Urolithin A is a natural compound formed by the bacteria in the colons of about half of elderly people after consuming certain foods. Urolithin A seems to improve the function of mitochondria (the "powerhouses" inside our cells). However, clinical research with urolithin A suggested only limited benefit in improving muscle health in middle-aged and older adults. Get the details (and also learn about the safety of urolithin A) in our full answer by signing in as a CL member.
Also see our information about other supplements, such as protein, for building and maintaining muscle (which also requires resistance exercise).