Thursday, March 16, 2006

Hyponatremia, hydration, saltation Update

What could be said with confidence a year ago remains true: symptomatic exercise-associated hyponatremia (EAH) is always associated with too-much-body-water. (Symptomatic hyponatremia means you have a low plasma sodium concentration AND you are sick.) Other aspects remained open to debate. In particular (1) how is it that some people, drinking more than they need but by no means grossly overdrinking, retain water and become hyponatremic? (2) What is the role of sodium loss? A year later, we have a reasonable answer to the first question. As for the second, if we don’t yet have a full answer, we do have some interesting new insights.(1) We retain water inappropriately because the hormone AVP (arginine vasopressin), the only human antidiuretic hormone, is inappropriately released. I have written a detailed explanation of this for a lay audience. It is available at http://www.ultracycling.com/nutrition/drinking_too_much.htmlIf you are unable to access this, I can e-mail you a copy.Please also see Verbalis references (below) and Noakes et al. 2005.This has some important implications:**Do not slavishly follow any drinking schedule. Even a drinking schedule which seems to be working beautifully will get you into trouble if (for whatever reason) AVP levels become inappropriately high and you retain water. (Drinking schedules were quite deliberately not given in the Consensus Statement referenced below, Hew-Butler et al. 2005)**Listen to your body, and believe what it is telling you: if you think you are bloating, you probably are. Use a scale to make sure! Is your weight up from when you started? If yes, then STOP DRINKING until you urinate the excess. (Note that is highly unlikely that it is too much salt that is making you bloat, Weschler, 2005.)**Urination frequency and volume do NOT always give reliable information as to hydration status. You can be overhydrated and yet retain water. Do not think that if urination ceases, you are becoming dehydrated. Put this observation in a context of other observations.(2) The experiments have not been done from which we can conclude that sodium loss is or is not an important cause of EAH. Keep in mind that water overload, all by itself, with no loss whatsoever of sodium, can cause hyponatremia (Weschler, 2005). Also, note that we cannot extrapolate from, say, sweat rates and sodium-in-sweat concentrations for shorter events to longer events. Nonetheless, an interesting theoretical approach suggests that sodium loss may well be part of the hyponatremia picture in long events and in warm weather (Montain et al, 2006). On the other hand, evidence has been presented (Noakes et al. 2005) which suggests that we have sodium stores (“osmotically inactive sodium”) available in time of need. These stores would be a short-term (one long event solution); they would likely not be sufficient for long-term (training for weeks in hot weather).Implication:**Think you need salt? One trick is to add salt to already salty things (soups, pickles, chips, tomato juice, V-8 juice, jerky). Another trick: add salt to watermelon or tomatoes.

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