In Detail
The largest driver of age-related metabolic slowing is loss of muscle mass. Muscle is metabolically expensive tissue — it burns calories at rest, supports insulin sensitivity, and provides the substrate that makes exercise effective. From roughly age 30 onward, adults lose 3 to 8% of muscle mass per decade unless they actively work against it.
Hormonal changes compound the effect. Declining testosterone and estrogen reduce the body's drive to build and maintain muscle. Subtle thyroid shifts and rising insulin resistance change how energy is stored. Sleep deteriorates, which raises cortisol and disrupts appetite regulation.
The good news: most of these factors are modifiable. Resistance training, adequate protein, sleep, and where appropriate, hormone optimization can substantially preserve metabolic function.