Fluid and Electrolyte Balance and Endurance Exercise:

What can we learn from recent research?

Ian R. Rogers, MD, FACEM

In the not too distant past, the conventional (42km) marathon was considered to be the limit of human endurance. In the last two decades, we have seen this "limit" challenged. It is now commonplace to see non-elite athletes competing in ultra-marathons, long-distance cycle events, and multidisciplinary "multi-sports" or "adventure sports" races. These new sporting disciplines have brought with them a whole new raft of previously unreported medical illnesses. The most important of these is exercise-associated hyponatremia. Much of the research on this subject has been from work done in the setting of the Ironman triathlon (3.8km swim, 180km cycle, 42km run). It is, however, applicable to a wide range of endurance exercises and provides important information about appropriate fluid and electrolyte balance for any form of wilderness endurance activity.

The first reports of exercise-associated hyponatremia appeared in the early and mid 1980s, during Ironman triathlons and ultra-marathon running races. More recently it has been reported in a broad range of endurance and wilderness activities including cycling, mountaineering, canyon walking, and all-night dance parties. When first reported, it was thought that exercise- associated hyponatremia was due to excessive losses of sodium in the sweat that could not be adequately replaced during exertion. Repeated studies have now shown this not to be the case. The common feature of all studies has been excessive fluid intake. This fluid is retained in the extracellular space and in particular the intravascular space. Effectively, it dilutes the sodium, which is largely an extracellular cation, and leads to the observed clinical effects of the syndrome. So exercise-associated hyponatremia is due to dilution from fluid overload.

The symptoms of hyponatremia are largely caused by the fluid accumulation and edema formation, especially in the brain. This means that the early symptoms can be relatively non- specific and include nausea and vomiting, lethargy, malaise, headache, and fatigue. These symptoms typically occur at serum sodium levels of 125-134mmol/l (the normal is 135-145mmol/l). It can be difficult to differentiate these symptoms in the field from a whole range of other medical problems such as dehydration, heat illness, hypothermia, and hypoglycemia. As the serum sodium becomes lower still (less than 125mmol/l) more specific symptoms develop. Prominent among these is marked confusion followed by seizures and coma. It is clear that women are substantially more at risk of the syndrome than men, though just why is uncertain. Exercise-associated hyponatremia should be suspected following prolonged exercise (more than 4 hours), when fluid intake has been high (and greater than expected losses), and when typical symptoms develop.

It is not usual to be able to measure serum sodium in a field setting, so treatment needs to be commenced on the basis of clinical suspicion. Milder cases should self-correct with rest and limitation of access to further fluid. The subject will then usually excrete the excess fluid in the urine. Severe hyponatremia (serum sodium less than 125mmol/l) is an emergency warranting medical evacuation. It is critical to withhold IV fluids as this will just worsen the dilution and hyponatremia. Patients who are having seizures or are comatose require intravenous hypertonic saline which is usually only available in a hospital.

Prevention is obviously preferable to treatment and is of particular relevance to a wilderness setting. It has now been shown that hyponatremia can be prevented in the Ironman triathlon by limiting access to education, fluid, and increasing salt intake. There is no reason to believe that a similar strategy is not also applicable to wilderness endurance activity.

This begs the question of what is the appropriate amount and type of fluid intake for endurance activity. It is important to realize that longstanding advice about appropriate fluid intake for exercise was formulated on research done on much shorter events when the "limit of human endurance" was much less. The applicability of this to longer events is questionable. The American College of Sports Medicine in its position statement, currently recommends a fluid intake during exercise of 600-1200 mls/hr. The fluid intake of most of the reported cases of exercise associated hyponatremia has been at the middle or upper end of this range challenging this as an appropriate fluid intake. A more realistic intake is likely to be 500-750mls/hr. Whether this fluid should contain salt as well remains unproven. Intuitively, it seems prudent to use a proprietary sports drink containing 20-30mmol/l of sodium if this is available, rather than just water.

While the old mantra, "If you donít drink you die" is not yet dead, it has certainly been challenged. We can no longer assume that excess fluid taken during prolonged exercise will just be passed out in the urine. Like most things in life, balance is the key and the balance is likely to be at a fluid intake not much above 500 mls per hour in most situations, unless predicted losses are very substantial.


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Ian is an Emergency Medicine physician in Verdun, Nedlands, Western Australian. He is a frequent contributor to the Journal and popular speaker at WMS conferences.

Wilderness Medicine Letter, Volume 18, Number 3, Spring 2001