Wednesday, October 20, 2010

Do Women Sweat Differently Than Men?

Does a hard-working woman sweat like a man? That question, which has figured in many an antiperspirant commercial, has received surprisingly little scientific scrutiny. But a new study published this month in the journal Experimental Physiologyprovides persuasive evidence that women do sweat copiously during exercise. (Did anyone really believe they misted?) But fit women seem to sweat differently than unfit people of either sex, and quite differently than fit men, a fact that has implications for sports performance. It also may have some bearing on what it has meant, since prehistory, to be female.
For the study, researchers at Osaka International University and Kobe University in Japan recruited a pool of trained athletes, male and female, as well as an age- and gender-matched group of untrained volunteers. The men and women in the two groups were similar in terms of weight. All of the volunteers rode stationary bikes in a physiology lab heated to a balmy 86 degrees. The beginning of the hourlong session was

leisurely; the pedaling intensity was only about 30 percent of each volunteer’s maximal oxygen capacity (VO2max). Then it became tougher, rising to 50 percent of each rider’s VO2 max, while the final 20 minutes required that the riders work at a strenuous 65 percent of his or her VO2 max. Throughout, researchers monitored how much perspiration the cyclists were producing on their foreheads, chests, backs, forearms and thighs. The researchers also determined how many sweat glands were active during each rider’s session. A person’s overall sweat rate depends on how many sweat glands his or her body activates and how much sweat is excreted at each gland.
What the researchers found was that the fit men, unsurprisingly, perspired the most, significantly more than the fit women, especially during the more intense exercise. But the athletic men weren’t using more sweat glands. The fit women had just as many glands active and pumping; they produced less sweat from each gland. Meanwhile, the unfit women, by a wide margin, perspired the least, especially during the strenuous cycling, and became physiologically hotter — their core temperatures rising notably — before they began to sweat at full capacity. These results, the scientists concluded, “revealed a sex difference in the effects of physical training on the sweating response” and, just as important, “a sex difference” in “the control of sweating rate to an increase in exercise intensity.” In other words, the women, whether fit or not, were less adept of ridding themselves of body heat by drenching themselves in sweat.
Sweating is one of those bodily functions that seem uncomplicated, but it remains not fully understood by physiologists, at least in terms of how (and why) gender affects the process. “We know that fitness changes the sweating response,” said Timothy Cable, Ph.D., a professor of exercise physiology at John Moores University in Liverpool, England, who has extensively studied female athletes and how they perspire. As someone becomes more fit, his or her body begins to sweat at a lower body temperature. This is important, because “the body has a critical core temperature,” Dr. Cable said, which occurs at about 104 degrees, after which the brain simply “shuts down the motor cortex.” Unbidden, your legs stop churning and you curl up on the sidewalk until your core temperature drops (or a kind passerby calls 911). Sweating delays the onset of this critical heat buildup by dissipating the excess heat through evaporation. If you start to sweat at a lower temperature and increase your sweating rate as you get hotter during hard exercise, you’re less likely to reach the critical temperature.
Scientists learned all that long ago. But they have yet to explain why women, even the fittest, don’t sweat as much as men, especially when they exercise at a high intensity. One answer, not yet definitively proved, seems to be “testosterone,” Dr. Cable said. Earlier studies suggest that the sweat rates of prepubertal boys and girls are roughly the same. Things begin to change when the sex hormones begin to circulate. Even then, men’s and women’s perspiration is similar in terms of water, salt content and smell, Dr. Cable said, strange as that may seem to anyone who’s been in close proximity to a young male athlete’s laundry hamper. (Sweat itself is odorless; the distinctive smell associated with sweating is produced by the waste products of bacteria feeding on the perspiration.) In an experiment conducted decades ago, male athletes were injected with estrogen and sweated less during subsequent exercise.
But does this difference in sweating rates, whatever its cause, have practical implications? “It appears,” said Yoshimitsu Inoue, Ph.D., a professor of physiology at Osaka International University and one of the authors of the study, “that women are at a disadvantage when they need to sweat a lot during exercise in hot conditions.” On the other hand, it may be that women, during evolution, had the good sense to get out of the hot sun, and their bodies adapted accordingly. The “lower sweat loss in women may be an adaptation strategy that attaches great importance” to preserving body fluids “for survival,” he wrote in an e-mail, while “the higher sweat rate of men may be an adaptation strategy for greater efficiency of action or labor.”
Dr. Cable agreed. “Prehistoric men followed the herds,” he said, whatever the temperature, while the women, cleverly, sought out the shade. “It’s not a bad survival strategy,” he said, even today.

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