Too Much Running Water?

Summarized by Robert W. Griffith, MD
June 3, 2005

Introduction

It seems like everyone goes around clutching a bottle of water, these days. Somewhere we've got the idea that we need to drink more water (preferably from a European spa town). And when it comes to exercising, water drinking knows no limits. Perhaps it's time to set some, however. A study reported in the New England Journal of Medicine shows that hyponatremia - too little sodium in the blood - occurs in a substantial number of marathon runners, and is linked to their fluid intake during the race. Here's a summary of the report.

What was done

Two days beforehand, runners in the 2002 Boston Marathon completed questionnaires about their experience and training program, their medical history, and how they planned to take fluids during the race. They were weighed just before the race. Then at the finish line they were weighed again, gave a blood sample, and answered further questions about their fluid intake and urine output during the race.

The blood samples were measured for serum sodium, to check for hyponatremia.¹ A serum sodium level of 135 mmol/L or less was considered to represent this condition. Statistical analyses were done to determine the likely risk factors for the development of hyponatremia.

What was found

Out of 766 runners who enrolled, 488 provided blood samples for analysis. Men outnumbered women, two to one; the men's average age was 40, the women's, 36. The women had a lower average body mass index (BMI) than the men (21.4 vs. 23.7).

Thirteen percent of the runners had hyponatremia at the finish line - 22% of the women, 8% of the men. Three had dangerously low sodium levels (119, 118, and 114 mmol/L).

The calculated 'predictors', or risk factors for hyponatremia were: weight gain during the race, consumption of fluids every mile, consumption of more than 3 liters of fluids during the race, an increased frequency of urination during the race, female sex, and a BMI below 20.

Overall, a third of the runners gained weight during the race. A 1 kg (2.2 lbs) increase in weight doubled the likelihood of hyponatremia; and 70% of those who gained 3 kg (6.6 lbs) or more developed the condition.

What this means

These findings confirm earlier reports that have suggested a link between excessive fluid intake and hyponatremia. But of course, the size of the risk demonstrated here was under 'extreme' conditions, i.e. a marathon race. How great is the risk for the average weekend warrior?

The clear link between weight gain and hyponatremia means that only a few marathon-like events are likely to present this problem. Not many amateur athletes undertake prolonged exertion (over 3 hours) in which they consume as much as 3 liters of fluid. And with improved knowledge, more and more marathoners and 'iron men' are aware of the risks, and act accordingly; analysis showed that it was the non-elite runners in Boston who bore the greatest risk. Other high-endurance activities, such as five-set tennis matches, may, however, present problems.

The authors of the study suggest that long-distance runners weigh themselves before and after training runs, and adjust their hydration strategy accordingly. Thirst and sweating rate may be better guides than actual measures of intake.

In this study, the type of fluid consumed was less relevant than the volume. However, the first link below gives excellent advice about the need for a balanced salt intake for endurance athletes.

Source

• Hyponatremia among runners in the Boston Marathon. CSD. Almond, AY. Shin, EB. Fortescue,  et al., N Engl J Med, 2005, vol. 352, pp. 1550--1556

Footnotes
1. Hyponatremia is an abnormally low concentration of sodium in the blood. It's also known as "water intoxication". Sodium is an electrolyte which is lost through sweat. The more one sweats, the more sodium is lost. Symptoms of hyponatremia are similar to those of dehydration - lightheaded, tired, cramping, nausea, headache, etc. If severe, it's a medical emergency.

Related Links
Sports Medicine Web: Heat and Hydration
Nutrition for Amateur Athletes
Aerobic Fitness May Preserve Mental Capabilities

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