and Electrolyte Balance and Endurance Exercise:
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
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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Massimino F, Hiller RE, Laird RH. Plasma electrolyte and glucose changes
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Murray B. Fluid Replacement:
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Speedy DB, Noakes TD,
Rogers IR, et al. Hyponatremia in ultradistance triathletes. Med Sci
Sport Exerc 1999; 31:809-815.
Speedy DB, Rogers IR,
Noakes TD, et al. Diagnosis and prevention of hyponatremia at an ultradistance
triathlon. Clin J Sport Med 2000; 10:52-58.
Speedy DB, Noakes TD,
Schneider C. Exercise associated hyponatremia: a review. Emerg Med 2001;
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.
Volume 18, Number 3, Spring 2001