A new study found that women who weren’t taking birth control pills (oral contraceptives) gained 60% more muscle than those who did. The study also found that the women taking contraceptives lost slightly less fat, though those findings didn’t reach statistical significance.
Birth control pills are one of the most popular contraceptives, with one study showing that 80% of sexually active women have taken them at least once. This is something, then, that could affect most women, especially at ages when they’re most eager to build muscle.
This is the latest of several studies looking at how birth control pills affect muscle and fat gain, but the sample size was fairly large, and the findings are fairly dramatic. Let’s take a look.
Birth Control Pills May Reduce Muscle Growth
A recent study by Riechman et al. looked at the results of 72 women, 20-years-old on average, after doing 10 weeks of weight training. They did full-body workouts and trained three times per week, exactly as we recommend. The control group gained around 60% more muscle than the group taking birth control pills. (Note that when we say “muscle,” we’re talking about gains in lean mass, which can include more than just contractile muscle tissue.)
On the bright side, there weren’t any major differences in strength gains. Mind you, muscle size and muscle strength are very, very closely linked, and so those gaining more muscle will inevitably grow stronger. I suspect that the reason this study didn’t find a difference was because these women were still relatively new to lifting weights. With new lifters, differences in coordination and muscle activation can overshadow differences in muscle strength.
This is a fairly good study. The sample size was quite large (72 women), they used the gold standard of body composition measurement (hydrostatic weighing), and the effect size was fairly big. It also agrees with the tentative findings of earlier studies (study, study).
WHY Would Birth Control Pills Affect Muscle Growth?
Birth control pills produce physiological reactions similar to those produced by androgens. If your androgen receptors are busy dealing with the androgenicity of the birth control pills, that can interfere with your androgen signalling, which can hurt your ability to build muscle.
In this study, we see that the women taking birth control pills with greater androgenicity gained the least amount of muscle. The effect size is quite large, too, with the control group gaining five times as much muscle as the women taking higher-androgenicity birth control pills.
The study also found that the women taking birth control pills had higher levels of cortisol, the stress hormone. It’s conceivable that the increase in cortisol also affected muscle growth (and fat loss).
Do Birth Control Pills Affect Fat Loss?
The control group lost nearly twice as much fat as the group taking birth control pills, but this finding didn’t reach statistical significance. So it’s not unreasonable to suspect that birth control pills could make it harder to lose fat, and this study indeed shows that trend, but it may just be a coincidence.
What’s really cool about this study, though, is that these women weren’t even trying to lose fat. They were on a muscle-building program. They were gaining weight. And yet, in both groups, birth control pills or no, the women all finished stronger, leaner, and more muscular than when they started.
A high-quality new study found that women who weren’t taking birth control pills gained 60% more muscle and lost twice as much fat as the women who weren’t. The good news, though, is that after ten weeks of lifting weights, eating slightly more protein, and eating in a slight calorie surplus, both groups gained strength, built muscle, and lost fat.
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