Hyponatremia

by Dr. Cathy Fieseler

For years, distance runners have been told to drink as much as possible. Lately, I’ve heard about runners dying from drinking too much water during marathons. I’m a bit confused; if I don’t drink enough, isn’t dehydration a problem? Do sports drinks protect me from the danger of too much fluid?

For years, the medical community has been concerned about dehydration associated with prolonged running, such as marathons, especially in warm weather. This concern has led to the plentiful aid stations in races and the development of sports drinks. It was felt that consumption of fluids while running would decrease the degree of dehydration.

In recent years, with slowing of average marathon times, the problem of hyponatremia (low blood sodium level) has come to the forefront for the general running population. During prolonged activities (typically longer than 4 and a half hours), it is possible to ingest a greater amount of fluid than is lost through sweat. The ultramarathon and endurance triathlon communities have been aware of this condition for years.

During these prolonged activities, the fluid consumed is absorbed at a greater rate than it is lost and in some cases, for reasons yet to be determined, excess fluid is not excreted in urine. This causes the level of sodium in the blood to be diluted to an abnormally low level. This problem can occur with ingestion of water or sports drinks. Although there is some sodium in the sports drinks, the level is small and cannot correct the deficit in the blood.

Hyponatremia is a very serious condition, which can be fatal. Unfortunately, the symptoms are nonspecific and can be confused with dehydration. Symptoms of hyponatremia include fatigue, swelling of extremities, nausea, headache, confusion, disorientation, seizures, coma and death. Swelling is the symptom that helps discriminate hyponatremia from dehydration. This is more than just the swelling that may occur in one’s fingers during a prolonged run; your shoes, socks and watch will become tight due to swelling. If you notice this occurring during a race, cut back on your drinking and ingest electrolytes capsules or salty food or drink (pretzels, chicken broth, etc.). If you don’t feel well, drop out; sodium levels are checked at many major races now and are critical in the appropriate treatment of endurance athletes.

Minor cases of hyponatremia are extremely common; most athletes with mild hyponatremia are asymptomatic and urinate out the excessive fluid during the hours following the event. One common finding with hyponatremia is minimal weight loss, or even weight gain following exertion.

To determine how much you should drink during long training runs and races, weigh yourself before running; keep track of how much you drink during the run. Weigh yourself again following the run. Each pound of weight that you lost represents a 16 ounce fluid deficit. A couple of pounds of this deficit are due to “metabolic waste”. For each gram of glucose stored in the muscle and liver as glycogen, 3-4 grams of water are stored. This will be lost during exercise and is not concerning for dehydration. Add to this deficit the amount of liquid ingested during the run to approximate your fluid needs. It’s a good idea to perform this test during different weather conditions to determine your needs at various times. If you sweat heavily, add salt to your diet or consider using electrolyte tablets.

© 2011 Savvy Runner Inc.

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