Hyponatremia at Sundown Marathon

What's that again you say? Hyponatremia means having low levels of sodium (or salt) in your bloodstream (<135 meq per litre).

At least 3 runners were known to be affected judging from the data my friend collected as part of his research from runners who participated in the 84km Sundown marathon last Sunday.

How serious is this? Well, there was one death from hyponatremia in the 2002 Boston marathon. Thankfully, there were no fatalities from this race. Most runners are aware of the dangers of dehydration, but not many are aware of the dangers of over hydration.

Usually, athletes who participate in marathon distances and above, half ironman distances triathlons and above are most at risk.

Sodium (or salt) is a very important electrolyte that is involved in muscle contraction and has a role in moving water through your cell walls and distributing it throughout your body. When you sweat, you lose salt and water, and if you sweat a lot, you can deplete your body's sodium stores. If you drink too much water and do not have enough enough sodium (or salt) in the body, the water cannot move from the gut to the blood stream so the water remains in your tummy.

A common complaint among athletes is that they have drank a lot yet still feel thirsty. They feel bloated, their tummy is swollen and they have a upset stomach. More severe symptoms can include cramps, giddiness, disorientation, vomiting, diarrhoea, convulsions, swelling in the brain and even death in severe cases. Trust me on this, I had most of the previous mentioned symptoms and even spent 2 days in the ICU in a Hong Kong hospital when I had hyponatremia while taking part in a 100km run in November 2000.

Some interesting numbers for your consideration. Sweat contains 2.25-3.4 grams of salt per litre. In a long race, you can easily sweat 1 litre an hour, so in a 12 hour race that can add up to 27-41 grams of salt. A general rule of thumb is that you should consume 1 gram of sodium per hour in a long event. You should also increase your salt intake in the days leading up to the event (try 10-25 grams of salt per day pre race). Now, those of you who know me know why I always sprinkle extra salt on my french fries and why I'm not afraid of eating chips. Well, now that I'm not training like before, I'd better not go crazy with my salt intake.

To get just one gram from sports drinks alone, you will need to drink 2.8 litres of Gatorade. Do you think you can drink that much for the duration of your race? Not many of us can. Even if you drink sports drinks (instead of water), it is quite likely you will sweat out more salt than you can drink, especially on a hot day. You will probably need to replace your salt specifically.

Hope this helps with your next long training session or race.

References

 Almond CSD et al, (2005). Hyponatremia among runners in the Boston Marathon. New Engl J Med. 352: 1550-1556.

Noakes, T (2002). Hyponatremia in distance runners: fluid and sodium balance during exercise. Curr Sports Med Rep. 1: 197-207.