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How much should I drink during exercise?

Drinking too much or too little can have dire consequences. Here's the lowdown on how much athletes need to gulp down

From Friday's Globe and Mail

Alex Hutchinson draws on the latest research to answer your fitness and workout questions in this biweekly column on the science of sport.

The question

How much should I drink during exercise?

The answer

When Canada's top marathoners faced off in the ING Ottawa Marathon last weekend, they were battling for a spot on the Olympic team in Beijing. Every second mattered, so getting details like hydration right was crucial.

"Marathon races, for the most part, are won and lost at the water table," says veteran Olympic coach Hugh Cameron.

And it's not just a question of making sure you drink enough. That is an important issue: Severe dehydration can drive up body temperature to dangerously high levels, resulting in potentially life-threatening heat stroke, and other conditions. But a handful of sudden deaths from hyponatremia, a dangerous low-sodium condition caused by excess hydration, has made it clear that it's also possible to drink too much.

Then there's a new debate swirling among exercise physiologists over whether the effects of mild dehydration hurt your performance as much as previously thought.

Here's what we know: Hydration is an issue for just about any exercise activity. A summer tennis match might cost you, on average, 1.6 litres of sweat per hour. Running in the winter might cost 1.5 litres per hour, and even swimmers lose about 0.37 litres per hour.

But these averages don't really tell you much, because individual sweat rates vary dramatically depending on exercise intensity, the weather, your fitness and even your genetics, says McMaster University researcher Brian Timmons, who studies exercise and hydration in children.

"It's a very individual issue," says Dr. Timmons. "But if you just let thirst guide your drinking, you won't match your sweat losses."

A study published last year in the International Journal of Sport Nutrition and Exercise Metabolism found that runners who were allowed to drink as much as they liked during a 75-minute run replaced less than one-third of the fluid they lost by sweating.

What's less clear is whether you really need to match your sweat losses. Decades of lab studies have suggested that you need to drink enough to ensure you lose no more than 2 per cent of your body weight - the threshold beyond which performance is thought to suffer.

Recently, though, some scientists have suggested that while letting thirst drive your drinking may not replenish all your lost fluids, it's unlikely to impair your performance.

They cite studies from running races showing that the highest finishers in the race also tend to be the most dehydrated. (That was certainly the case for one of Mr. Cameron's athletes in the 1980s, four-time national marathon champion Mike Dyon, who didn't drink at all during races because of stomach problems.)

As a result, Tim Noakes, the South African doctor who first raised alarms about hyponatremia in the 1980s, advocates a simplified approach to hydration: "If you are thirsty, drink; if not, do not," he wrote last year. "All the rest is detail."

But for those who want a more scientific approach, the key to getting enough - but not too much - water is to know your personal needs in advance. In preparation for the Ottawa race, Mr. Cameron's athletes experimented for months during their training runs, figuring out how much fluid they need in different weather conditions.

It doesn't have to be that complicated, Dr. Timmons says. Simply weighing yourself before and after exercise a few times can give you some idea of how much you should expect to drink.

You may be able to get away with drinking less - but why risk it?

Alex Hutchinson is a former member of Canada's long-distance running team, and has a PhD

in physics.

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The sweat test

In order to assess your rehydration needs per hour of exercise, you need to calculate your sweat rate.

THE TEST:

- Do a warm-up run to the point where perspiration is generated. Urinate if necessary.

- Weigh yourself naked on an accurate scale. Do it nude, since your clothes may absorb some sweat.

- Run for 75 minutes at an intensity similar to that of the targeted race. Drink a measured amount of a beverage of your choice during the run.*

- Weigh yourself in the buff again on the same scale after the run.

*Do not urinate during the run (unless you choose to measure the amount of urine).

(Don't forget that sweat rate varies depending on the conditions and the activity, so you can't assume that what you measure for one sport will apply to another.)

THE RESULTS (example):

Pre-exercise weight: 60.0 kg

75-minute cardio session, 500 ml of water

Post exercise weight: 58.4 kg

Difference: 1.6 kg corresponds to 1.6 litres of sweat if you drank 500 ml of water, then your total fluid losses are 2.1 litres in 75 minutes, or 1.68 litres per hours.

AVERAGE SWEAT RATES AND VOLUNTARY DRINKING RATES

(LITRES PER HOUR)

Sweat rateVoluntary drinking rate
Tennis (males)1.61.1
Swimming (males and females)0.370.38
Half-marathon run (males)1.490.15

NOTE: Men generally sweat more than women.

SOURCE: AMERICAN COLLEGE OF SPORTS MEDICINE

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