Thinking about water and salt while running in the woods

I signed up to run a 77-mile ultramarathon in the highlands of Pennsylvania. The weather forecast predicted a high temperature of 85 degrees with relative humidity between 90% and 100%. I knew the day would be especially challenging for me. I had not been able to train in or acclimate to heat and humidity where I reside in arid western South Dakota. We were permitted 24-hours to run the entire distance. If we did not make it through each of the four checkpoints by the designated time, we would be pulled from the course and considered “DNF” or “Did Not Finish.”

As the hours and hours go by, one thought that comes to mind constantly is fluid status. Am I drinking enough? Am I drinking too much? Drink too little and dehydration results. By the time the mistake is recognized, it would be too late and my race would be over. On the other hand, if I drank too much I could risk hyponatremia — a potentially lethal condition.

When long distance running became popular in the 1980s and 1990s, runners were cautioned about the hazards of dehydration. They were warned: Drink! Even if you don’t feel thirsty! As a result of this well-meaning, but mistaken advice, a number of runners died from severe exercise-associated hyponatremia. Since then, exercise associated hyponatremia and the prevent of it has become better understood.

Risk factors for exercise-associated hyponatremia include: Low body weight, female gender, exercise duration of 4 hours or more, inexperience, and excessive intake of fluids. Individuals with exercise associated hyponatremia have been found to have less than maximally suppressed vasopressin. In addition, non-aspirin nonsteroidal anti-inflammatory drugs may predispose to hyponatremia — presumably through inhibition of prostaglandins which enhances the effects of vasopressin.

It can be difficult to monitor fluid status during extreme endurance activity. Thirst is helpful; however, it becomes less reliable as fatigue sets in. The frequency and color of urine can be a useful marker of hydration status. Some ultramarathon races require runners to check in at medical tents and be weighed at scheduled intervals. Too much weight loss compared to baseline suggests dehydration. The runner is forced to stop and drink. Too much weight gain suggests overhydration — the runner may also be pulled from the race until excess fluid is urinated.

High salt foods are frequently offered at the aid stations of ultraendurance events. Speaking from personal experience, these foods taste exceptionally good during periods of extreme endurance activity. However, increasing oral sodium intake has not been shown to prevent exercise-associated hyponatremia. In addition, despite the heavy marketing to the contrary, no sports drink has been shown to improve serum electrolyte levels or prevent hyponatremia.

The best method to prevent exercise associated hyponatremia is to pay attention to fluid intake and avoid overhydration. Several groups have offered specific guidelines regarding how much fluid should be consumed per hour. The problem is that there is great variation in actual requirements between individuals related to weather conditions, training, experience, acclimatization, and rate of perspiration.

I sipped small amounts of water from my bottle and nibbled on small amounts of food throughout the day. I struggled to maintain my pace as I climbed and descended hill after hill. Darkness fell. I could only see the small area of forest lit up by my headlamp. I arrived at the 53.2 mile checkpoint only to be told that I had no choice, but to stop because I was 5 minutes past the cut off time. By that time, 50% of the runners who had started with me that morning had dropped from the race somewhere in the miles behind me. I heard of at least one who required formal medical attention.

It felt strange to stop just as I began feeling stronger in the cool temperatures of the evening. Nevertheless, I took great pride in knowing that I had succeeded in keeping my hydration “just right” despite the difficult conditions. I am certain that I would have not been able to have made it as far as I did had I not heeded my own advice. Knowledge never guarantees success in application. Some lessons are best learned through personal experience.