume is increased rather than decreased relative to that seen with administration of water. Probably because the intergame interval for competitive soccer players is short, replenishment of glycogen reserves proceeds quite slowly. Moreover, this process does not seem to be helped by ingestion of either glucose polymers or a high carbohydrate diet.