Exercising al fresco is one of the great pleasures of summertime. But heat waves and humidity can turn a refreshing long run into a sweat-drenched experiment in heat exhaustion.
Overheating 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 pushing and internal temperatures pass 104 degrees, the athlete risks "organ failure and death from heat stroke," says Dr. Aurelia Nattiv, professor in UCLA's Department of Family Medicine, Division of Sports Medicine.
Scientists are learning more about the factors that influence overheating - and ways to help the athlete avoid it. Just how hot and bothered you get on the inside depends on a number of factors: body size, fitness level, intensity of exercise, the heat and humidity of the environment, and how acclimated you are to exercising in hot weather.
For 46-year-old Laura Garcia, a legal secretary and avid runner, the worst overheating experience of her life came during the 2004 L.A. Marathon. Temperatures were in the 90s. Scores of runners ended up in medical tents. "It was unbelievably difficult," she says. "I could feel my muscles start to seize."
Working out uses energy we derive ultimately from food that we eat. A mere 25 percent of that energy ever leverages muscle force. The rest goes to waste - as heat.
Fortunately, the human body comes equipped with heat-loss mechanisms. As core temperatures rise, sweat glands pump water through the skin. It evaporates into the air, taking a thwack of body heat with it.
Sweating's not the only way we have to cool down. Higher body temperatures cause the heart to pump more blood to the skin. Skin blood vessels dilate, exporting more heat.
When the temperature passes 100 degrees, we actually begin to absorb heat from the environment - that's on top of the heat we're absorbing from the sun.
Humidity adds an extra whammy. If the surrounding air is heavy with water, sweat cannot evaporate off the skin.
Train for the heat
There are factors the athlete can more readily control. Among the most important: Take time to acclimate to the heat.
"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 season 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. Most heat illness cases occur in people not used to working out in the heat, Casa says - such as in the first really hot days of summer.
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.
The American College of Sports Medicine recommends athletes ensure they are hydrated before exercise, keep hydrating during exercise and rehydrate after. 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.
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."
But 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 should not drink beyond thirst because they may develop hyponatremia, a dangerous dilution of body salts that killed a 2002 Boston marathon runner.
But advocates of hydration during exercise say that Noakes' suggestions 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 more than they sweat out, whereas most people working out in hot weather are in little danger of drinking too much.
Everyone has to find a strategy that works for them to ensure adequate, but not excessive, hydration.
Make sure your fluid is cold. A team led by David Jones, professor of sport and exercise sciences at the University of Birmingham, had eight men cycle to exhaustion in 93-degree heat. They found those who drank cold fluids biked seven minutes longer than those given warm drinks.
Paradoxically, after exercise, drinking cold water might be worse for hydration. "It satiates you more so you drink less," says co-author Toby Mundel. Drink room-temperature liquid.
Sports drinks have an advantage over water - they contain salts and sugars that are depleted by exercise and sweating.
Anna Gosline is a freelance writer, and Jeannine Stein is a staff writer for the Los Angeles Times.
HANDLING THE HEAT
Warm-up. A few minutes of warm-up prepares the body for exercise, literally heating up the body's inner temperature.
Stretching. Do this after warm-up, because stretching cold, stiff muscles can cause injuries. Don't force stretches.
Cool-down. After your cardio, it's important to allow the body to cool down, even if it wasn't a particularly sweaty workout. This allows the body's core temperature and heart rate to return to normal.
Find some shade. If it's a hot, sunny day, head for a cool room or the shade. Use an ice pack or take a cool shower or bath. Make sure you're drinking enough water to fend off dehydration.