Or what is an ultrarunner to do?
The NEJM article I discuss in the previous post deals with hyponatremia in runners in the Boston Marathon. Risk was increased with longer finishing times, presumably because it gave more time to get into trouble. The highest risk group was runners who took more than 4 hours to finish.
How much higher is the risk in an 100 mile run where the winner might take 16 hours (and the conditions generally more extreme)? One would guess higher, but it might not be so bad. In an ultra, the pace is slower, and the races last long enough that taking a break to urinate is no big deal (some runner try to go without stopping, the "Western States Wiggle"). The participants are generally better trained as well.
As the old saying goes "the dumbest kidney is smarter than the smartest nephrologist." This is why we don't have to worry about our fluid and sodium intake all the time. We take in whatever we feel like and our kidney secretes more or less sodium (and other electorlytes) to keep us in balance. Pretty nifty (and nephrologist are mostly really smart).
Hopefully in an ultra, you take in enough salt and water and go slow enough that your kidney sorts it out. But not always.
I've had two experiences in ultras that suggest hyponatremia, both 100 milers. At Vermont in 2002, the day after the race (I ran fast enough to finish in the early morning hours) I had pitting edema in my lower legs (pitting means you push your finger in and it makes a dent that stays there for a few minutes). At Western States in 2003, by the 20 mile point or so my hands were very swollen and I hadn't urinated at all (4+ hours). I also gained about 5 lbs. I was also having trouble breathing, but that might just be the altitude. As I went on, and lost altitude, I started urinating and felt better, but kept most of the weight until the end.
I do regularly take Suceed caps, which are modified salt pills during both races, and all my ultras for that matter. I think they help by replacing sodium you lose sweating, but at this point is it a bitconfusing what the best hydration/electrolyte strategy is (except to make sure you pee!)