honoluluadvertiser.com

Sponsored by:

Comment, blog & share photos

Log in | Become a member
The Honolulu Advertiser
Posted on: Sunday, June 17, 2007

Exercising in summer heat requires precautions

By Anna Gosline and Jeannine Stein
Los Angeles Times

WHEN DRINKING COLD FLUIDS IS OK

Make sure whatever fluid you drink before and during exercise is icy cold. A team led by David Jones, professor of sport and exercise sciences at the University of Birmingham, England, had eight men cycle to exhaustion in 93-degree heat. They found that those who drank cold fluids biked seven minutes longer than those given warm drinks. They also had slightly lower temperatures and heart rates, and drank 1.4 quarts of fluid compared with 1 quart of room-temperature water.

Paradoxically, after exercise, drinking cold water might be worse for hydration. "It satiates you more, so you drink less," says co-author Toby Mundel. That's when you should drink something at room temperature.

— Anna Gosline and Jeannine Stein

spacer spacer

Exercising al fresco is a great pleasure. The scenery and fresh air can make a long run go by faster. But summer heat waves can turn refreshing outdoor exercise into a sweat-drenched experiment in heat exhaustion.

Overheating, the mild form, causes fatigue and dizziness. That's annoying enough. As internal temperatures rise above 100 degrees, athletes may experience cramps, headaches, nausea and vomiting. By the time core temperatures reach 104, the body rebels from hyperthermia. If the athlete keeps on pushing and internal temperatures pass 104, the athlete risks "organ failure and death from heat stroke," says Dr. Aurelia Nattiv, professor in the Department of Family Medicine, Division of Sports Medicine at the University of California-Los Angeles.

Scientists are learning more about what influences overheating — and ways to help an athlete avoid it. Just how hot and bothered you get on the inside depends on body size, fitness level, intensity of exercise, the heat and humidity of the environment, and how acclimatized you are to exercising in hot weather.

Some tips science offers are unsurprising: Lower the intensity of exercise; wear the lightest, littlest clothing possible.

Others are more nuanced, or evolving: cool drinks are best during workouts, but afterward, warmer's better (if, that is, you drink at all during workouts; not all scientists agree that it's needed, or advisable).

Immersion in an ice-cold bath before exertion is helpful. And caffeine, long thought to be a no-no because it contributes to overheating, may be fine on race day.

Follow the advice here and those outdoor runs can still be a pleasant — if unavoidably sweaty — part of summer.

COOLING SYSTEMS

Working out uses energy we derive from food. A mere 25 percent of that energy leverages muscle force. The rest goes to waste — as heat.

Fortunately, the human body comes well equipped with heat-loss mechanisms. As core temperatures rise, sweat glands pump water through the skin. Sweat evaporates, taking body heat with it.

Higher body temperatures cause the heart to pump more blood to the skin. Skin blood vessels dilate, exporting more heat.

As anyone running in midday heat knows, these mechanisms can be severely impaired by weather. "Exercise in the heat poses a formidable challenge to the body's ability to control its internal environment," says Susan Shirreffs of Loughborough University in Britain. As the difference between body temperature (98.6 degrees) and ambient temperature shrinks, heat moves less readily to the air.

When the mercury passes 100, we actually begin to absorb heat from the environment.

Humidity adds an extra whammy by keeping sweat from evaporating.

Other factors determine how hot we get — such as body size. In a 2000 study, Frank Marino of Charles Sturt University in Australia tested 16 trained runners whose body weights ranged from 121 to 198 pounds. The lighter runners produced and stored less heat at the same running speeds, probably because smaller bodies require less effort to move and have a greater ratio of surface area-to-volume to dissipate heat.

This doesn't mean larger-framed athletes must exercise in the confines of a humidity-controlled, air-conditioned gym. Merely being fit helps, too. The stronger the cardiovascular system, the easier and more efficiently it pumps blood to the skin, where it can dump excess heat, says Glen Kenny of the University of Ottawa.

Regular exercisers start sweating at a lower core body temperature — and sweat more.

So if you can't be small, be fit. And while you're at it, shed excess body fat.

Fit and lean people aren't just better at cooling down, they also seem able to withstand greater heat. A 2001 study gave 24 men and women of either high or low fitness and fatness an extreme heat tolerance test — they had to exercise in nearly impermeable protective gear under hot conditions. The fit, lean men and women exercised, on average, 45 minutes longer, even with body temperatures slightly higher than unfit, fat subjects.

It's hard to control all factors that play into overheating. For example, "Some people are just genetically heavy sweaters," says Larry Kenney, professor of physiology and kinesiology at Pennsylvania State University.

And people with diabetes are at a disadvantage because they often have decreased blood flow. This means less hot blood can be pumped to the surface to help with heat loss.

Even menstrual cycles affect heat balance. During the follicular phase (after the menstrual phase and before ovulation at day 14), women have a significantly lowered body temperature, a lower threshold for sweating and increased blood volume.

There are factors the athlete can more readily control. Among the most important: Take time to acclimate to the heat.

STAY HYDRATED

"Your body just does a lot of things to fine-tune itself to hot exercise," says Douglas Casa, director of athletic training education at the University of Connecticut. People who regularly exercise in the heat have a lower resting body temperature, decreased heart rate and quicker and more generous sweating.

This doesn't happen overnight. To prep for summer athletics, it takes 10 to 14 days of regular exercise in the heat, slowly building up to intense workout at the hottest times of the day.

Heat acclimation is quickly lost. One week without activity in hot weather can strip away that hard-won adaptation.

There are practical lessons here. Ian Murray, head coach of L.A. Tri Club, which provides services and support for Los Angeles-area triathletes, advises people to take weather into consideration when training, mimicking the conditions expected on race day. If the marathon portion of a triathlon doesn't start until the afternoon, during peak daytime temperatures, he'll instruct athletes to take runs in the heat of the day to properly prepare.

Sports physiologists also stress the importance of proper hydration. As water content drops, less is left for sweat — meaning less sweating and less cooling. Plasma blood volume also drops and less blood flows to the skin.

Classic laboratory studies from the 1970s and '80s revealed that being dehydrated while exercising in heat leads to higher core temperatures and a faster core temperature rise. The cardiovascular system also begins to suffer strain.

The American College of Sports Medicine recommends athletes ensure they are hydrated before exercise, keep hydrating during exercise and rehydrate afterward. But because there is no formula that fits everyone, because of huge differences in factors such as sweat rate, the ACSM suggests athletes try to replace fluid as it's lost.

It also stresses that relying on thirst will not do the trick, because thirst kicks in only when 1 percent to 2 percent of body weight is lost. Above that, many people find they aren't really thirsty enough to fully replace fluids they sweat out.

Failing to drink during a short, one-off bout of exercise may not matter if you began well hydrated. But if you're doing continual training over several days, or several exercise sessions in one day without adequate rehydration in between, there's a heat-illness risk, Casa says.

A strategy? Casa suggests exercisers drink while working out and weigh themselves before and after: "If you weigh less, drink a little more. If you weigh more, you overdid it." Monitoring urine color also can help track hydration. Anywhere in the region of lemonade color is good, but when it gets to the appearance of apple juice, it is time to drink up.

There's some disagreement on the issue of hydration. Dr. Timothy Noakes, professor of exercise and sports science at the University of Cape Town in South Africa, maintains that marathoners shouldn't drink beyond thirst lest they overdo their intake and develop hyponatremia, a dangerous dilution of body salts that killed a 2002 Boston marathon runner.

What's more, there is evidence that the amount of fluids consumed during exercise has little effect on core body temperature in real race conditions. A 2006 study led by Chris Byrne, a lecturer in exercise physiology at England's University of Exeter, tracked 18 acclimatized runners in Singapore who had ingested a telemetric sensor that tracked core body temperature.

By the race's end, the athletes lost an average of 1.6 quarts of sweat per hour, and replaced 6 percent to 73 percent of that loss. But the runner with the highest body temperature — 107 degrees — was also the runner who reported drinking the most.

"If we interpret our findings that fluid intake is not important" for cooling core temperature, Byrne says, "that goes against the conventional view."

Noakes argues that our bodies evolved to run in hot climates with little opportunity for rehydration, and that thirst works well as a gauge for every other animal on the planet.

But advocates of hydration during exercise say that Noakes' suggestions best protect just a small fraction of athletes — people doing lengthy, low-intensity exercise, such as back-of-the-pack marathoners. These people are more likely to drink up more than they sweat out.

FIND YOUR STRATEGY

Everyone has to find a strategy that works for them to ensure adequate, but not excessive, hydration.

Sports drinks have an advantage over water — they contain salts and sugars that are depleted during exercise and sweating. Some authorities caution against drinking caffeinated beverages during hot-weather exercise. Caffeine is a stimulant that increases heart rate and metabolism and was thought to crank up heat production and throw off fluid and salt balance.

But a 2006 study of 59 men by the University of Connecticut's Casa and colleagues found taking 3 or 6 milligrams of caffeine per kilogram of body weight did not raise body temperatures or affect heat tolerance.