WOMEN'S HEALTH
May 7, 2009

The Pill and Muscle Mass

Women who use birth control pills have a more difficult time building muscle mass than those who do not.

If you're a woman who isn't seeing the kind of strength gain you'd expected from an exercise program, a recent study suggests your birth control pill may be partly to blame.

Researchers from Texas A&M University found that a 10−week resistance exercise program was significantly less likely to improve lean muscle mass in young women who were taking oral contraceptives than in those who were not.

Elite athletes, who must add muscle mass to incrementally increase strength, would be more likely to see performance benefits from going off birth control pills.

Of 73 female recreational athletes between the ages of 18 and 31, the 34 who were taking birth control pills improved their lean muscle mass by 2.1%, while the 39 who were not taking oral contraceptives improved by 3.5%. The findings were presented last month at the annual meeting of the American Physiological Society.

More detailed analysis suggested that the effect of an oral contraceptive on muscle gain is determined by the action of the hormone progestin in the drug. Progestin has the ability to block the effects of testosterone and the degree to which the progestin in the drug does this is known as its androgenicity. Testosterone, which promotes muscle development and strength, is produced by the ovaries in women, though in much smaller amounts than are produced by the testes in men. Oral contraceptives with low androgenicity for progestin were associated with a 2.5% gain in lean muscle mass, versus just 0.5% for pills with moderate or high androgenicity.

"This effect makes sense biologically," said Steven E. Riechman, PhD, an assistant professor of kinesiology at Texas A&M and lead author of the study. "Progestin is really the hormone of pregnancy, where everything's focused on the developing fetus. Of course you don't want to be building muscle when the fetus is the priority for nutrients and growth."

The researchers found that oral contraceptive use did not significantly affect strength gains during the 10−week program, which is indicative of the fact that neurological factors as well as hormonal factors affect strength gain in non−elite athletes, Riechman said. Elite athletes, who must add muscle mass to incrementally increase strength, would be more likely to see performance benefits from going off birth control pills.

But oral contraceptive use could also affect recreational athletes' ability to improve strength in the long term, Riechman said. "I don't think it's a major concern for people pursuing strength training just for fitness. But it's going to be a little harder for them," he said. "They're going to find that the effort they're putting into training is not paying off as much as they would like."

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