FRIDAY, April 17 (HealthDay News) -- Lower hormone levels in women who exercise regularly and take birth-control pills may result in less muscle mass increases, a new study suggests.

But the muscle-mass differences between women taking "the pill" and those not taking the pill did not affect performance.

At this point, the finding, from a study scheduled for presentation Friday at the annual meeting of the American Physiological Society in New Orleans, is no reason to stop taking oral contraceptives, experts said.

"It is premature to say anything conclusively at this point," said study lead author Chang Woock Lee, a doctoral candidate in the department of health and kinesiology at Texas A&M University, College Station. "Vigorous future studies with more stringent control and clever design will be definitely needed to confirm the results and/or elucidate the underlying mechanism conclusively."

Others agreed.

"It's just one small, a very small, group of women. If you think of how many women actually are on the pill, how significant is the difference in terms of patients noticing anything or even an actual health effect?" said Dr. Jennifer Wu, an obstetrician/gynecologist at Lenox Hill Hospital in New York City. "This might make a difference for a high-performance athlete in a competition, but, for your normal patients who have a healthy exercise routine, this might not make a difference."

"I don't disagree with the statistical significance, but the clinical significance is very questionable," added Dr. Amanda Weiss-Kelly, director of pediatric sports medicine at Rainbow Babies & Children's Hospital, part of University Hospitals Case Medical Center in Cleveland. "The difference didn't translate into improvement in performance," she added.

There is some existing research into the effect of oral contraceptives on body composition, Weiss-Kelly said, but most studies have been small, and results have been conflicting.

The authors of the new research studied two groups of 18- to 31-year-old women: 34 who were on the pill and 39 not on the pill. All were active and healthy and took part in a 10-week resistance-exercising training program (three times a week) as well as analysis of their body composition both before and after the program.

Women not taking oral contraceptives gained more than 60 percent more muscle mass than those on the pill.

There were other changes noted in participants on the pill, including reduced concentrations of the hormone DHEA, which Lee explained, is an anabolic hormone and therefore builds muscle.

But some of these changes would be expected in women taking contraceptives, Weiss-Kelly said.

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