Young women seeking a sculpted, muscular silhouette may want to avoid taking oral contraceptives, U.S. researchers said on Friday.
They found women who were not taking birth control pills gained 60 percent more muscle mass after a 10-week weight training program than those who were.
The study, led by Chang-Woock Lee and Steven Riechman of Texas A&M University in College Station and Mark Newman of the University of Pittsburgh in Pennsylvania, will be presented at the American Physiological Society meeting in New Orleans this weekend.
The researchers studied 73 generally healthy women between 18 and 31 who completed a whole-body resistance exercise training program. About half took the pill and half did not.
The women were encouraged to eat at least half a gram of protein per pound of body weight each day — about a third more than recommended by U.S. nutritional guidelines — to ensure they got enough protein and calories to build muscle.
Both groups exercised three times a week under the supervision of exercise physiologists, performing the same number and intensity of exercises.
At the end of the 10 weeks, the women who were not taking oral contraceptives had built significantly more lean muscle.
And blood samples before and after the training period showed the women on the pill had lower levels of muscle-building hormones such as testosterone and far higher levels of muscle-breaking hormones such as cortisol.
In a statement the team said that oral contraceptives appear to have a negative effect on women's ability to build lean muscle, but they added that future studies were needed to explain why.