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A recent study included 73 women aged between 18 and 31 who were divided into two groups - one taking oral contraceptives and one not taking them.
Each group completed 10 weeks of resistance training, supervised by exercise physiologists.
Before and after training, blood samples were taken to determine levels of anabolic and catabolic hormone levels.
Anabolic hormones influence muscle growth, while catabolic hormones, such as cortisol, can destroy muscle fibres. Women not taking birth control pills added more than 60% more muscle mass than women who were taking the drugs.
Also, fasting levels of anabolic hormones were lower and plasma concentrations of cortisol were higher in the contraceptive group.
However, gains in strength and arm and leg circumferences were about the same in both groups. The study was presented recently at the annual meeting of the American Physiological Society.