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Advice from the Food Standards Agency of the UK is to limit salt intake to six grams of salt a day, an amount that would not cover what some footballers lose in an hour according to one study. Pamela Hinton, Assistant professor of Nutritional Sciences at the University of Missouri-Columbia, outlines the amount of salt that should be replaced during and after exercise and provides a recipe for a home-made sport drink has the optimum sodium content for sustained performance.

General health guidelines in most countries advocate the reduction of salt intake for good health. However, this advice may be too simplistic for athletes who lose significant amounts of salt during exercise. A study (commissioned by the Salt Manufacturers Association) on professional footballers in the UK found that some players lose as much as 10 grams of salt in a 90-minute training session.

Athletes have higher fluid and sodium requirements than sedentary individuals. For the general population, the recommendation is to limit sodium intake to 2.3 grams per day, which is equivalent to 5.8 grams of salt. The rationale for this guideline, is that excessive sodium intake causes high blood pressure in individuals who are 'salt sensitive'. Because most people only require 1.5 grams of sodium per day, it makes sense from a public health perspective to recommend reduced intakes. Athletes, however, need significantly more sodium than their inactive counterparts; the exact amount varies greatly between individuals, depending on sweat volume and sweat sodium concentration.

Some athletes may require more than 10 grams of sodium per day to make up for the amount lost in sweat. Athletes also require more fluid than sedentary individuals - up to 10 litres per day. Both dehydration and sodium depletion adversely affect athletic performance. However, it is difficult to differentiate between the two because they occur simultaneously and have similar negative consequences.

Dehydration and hyponatremia

Dehydration due to an imbalance between fluid loss and intake is the most common cause of heat-related illness in athletes. Athletes may lose water at a rate of 0.5-1.5 L/h and up to 6-10 percent of their body weight. Water is lost from all fluid compartments, resulting in decreased sweating and impaired heat dissipation.

The decline in blood volume decreases blood pressure and cardiac output. Heart rate increases by 3-5 bpm for every one percent of body weight lost to compensate for decreased stroke volume. Skin blood flow is also decreased, further reducing the ability to decrease body temperature. Symptoms of heat-related illness are headache, nausea, dizziness, apathy, confusion, exhaustion and chills. Performance declines markedly due to decreased muscle perfusion. Paradoxically, gastric emptying is slowed, impairing fluid absorption and restoration of fluid balance. The risk of heat-related illness is increased by exercise in hot and humid environments, the use of diuretics, and by increasing age.

A disproportionate amount of fluid lost in sweat is from the extra-cellular fluid (ECF), the fluid outside of the cells, including the blood plasma. The average concentration of sodium in sweat is 1150 mg per litre, but can vary greatly (450 mg to 2300 mg per litre). Assuming a sweat rate of 1.5 litres per hour, an athlete with sweat of average saltiness would lose about 1700 mg of sodium per hour. Excessive sweating, combined with consumption of plain water in copious amounts (e.g., 10L in 4 hours), results in a sodium deficit, i.e., dilutional hyponatremia.

The symptoms of hyponatremia are disorientation, confusion, seizure, and coma. This condition is quite rare and most often occurs in marathon and ultra-marathon type events lasting longer than three hours and in individuals who ingest a large volume of fluid without electrolytes.

Exercise-associated muscle cramps (EMAC)

The belief that dehydration and the concurrent electrolyte imbalances, secondary to heat stress, cause muscle cramps is prevalent. However, if this were the case, one would expect widespread, as opposed to localised, muscle cramps. EMAC are usually localised to a specific muscle group, rather than globally affecting all skeletal muscles. A recent study of ultra-marathon runners found no differences in hydration status or blood electrolyte concentrations between runners who suffered cramps and those that did not. Similarly, a study of cramp-prone athletes found that the incidence of cramps was not affected by hydration status. An alternative explanation for EMAC, is that neuromuscular fatigue causes the muscle to become 'hyper-excitable' so that the muscle does not relax, but remains contracted. Regardless of the cause of cramps, dehydration and sodium depletion negatively affect performance.

Fluid and sodium intake during exercise

Ideally, athletes should drink 8-12 ounces (240-350ml) of fluid every 15-20 minutes during exercise. If a training session or competition exceeds one hour, a commercial fluid replacement beverage that contains carbohydrates and sodium is superior to plain water.

Exogenous carbohydrate maintains blood glucose concentrations, so glycogenolysis is delayed. Sodium increases the palatability of the beverage and enhances fluid consumption and replacing some of the sodium lost in sweat will reduce the risk of hyponatremia. The recommended concentration of sodium in a fluid replacement beverage is 500-700 mg per litre. Most sports drinks contain sodium, although the amount varies from 300 to 650 mg per litre.

An alternative to commercial fluid replacement beverages is easily prepared by adding ¼ - ½ teaspoon of salt to one litre (32 ounces) of water, which is equivalent to about 600 and 1200mg of sodium per litre. Salt (sodium chloride) tablets are available, but eight ounces of fluid (250mL) must be consumed with every 200mg of sodium so that the concentration of sodium in blood does not rise too rapidly. Salt tablets are more effective and better tolerated (they may cause gastrointestinal problems in some people) if they are crushed and mixed with water.

The fluid that is consumed must be emptied from the stomach and absorbed from the intestine to be of any benefit. The rate of gastric emptying can reach one litre per hour and is maximized when gastric volume is high (>600mL), solutions are hypotonic, and the carbohydrate concentration is 4-8 percent. The rate of fluid absorption is negatively affected by high intensity exercise (>80 percent maximal oxygen consumption, VO2max), carbohydrate concentrations that exceed 8 percent, and dehydration (>4 percent BW).

Fluid and sodium intake post-exercise

Rehydration after exercise is important because most athletes do not consume enough fluids during exercise to replenish the fluid lost in sweat and respiration. In general, an athlete should consume 24 ounces of fluid (709 millilitres) for every pound of weight lost during an exercise session. The excess fluid consumption is to offset the 'obligatory urine losses' that occur when a large volume of water is consumed within a short period of time. Obligatory urine losses can be minimised by drinking a beverage that contains sodium and by eating foods that are high in sodium after exercise: pretzels, pickles, pizza, cheese, tomato sauce, soy sauce, tomato juice, canned soups, and ketchup.