Overhydration and Hyponatremia
By Lulu Weschler
Lulu Weschler is an ultracyclist who participated in the 1st International Exercise-Associated Hyponatremia Consensus Development Conference, Cape Town, South Africa 2005.
Overdrinking leading to hyponatremia is the suspected cause of the death of a young Washington DC policeman during a bicycle training program.
Every death, and every serious case of hyponatremia during or after exercise thus far reported has involved over-hydration. To be sure, you lose sodium during exercise, but by far the dominant factor in exercise-related hyponatremia is over-hydration.
Hyponatremia means that when you divide the amount of sodium by the volume of blood plasma the number you get is too small. This number is called plasma sodium concentration. (Hypo means too small; -natremia means sodium status.) Theoretically, there are two ways to make this number too small: by decreasing the amount of sodium or by increasing the volume of fluid. Thus far, in symptomatic exercise-related hyponatremia cases, the increased volume of fluid caused the hyponatremia, not the amount of sodium being too small.
Note that over-hydration all by itself (regardless of whether or not sodium is "washed out") can cause hyponatremia by diluting the sodium. When the dilute blood gets to the brain, water seeps into brain cells and causes swelling. In hyponatremia deaths, brain swelling is the killer.
Overhydration can happen not only when you grossly overdrink, but also when you are moderately overdrinking, and for reasons that we are just now beginning to understand, retaining the overload that you would urinate at rest. Overdrinking a sports drink with electrolytes can cause overhydration and hyponatremia, because a sports drink has a much lower concentration of sodium than blood.
Therefore, take seriously any sign that you are putting on water weight during a ride. Weighing yourself before and after a ride is a good way to sort out your hydration needs. You should never finish with a weight higher than when you started. Other signs of over-hydration include evidence of bloating: puffiness in the hands or feet (at the sock line, watch, rings) or short line, "boggy" feeling flesh at the ankles, headache (especially noticeable when you ride on a bumpy road), looking like and/or feeling like the Michelin Man.
Since it is the brain swelling that kills, signs of weight gain plus any change in mental status (confusion, memory loss, disorientation) or any neurological symptom (incoordination, speech slurring) give a presumptive diagnosis of hyponatremia and represent a dire medical emergency. One other warning sign: nausea and vomiting are very often seen early in the development of hyponatremia.
What to do? Stop drinking. What you want is for urination to dump the fluid overload. Ingesting some concentrated salt could help get urination started. The recipe used by the Medical Staff at the Boston Marathon uses concentrated bouillon, one bouillon cube per ounce of water. This is the one exception to the no-drinking rule: use a very small amount of water as a delivery vehicle for salt. Other remedies include V-8 or tomato juice to which salt is added. Improvise ways to get some salt in. Then wait eagerly for urination to start.
Do not drink any sports drink: the concentration of sodium in sports drinks is too low, and the additional fluid will make the water overload worse. Do not resume drinking until you are certain that you have gotten rid of the overload of water.
Lulu is an author of the Exercise-Induced Hyponatremia Consensus Statement, which is available at www.cjsportmed.com (July, 2005).
Copyright 2005 by the UltraMarathon Cycling Assocation, Inc.
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