Hyponatremia I

is a word that strikes fear into sports-medicine docs and medically aware long-distance runners. It just means low blood levels of sodium.

For reasons that are not clear, some runners retain water which dilutes out the sodium in their blood. Basically, runners drink a lot while they run, but don't urinate, losing water and sodium only by sweating. Most drinks, even sport drinks, have low sodium contents relative to blood, so runners can end up taking in too much water and not enough salt.

This can go unnoticed or can progress to big-time problems like seizures, confusion (mental status changes) and, worst, swelling of the brain (cerebral edema) that can cause death (never a good outcome in a healthy runner).

This issue jumbed into the public spotlight in 2002, when a previously healthy young women died during the Boston Marathon.

Despite being an MD and an ultrarunner, I don't really know much about this issue. What studies have been done have been mostly small and retrospective (looking back at patients after they got hyponatremia).

This weeks, New England Journal of Medicine (NEJM) has an excellent prospective article of more than 400 runners in the 2003 Boston Marathon. Most of the authors were a year behind me in residency, and they did the study as sort of a "class project." (ed: NEJM article, quite a class project; indeed).

They signed up runners before the marathon, then drew blood samples after and measured sodium levels. Concerningly, they found that 13% of runners had low sodium (<135)>4 hours, which is not that slow), gaining weigth during the marathon and being either thin or fat were the primary factors associated with increased risk. Females were at increased risk but this seemed more related to slower times and weight gain than to sex itself.

One weakness of the study is they don't provide follow up on those runners (did they go to the hospital, feel bad, or never notice anything).

In an accompanying commentary, Drs Benjamin Levine and Paul Thompson point out that overall marathon running is pretty safe:

In actuality, marathoning is a reasonably safe sport, with less than one death per 50,000 participants. Deaths that occur during less extreme physical activity and in previously healthy persons are usually caused by cardiac disease — predominantly, congenital problems such as hypertrophic cardiomyopathy or coronary anomalies in young athletes and atherosclerotic coronary artery disease in persons older than 35 years of age

but point out the paradox of hydration. Copious water intake during strenuous exercise, especially in hot weather, helps prevent problems like heat stroke, kidney failure and dehydration (obviously) but may lead to hyponatremia. So what should runners do?

One problem is that fluid requirements probably differ markedly among runners, summed up nicely in the original article:

Because runners vary considerably in size and in rates of perspiration, general recommendations regarding specific volumes of fluids and frequencies of intake are probably unsafe and have been superseded by recommendations favoring thirst or individual perspiration rates as a primary guide.

They note periodic weighing may also help. Interestingly, may ultras, especially 100 milers, periodically weigh runners, but usually looking for weight loss, not weight gain.

So I think runners need to drink to thirst, not excessively, but that is not much to go on.

Use of sodium containing drinks (such as Gatorade) would intuitively seem a good idea, but didn't seem to protect against hyponatremia in this study.

Not addressed in this study is use of sodium supplement (salt pills) which are widely but not universally used by ultramarathon runners.