Young women who use oral contraceptives (OC) may not get as much out of their weight-lifting routine as women who are not on the pill, according to a study released today, which suggests that OC use impairs muscle gains from resistance exercise training in women.

"The factors that explain the differences in the magnitude of the responses to resistance exercise training between individuals are largely unknown," Chang-Woock Lee, from Texas A&M University in College Station stated.

"The present study is meaningful in that we have identified a potential new factor that may be independently associated with the characteristics and variability of muscle responses to a controlled resistance exercise training program," the researcher added.

In the study, 73 generally healthy women between 18 and 31 years old participated in whole-body resistance exercises three times per week for 10 weeks. Thirty-four of the women used oral contraceptives and 39 did not. The women were encouraged to eat enough protein to promote muscle growth.

According to the researchers, there were marked differences in lean muscle mass gains between the two groups. Lean muscle increased by just 2.1 percent in OC users compared with 3.5 percent in non-OC users.

Lee and colleagues presented their findings Friday at the American Physiological Society meeting, part of the Experimental Biology 2009 scientific conference underway in New Orleans.

In a prepared statement from the meeting, the researchers acknowledge being "surprised at the magnitude of differences in muscle gains between the two groups, with the non-OC women gaining more than 60 percent greater muscle mass than their OC counterpart."

Other muscle responses, such as strength gains and arm/leg circumferences, were similar between the OC and non-OC users. 

Tests on the women also showed that blood levels of three muscle-building hormones were significantly lower and one muscle-breaking hormone was significantly higher in OC users than non-OC users, Lee said. These findings "could help explain" why OC users showed diminished muscle gains from resistance exercise training.

Summing up, Lee said: "Numerous health and performance benefits including improved exercise/athletic performance, body composition, esthetic beauty, and self-image can be attained from the increased muscle mass and strength associated with resistance exercise training. OC users may not be able to fully enjoy those benefits while experiencing impaired exercise performance and difficulties achieving athletic goals due to diminished muscle responses they get from resistance exercise training.