For years, scientists have disagreed about what causes a healthy, well-trained athlete to pass out suddenly from heat stroke during exercise on a hot day. But recent data show that the cause is a sudden rise in body temperature and not severe dehydration.
Almost every time an athlete collapses suddenly in an endurance event, he or she is near the end of the competition, or has just stopped. Sudden collapse rarely occurs during competition. This fact supports the argument for a sudden rise in body temperature:
When you exercise, your heart pumps tremendous amounts of blood from your hot, exercising muscles to your skin, where the heat can be dissipated. When you exercise vigorously, your heart beats near its maximum rate, somewhere around 200 beats per minute. Immediately after a well-conditioned athlete stops exercising, his or her heart rate may slow to 120 beats. One minute later, it has usually dropped below 90. Since the athlete's heart is beating less often, less blood is being pumped to the skin. There is less cooling; subsequently, body temperature rises.
During exercise, an athlete's body temperature can rise safely to around 104 degrees, but after exercise, the temperature can rise to more than 105 degrees. This increase can "cook" the athlete's brain, leading to a loss of consciousness.
Heat stroke is treated by immediately pouring any available liquid over the victim so evaporation can cool the body.
But heat stroke should never happen; you can tell when it's coming. First, your muscles burn and hurt. Then you start to have difficulty breathing; each breath burns your lungs. By the time your brain is affected -- you may feel dizzy, your ears may ring, your head may ache, your vision may be blurred and you may have a feeling of impending gloom -- it may be too late.
Q: Is it safe for a woman to exercise during pregnancy?
A: Although all the evidence isn't in yet, the current recommendation is that exercising during pregnancy appears to much safer than previously believed.
In the past, doctors had two major concerns about exercise and pregnancy: that a woman could exercise so vigorously that her blood levels of oxygen would drop and the unborn baby's brain would be damaged from a lack of oxygen; and that her temperature would rise high enough to damage the baby's brain.
But there is no evidence that a mother's hard exercise will damage an unborn baby's brain from lack of oxygen. A study with sheep showed that severe, forced exercise caused no fetal brain damage.
And while there are studies showing that high fever during the first month of pregnancy can cause brain and spinal cord damage in the fetus, these studies were reported on pregnant women whose high fevers were caused by infections.
It's true that after 10 minutes of fast running on a warm day, a woman's temperature may rise to more than 101 degrees. (Her temperature can rise above 101 degrees after 12 minutes in a hot tub.) But a recent study from the University of Vermont showed that pregnancy, by itself, helps keep women's temperatures from rising too high: At the same level of exercise intensity, women's temperatures were significantly lower during pregnancy than they were before pregnancy.
Q: I know President Bush won't eat broccoli, but I love it and know it's good for you. Will microwaving broccoli destroy its benefits?
A: Researchers at Johns Hopkins have found that broccoli contains a chemical -- sulforaphane -- that may be the most powerful chemical yet to stimulate both animal and human cells to produce cancer-fighting enzymes. And the chemical is not destroyed by microwaves.
The research confirms earlier data showing that people who eat cruciferous vegetables -- broccoli, kale, kohlrabi, brussels sprouts, cauliflower and cabbage -- have a lower incidence of cancer.
Vegetables are loaded with chemicals that stimulate your body to produce a vast array of enzymes, some of which are good for you and others that can be harmful. Phase I enzymes change harmless substances into oxidants that can damage the genetic material in cells, thereby leading to cancer. Phase II enzymes prevent oxidants from damaging genetic material, thus preventing cancer. Phase II protective enzymes are found in cruciferous vegetables.
In the future, scientists may be able to isolate and purify sulforaphane so it can be taken in pill form. To get it now, though, you -- and the president -- need to eat cruciferous vegetables.
Dr. Mirkin is a practicing physician in Silver Spring specializing in sports medicine and nutrition.