The study further finds that use of oral contraceptives (OC) is also related to lower hormone levels in women.

The study was conducted by lead author Chang-Woock Lee and Steven E. Riechman of Department of Health and Kinesiology, Texas A&M University, College Station, TX; and Mark A. Newman, Human Energy Research Laboratory, University of Pittsburgh, Pittsburgh, PA.

The findings of the study are based on 73 healthy women aged between 18 years and 31 years who took part in whole-body resistance exercises thrice a week for a period of 10 weeks. All of them were asked to include enough protein in their diet to help them in muscle development.

Out of 73 women, thirty-four were on birth control pills while the remaining thirty-nine did not use any pills. The researchers noted a significant difference in the muscle growth between the two groups. In women using OC, the muscle growth was just 2.1 percent as compared to 3.5 percent growth of muscle mass in non-OC users.

Chang-Woock Lee, the lead author of the study, said, “The factors that explain the differences in the magnitude of the responses to resistance exercise training between individuals are largely unknown.”

He further said, “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.”

“We were 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,” revealed the researchers.

Tests also revealed remarkably lower blood levels of three muscle-building hormones but higher blood level of one muscle-breaking hormone in OC group as compared to the non-OC group, revealed Lee.

However, the researchers noted a similarity in the arm/leg circumferences and strength gain in the two groups.

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.”

However, the experts do not, in any way, find any reason for women to stop taking birth control pills. “It is premature to say anything conclusively at this point,” affirmed Lee.

Lee added that “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.”

Dr. Jennifer Wu, an obstetrician and a gynecologist at Lenox Hill Hospital in New York City, quipped, “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?”

“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,” Wu added.

The findings of the study were presented Friday at the 122nd annual meeting of the American Physiological Society, which is also participating in the scientific conference Experimental Biology 2009 being held in New Orleans.