It’s not in your head: Building muscle and sculpting that toned look is harder with each passing year. Beginning in your 30s, your body starts to lose 3-5% of its muscle mass each decade, as the process of building new muscle (muscle protein synthesis slows). Not to mention, many women enter peri-menopause in their late 30s or early 40s, and a decline in estrogen1 can negatively affect body composition, whereas an increase in cortisol can lead to fat storage.
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