As mice get older, their bodies make less of a peptide called MOTS-c, especially in muscles and blood—and when they fast, levels drop too, hinting that this peptide responds to energy needs.

When mice fasted, MOTS-c levels dropped in muscles and blood but stayed the same in brain and heart.

Old mice that got MOTS-c injections responded to insulin just as well as young mice, meaning the peptide might fix the insulin resistance that comes with aging.

MOTS-c injections made muscle cells more responsive to insulin by activating AMPK and increasing GLUT4, which helps move glucose into cells.

MOTS-c made muscle cells produce more GLUT4 (a sugar transporter) and turned on AMPK (an energy sensor), two key players in how muscles use sugar.