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

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.

MOTS-c didn’t make mice move more, so its effects on weight and metabolism aren’t just because they were exercising more.