Skeletal muscle is a metabolically active tissue that lives in a constant balance between protein synthesis and protein breakdown. In adults, this balance tilts slowly in favor of breakdown starting in the 30s: research summarized by the National Institute on Aging estimates adults lose roughly 3 to 8 percent of muscle mass per decade after age 30, with faster losses after 60.
Anabolic resistance is a second driver. As you age, your muscles become less responsive to the protein you eat and to a normal training stimulus, so the same lunch or the same workout builds less muscle at 55 than it did at 25. Layered on top are disuse (post-illness, post-surgery, long winters spent indoors) and hormonal shifts in estrogen, testosterone, and growth hormone that reduce the body's ability to rebuild tissue.
A newer contributor is medication-driven lean mass loss. GLP-1 receptor agonists like semaglutide and tirzepatide produce significant total weight loss, but a meaningful fraction of that weight can come from muscle unless the patient is actively resistance training and eating enough protein. Our medical weight loss program is built around protecting lean mass while you lose fat.
