Exercise is supposed to lift your mood, strengthen your bones and slim you down. But if you're a young woman on birth control pills, a workout can spell trouble for your bones.
That is the unexpected finding of research at Purdue University into what happens to women's bodies when they exercise moderately.
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"Birth control [pill] prevents the exercise from reaching its maximum effectiveness in protecting bone," said Connie Weaver, professor of foods and nutrition at Purdue.
But there is good news too: The correct amount of calcium in your diet will forestall any bad bone effect from oral contraceptives.
Researchers studied 180 women, between 18 and 30, for up to two years. One group did little exercise, while another group had to lift weights and engage in cardiovascular exercise for up to four hours a week.
Even though the women were paid for the study, only 55 completed it, Weaver noted. The results are in a recent issue of the journal Medicine and Science in Sports and Exercise.
The researchers were first surprised to find that many of the women were already losing bone mass - 1 percent to 2 percent a year - from their hips and spines, even at their young ages. "We don't expect premenopausal women to be losing bone at all, so it was a big shock," Weaver said.
But she said, "The exercising women had better bones than the sedentary women," unless the exercisers were on birth control. Then, for reasons that aren't clear, their bones were in worse shape than those of the couch potatoes.
"We think birth control is holding oestrogen at a low constant level, reducing the spikes in oestrogen that help bone," Weaver said.
But proper intake of calcium kept the bone problems at bay in all the women. For that reason, young women shouldn't stop going to the gym, she said.
"Don't stop exercising," she urged. "Keep on exercising for your overall health, but make sure you get enough calcium when you do."
Calcium could be just one part of the whole dietary picture, said Dr Anne Zeni Hoch, an assistant professor of physical medicine and rehabilitation at the Medical College of Wisconsin and an expert on how exercise affects the female body.
Previous studies have shown that diet in general, not just calcium, affects bone mass in female athletes, she said. The Purdue research may not have taken that fully into account, she added.
Young women "need to be consuming enough calcium, but they also need to consume enough calories," she said.
Regardless of what causes bone loss, the results could be catastrophic, she said. Losing minerals inside bone could lead to a higher risk of fractures.
What to do If you're a young female athlete in your teens or 20s, pay special attention to your diet, especially your calcium intake. You don't need to eat more calcium than other women, but it's more important that you get enough.
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