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 UCLA's Department of Family Medicine, Division of Sports Medicine.
Overheating: A June 4 Health article on ways to prevent overheating during exercise said that a study found taking 3 or 6 milligrams of caffeine daily did not raise body temperature or affect heat tolerance. The correct numbers are 3 or 6 milligrams of caffeine per kilogram of body weight. The same article said that in another study, dehydrated cyclers lasted an average of 13.6 minutes, and hydrated cyclists 19.6 minutes, in a cycle-to-exhaustion test. The correct numbers are 13.9 minutes and 19.5 minutes. —
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 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 to indulge in on race day.
Follow the advice on these pages and those canyon runs can still be a pleasant — if unavoidably sweaty — part of summer.
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. It was the second of seven that she's run. 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."
She took advantage of spectators who were cooling people down with their garden hoses. It did make her feel cooler. But at mile 18 or 19, she says, "I was done. I could just feel that overwhelming heat, like I was going to fall over. I really scaled back. I walked, I sipped water consistently, and drank Gatorade . Around mile 22 or 23 I thought, 'I don't think I'm going to make it.' " (She did.) "When you see people around you dropping like flies, it's scary."
Working out uses energy we derive ultimately from food that we eat. A mere 25% of that energy ever 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. 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.
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 — that's on top of the heat we're absorbing directly from the sun.
Humidity (a problem occasionally in L.A. and routinely elsewhere in the U.S.) adds an extra whammy. If the surrounding air is heavy with water, sweat cannot evaporate off the skin.
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. Thus, lighter runners can run faster or farther before reaching exhausting core heats.
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 also start sweating at a lower core body temperature — and show an increased sweat rate too. So if you can't be small, be fit. And while you're at it, shed excess body fat, which strikes a double blow against heat tolerance. It adds more weight to move and insulates — like any clothing that's bulky or doesn't breathe — making it harder for heat to escape.
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 in to 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.
"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.
Heat acclimation is quickly lost. One week without activity in hot weather can strip away those hard-won adaptations.
There are practical lessons here. Ian Murray, head coach of L.A. Tri Club, which provides services and support for L.A.-area triathletes, advises people 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.
Garcia learned that lesson after her 2004 marathon experience. "I used to run in the morning, or after work at night," she says. "Now on the weekends I'll go out at noon and do 10 miles. I know it's going to be harder."
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.
Dehydration can cut performance, speed and the time to exhaustion. A study published in February found that men who were dehydrated by a two-hour ergometer ride (losing an average of 2.5% body weight in fluid) performed poorer than controls (who drank enough water to maintain body weight) on a subsequent cycle-to-exhaustion test. The four dehydrated cyclers lasted an average of 13.6 minutes, the hydrated men 19.6.
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.
It also stresses that relying on thirst will not do the trick, because thirst kicks in only when 1% to 2% of body weight is lost. Above that, many people find they aren't really thirsty enough to fully replace fluids they sweat — and, indeed, many studies report that athletes fail to do so.
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 can also 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.
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 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% to 73% 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, 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 icy 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. 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 coauthor Toby Mündel. Drink room temperature liquid.
Sports drinks have an advantage over water — they contain salts and sugars that are depleted by 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 recent studies have found caffeine to be safe in the heat. A 2006 study of 59 men by University of Connecticut's Casa and his colleagues found that taking 3 or 6 milligrams of caffeine daily did not raise body temperatures or affect heat tolerance.
One last tip: Consider cooling your body before exercise. This might be a tad too much effort for the average recreational athlete — but a lowered core temperature will keep you cooler longer and may improve endurance. (But bear in mind that this might stiffen muscles.) A 1999 study by Marino found that among seven trained cyclers, pre-cooling in a cold-water bath allowed them to bike for almost a kilometer more under hot, humid conditions.
Exercising in the heat will always be less comfortable than working out in milder temperatures. It also takes more preparation. You've got to ensure that you're well hydrated before exercise and fully hydrated after. You have to check the weather and the humidity and try to exercise at milder times of day — and keep your pace slower and work out for shorter stints at summer's start, when you're still used to the balminess of spring.
"The key," Nattiv says, "is planning ahead and educating athletes. The majority of heat illnesses are preventable."
Anna Gosline is a freelance writer and Jeannine Stein is a Times staff writer.
Set a routine
An ideal cardiovascular workout should consist of warm-up, stretching, exercise and a cool-down. Each phase is important. Make sure to schedule all phases into your workout, shortening your cardio if necessary.
Warm-up. A few minutes of warm-up prepares the body for exercise, literally heating up the body's inner temperature. Light cardio — slow jogging, walking, leisurely bicycling — raises the heart rate, pumping blood to the muscles.
Stretching. Do this after warm-up, because stretching cold, stiff muscles can cause injuries. Don't force stretches. Do them gently, focusing on ones that will be used during exercise.
Cool-down. After your cardio, it's important to allow the body to cool down, even if it wasn't a particularly sweaty workout. (How hot you'll get depends on factors such as the weather and workout intensity and duration.) This stage allows the body's core temperature and heart rate to return to normal.
Begin by slowing the tempo of your activity. If coming off a run, return to a slow jog, then a walk. If swimming, continue at a more relaxed pace, or simply move around in the water. The cool-down activity can differ from the cardio one. After running on a treadmill, it's OK to hop over to a stationary bike and pedal slowly. While the muscles are still warm you can also do some stretching. Stretch while moving, not standing still.
Don't stop completely. This can cause blood to pool in the legs, leaving less blood to circulate to the brain, causing dizziness.
If you're experiencing extreme fatigue and must lie down, make sure your legs are elevated and moving. This will help maintain cardiac function and blood pressure.
If it's a hot, sunny day, head for a cool room or some shade. Use an ice pack, take a cool shower or bath, or plunge into the ocean or pool to cool core temperatures. Make sure you're drinking enough water to fend off dehydration.
— Jeannine Stein