A runner's guide to hyponatremia

Today in the pathology residents room, talk turned from the usual discussion of cute pets and rare tumors to something that has apparently been making headlines in the marathon world: hyponatremia. Hyponatremia, or too little ("hypo") sodium (Na) in the blood ("emia"), is a condition that results when the concentration of plasma sodium decreases, leading to all sorts of problems. On of the possible causes of this decrease in concentration is by dilution - if you drink a lot of water, and you don't pee it out, all of the ions in your blood (read "electrolytes") will be diluted. And because sodium plays such an integral role in most physiologic functions, for example nerve transmission and cardiac rhythm, and because sodium concentrations are closely regulated to control the fluid content of every single cell in the body, becoming hyponatremic can have really disastrous consequences.

Like, all your brain cells swell up and you go into a coma. Or die.

And apparently, marathon runners have been dying of this. Because your body drastically decreases its urine output during extended exercise (because most of your blood is re-routed from the kidneys to the working muscles) runners are trapped in this dangerous cycle of sweating off sodium and then diluting the blood with water, without peeing off the excess. Interestingly, its the inexperienced, slow runners who are dying, because they are running for longer and they drink a lot more water. We are all taught that the worse thing that can happen to you while you are exerting yourself is dehydration. So everyone drinks a lot of water to stay hydrated. Oops! Turns out that's way worse for you. No one has ever died from dehydration after the Boston marathon. But a couple people a year die from hyponatremia.

Crazy, huh? Read about it here, or read the original study here.

Oh, and I know what you're thinking: Shouldn't you just drink gatorade or a similar sports drink to replenish your electrolytes? Nice thought, hot shot, but gatorade, though it does have some sodium, only has about one fifth the concentration in blood. So you're still putting way more water in than sodium. Nice try, though.