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Researchers are finding that the differences between the sexes go beyond muscle mass and testosterone when it comes to physical activity. Men and women also differ in energy metabolism, lung function and other factors during exercise.
By MSNBC contributor
updated 4/29/2008 8:48:56 AM ET 2008-04-29T12:48:56

At the summer Olympics in Beijing, world records likely will be broken, inevitably leading to much speculation about the limits on athletic achievement. Just how much farther can the human body go without steroids or other performance-enhancing substances? And will women ever be able to perform at the level of men — or beyond?

While women have made tremendous gains in athletic performance, experts generally agree that the world's top female athletes will probably never outperform their male counterparts. Men are bigger and have more muscle and higher levels of that powerhouse hormone testosterone than women. "I don't think they really are ever going to catch up," says Danielle Day, an assistant professor of exercise and sport performance at the University of New England in Biddeford, Maine, who follows the topic.

But as researchers have begun to unravel the differences between the sexes with regard to physical activity, they're learning that there's more to the story. Men and women also differ in energy metabolism, lung function and other factors during exercise.

And one day, researchers say, their findings may pave the way to sex-specific training regimens for athletes and recreational exercisers as well.

"We're not the same. Our physiological systems are not identical," says Earl Noble, director of the school of kinesiology at the University of Western Ontario in London, Ontario, Canada. He likens exercise to a drug "that needs to be given in the appropriate dose and intensity and should be varied for sex differences."

Researchers are just beginning to sort out these differences. Until about 15 years ago, most exercise studies involved men, and researchers assumed that what happened in men happened in women, too, says Day, who along with Noble and other researchers authored a series of articles on the subject in the April issue of Medicine & Science in Sports & Exercise, a journal published by the American College of Sports Medicine.

Since then, many more studies have focused on sex differences in exercise, says Day, and the research has turned up some surprising results with potentially important implications.

'Tough pill to swallow'
One real-world application of such research may be the development of workouts that optimize weight-loss for women and men.

Research confirms what frustrated women have long known to be true — that females have a harder time losing weight through exercise than men, says Day. Men's bodies respond more favorably to exercise, whereas women's bodies go into survival mode, slowing metabolism and hanging onto fat. She and other experts say that's an evolutionary response intended to keep a woman healthy for child-bearing.

"Exercise is similar to short-term starvation," says Dr. Mark Tarnopolsky, a professor of medicine at McMaster University in Hamilton, Ontario, Canada. From an evolutionary perspective, he says, women need to withstand starvation to survive and have babies.

Interestingly, women, who naturally have higher body fat, actually burn more fat during longer periods of exercise than men, says Tarnopolsky. "We think that the female hormone estrogen is a major player here in determining what fuels the body uses during exercise," he says. When researchers have given estrogen to men, they've found that men burn more fat during exercise, too.

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The research counters historical beliefs that exercise, particularly endurance exercise, is bad for women, says Tarnopolsky, noting that it wasn't until 1984 that women were allowed to compete in the Olympic marathon. Women actually seem well-suited to it because they burn more fat, which is a longer-lasting fuel source than carbohydrates, he says.

The findings also indicate that "carbo-loading," the practice of consuming more carbohydrates in preparation for a race, may not be as important for women, since they rely less on carbs than men, he says.

But unfortunately for women wanting to slim down, the findings do not mean that women lose more weight than men just because they burn proportionately more fat, he says. "It's not so much what you're burning acutely; it's what's the balance [of calories in versus calories out] over 24 hours. It's a tough pill to swallow."

Women may actually need to do more exercise than men to lose weight, says Day. "Women are probably going to have to work a lot harder and incorporate interval training to challenge them more."Another way to kick things into high gear is to cross-train, meaning incorporating a variety of activities — such as hiking, swimming, spinning, hip-hop dancing and a high-impact aerobics class — into the mix to keep the body challenged.

Men, on the other hand, may find weight-loss success with a simpler and even a less-intense approach.

Matters of the heart
When it comes to matters of the heart, women have the smaller organ, which can lead to less cardiac output during exercise. But there are other factors at play.

Estrogen in women helps protect against heart attacks before menopause. However, when women have heart attacks, the events are generally more severe than in men, causing more tissue damage to the heart, says Noble.

Animal research by Noble and others may help explain why. Studies show that estrogen can counteract the effects of a protein called Hsp70 that is produced in response to fairly intense exercise (such as jogging or playing squash) and helps protect the heart against tissue damage. But the protein does not have its effect in the presence of high levels of estrogen.

"Males who exercise have a significant protection against cardiovascular-related issues," says Noble.

Because a premenopausal woman's estrogen levels fluctuate during the month, she may derive the most benefit from this protein if she exercises when her estrogen levels dip, explains Noble. The protein's protection appears to last about three to four days. Getting regular exercise on all or most days of the week may help ensure a woman harnesses the protection.

Another sex difference is lung capacity. It's long been known that women have smaller lungs than men, but researchers have only recently discovered that this can limit exercise tolerance in women, says Craig Harms, an associate professor of kinesiology at Kansas State University in Manhattan, Kan.

In other words, he says, "they're going to notice they're breathing a whole lot more" than men for a comparable amount of activity. So a woman may be working just as hard as a man, but based on how hard she's panting, it's going to feel harder to her. This could make exercising in the heat or humidity, for instance, or at high altitudes more tolerable for men than women.

As researchers learn more, they envision a day when people will be given an "exercise prescription" based on their age, sex, overall health and fitness level.

But for now, the main thing is for both sexes to keep moving.

"Any kind of exercise is beneficial," says Noble, "and inactivity is a risk."

© 2013 msnbc.com

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