Drink to Your Health

Hydration Hall of Fame

The medical literature on sweat rates and fluid consumption tells the
story of many amazing athletes. A few highlights:

Ride Across America: A cyclist in the Ride Across America once consumed 28 liters (nearly 60 pounds) of fluid in 1 day. It just about killed him.

Alberto Salazar: During the 1984 Los Angeles Olympic Marathon, Alberto Salazar lost 11.9 pounds (8.1 percent of his body weight) while running 2:14:19 for 15th place.

Tour de France: Riders in the Tour typically consume about 44 ounces of fluid per hour of racing. Research shows that, for several key reasons, cyclists can consume more fluid than runners while going hard.

Ultramarathoners: In a 100-mile race in temperatures that reached 100 degrees, 13 finishers consumed 19.4 liters of water in 26 hours, for an average rate of about 24 ounces per hour.

An ultra ultramarathoner:
In 195 days of running more than 8,000 miles around Australia (44.5 miles a day), Gary Parsons maintained his weight and fluid balance by drinking about 6.4 quarts a day.

Hydration, Performance, and Risk
Dehydration diminishes performance, because it thickens the blood, decreases the heart's efficiency, increases heart rate, and raises body temperature. But a modest dehydration is a normal and temporary condition for many marathoners, and doesn't lead to any serious medical conditions. Excessive fluid consumption, on the other hand, can prove deadly.

The long and sweaty road: The first dehydration studies with marathoners were done at the Boston Marathon in the 1960s, during an era when runners were advised to avoid water-drinking because it caused stomach cramps. At any rate, race organizers provided no fluids en route. Result: The runners lost 5 to 6 percent of their body weight through sweating, but apparently suffered no particular harm.

A full tank: Since then, a substantial body of research has shown that anything more than a 2-percent dehydration will worsen performance, and everyone agrees that it makes sense to limit dehydration as you run. Some runners can even train themselves to drink more. Studies have also shown that the more fluid in your stomach, the more that reaches your blood, where you want it. Hence, the good advice to run with a comfortably full stomach and to "top off your tank" frequently.

Out of the lab, onto the road: Nonetheless, in the real world, the winners of any given marathon are probably the most dehydrated runners on the course. At sub-5-minute pace, they produce tremendous amounts of heat and sweat, and have little time for drinking. Even most runners who finish in 2- to 4-hours will sweat about twice as much as they drink. This can easily lead to a greater-than-2-percent dehydration. It's the drinking that limits us; the body doesn't like to run hard and drink hard at the same time. (At about 4-hour pace, it seems, runners are going slow enough, perhaps with walking breaks, that they can drink sufficiently to avoid most dehydration. Those athletes completing 5- and 6-hour marathons, and Ironman-type triathlons that last twice as long, can actually overhydrate.)

The sweat-rate paradox: As we get fitter, we sweat more. This means that we dehydrate faster--a cruel blow, it would seem. Of course, the body is smart. It knows that it can cope with modest dehydration. Heatstroke is the serious danger. So the body increases your sweat rate as you get fitter, because sweat promotes cooling, which helps hold heatstroke at bay.

The heatstroke threat: If you read the sports pages, you've seen the headlines about athletes dying from heatstroke. Usually, they're football players returning to training camp in the high heat and humidity of August. These guys are big, muscular, out-of-shape, and unable to deal with the heat/humidity combo. Dehydration can contribute to heatstroke, which is one of the prime reasons why all athletes are admonished to drink regularly.

The dog days of August: But dehydration doesn't cause heatstroke--it's more the big-guys-out-of-shape-in-August syndrome, particularly the high humidity. Marathoners rarely suffer from heatstroke because we're smaller, thinner, better conditioned, and less motivated (we're not going to lose a multimillion dollar contract if we don't impress the coaches with our hustle). We run our Houston and Miami marathons in the early morning of the winter months, not in August. Indeed, heatstroke is a bigger threat to college or Olympic 10,000-meter runners forced to compete in hot, humid track meets than it is to most recreational marathoners.

Hyponatremia deaths: As marathoning has boomed, and particularly as it has attracted more women and recreational runners, an entirely new health risk has intruded on our sport. It's called hyponatremia. Hyponatremia means "low blood sodium," but it's caused by excessive fluid consumption, which lowers the concentration of sodium in the blood. In extreme cases, hyponatremia can lead to brain seizures and death.

Last year, both the Boston Marathon and the Marine Corps Marathon had their first-ever fatalities attributed to hyponatremia. Hyponatremia is also beginning to appear in other endurance athletes, including ultramarathoners, Ironman triathletes, long-distance hikers, Army recruits, and even Iditarod cyclist-runners competing in sub-zero temperatures in Alaska. While no one knows for sure how many runners typically become hyponatremic, the 2002 Boston Marathon produced a 13-percent incidence rate. And the condition can be very serious.

Hyponatremia risk groups: Women, women, and women. This is not a sexist slam; it's a warning. Men can also drink excessively, and suffer from hyponatremia, as has happened in the Army. Nonetheless, a high percentage of exercisers suffering from hyponatremia have been women, including the Boston and Marine Corps deaths last year.

Why women? As already noted, they're smaller and less muscular than men, so they don't sweat as much. They also have a smaller blood plasma "tank" than men, which is easier to overfill. Many women are new marathoners who are happy to finish in 5 hours or more. They have heard that marathoners must drink as much as possible, so that's what they do. They reach the 20-mile mark exhausted (who doesn't?), and think, "If I can force myself to drink more, I'll feel better." It's a recipe for disaster.