A team of sport scientists have warned endurance athletes about the risk of over-hydrating


Long distance runners, cyclists and triathletes should trust their thirst as a guide to how much fluid they need to drink, says an international panel of sport scientists.

The 17 experts warn that advice to drink before you feel thirsty is only appropriate to a narrow set of circumstances, and that the dangers of drinking too much fluid are the principal cause of hyponatremia, a condition that can be serious and even fatal.

Working together, the scientists have produced a “consensus statement” on hyponatremia, with the aim of launching an educational campaign to raise awareness of what they call “a preventable and treatable fluid imbalance.”

Their conclusions are published in the Clinical Journal of Sport Medicine, and make essential reading for athletes involved in endurance events, from ultra-runners to Ironman triathletes and long distance cyclists. To understand the issue more deeply, here are 10 key findings from their statement:

1) What is exercise-associated hyponatremia (EAH)? It’s a serum, plasma or blood sodium concentration below a normal reference range, for anything up to 24 hours after an event. For most laboratories, this is a sodium concentration of less than 135 mmol/L.

2) What causes EAH? It can result either from a loss of sodium and potassium; or a relative excess of total body water; or a combination of both. In most instances the single most important risk factor is a sustained, excessive intake of fluid. Whether water or sports drinks, this volume of fluid exceeds the amount of fluid lost through sweat, breath and urine, so that a positive fluid balance builds in the body.

Athletes who finish events at the same weight or heavier than they were at the startline are at risk. As a rough guide, a marathon runner who loses less than 0.75kg during the course of a race is at risk of EAH.

3) How far do sodium levels have to dip? Falls of 7% to 10% in sodium levels within 24 hours can lead to EAH.

4) When does EAH develop? Research indicates hyponatremia comes on during and after endurance events. As a precaution, weigh yourself before and after training to assess your sweat rates and fluid replacement needs. Some weight loss will be due to energy expenditure, and this increases during longer and more intense exercise. If you have gained weight during training, it indicates that your fluid intake has been in excess of your fluid losses and that water overload is present.

5) What are the symptoms of EAH? Symptoms vary depending on the severity of the hyponatremia from mild lightheadedness and nausea, to headache, vomiting, confusion and even seizure. However, many athletes finish endurance events with sodium levels depleted to the point where they are hyponatremic, but they experience no adverse symptoms.

In a study of runners during the course of an ultramarathon, 67% were hyponatremic (asymptomatic) at some point during the race, but only 27% finished with sodium concentrations of below 135 mmol/L; 40% had self corrected their sodium levels before the end of the race. Ironman triathletes also suffer from the condition, with up to 25% of competitors suffering from EAH.

6) Is it fatal? Complications associated with EAH have led to at least 14 athlete related deaths since 1981.

7) What types of event can lead to EAH? Military training exercises, marathons, Ironman triathlons and ultra marathons are the most common causes of EAH, but it has also been diagnosed at training camps for rowers, after rugby league matches and among long-distance cyclists. Slower finishers in both half marathons (2 to 3 hours) and sprint triathlons (2 hours) have also experienced EAH.

8) Will a sports drink solve the problem? The sodium in hypotonic sports drinks can stem the depletion of sodium from the body, but its concentrations in most drinks (10-38 mmol/L) is much lower than in blood (140 mmol/L). Consequently, excessive fluid intake tends to overwhelm the protective effect of the sodium content of drinks.

9) Who is most at risk? Smaller athletes and those who take longer to complete events are most at risk. Women have a higher incidence of EAH.

10) How do I avoid EAH? Drink less. “Thirst should provide adequate stimulus for preventing excess dehydration and markedly reduce the risk of developing EAH in all sports,” say the experts in the Clinical Journal of Sport Medicine.

Their consensus statement warns that advice to start drinking before you feel thirsty was intended for events that create high sweat rates where dehydration could evolve rapidly, but that this is a poor guide for lower sweat races. “Excessive fluid replacement beyond thirst (whether water, sports drinks or other hypotonic fluids) is not a panacea for all instances of fatigue, collapse, muscle cramping, or exertional heat stroke.”

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