Doctors routinely measure a patient's body mass index, or BMI. And if that weight-to-height ratio points to obesity, the doc might prescribe exercise, to shed the extra pounds. But when it comes to longevity, a focus on weight loss may be misplaced. Because BMI isn't actually a very reliable indicator of life span. A more useful measure, some physicians say, might be muscle mass.

Researchers analyzed BMI and muscle mass data from more than 3,600 seniors in a long-term study. And they tracked which seniors had died, a decade later. Turns out BMI wasn't much good at predicting chance of death.
 
But muscle mass was: more muscle meant better odds of survival. The study appears in The American Journal of Medicine. [Preethi Srikanthan and Arun S. Karlamangla, Muscle Mass Index as a Predictor of Longevity in Older-Adults]
 
There's no cause-and-effect here—just correlation for now. But study author Preethi Srikanthan, of U.C.L.A., has this recommendation: "Get up and start moving. Focus on trying to maintain the maximum amount of resistance training that you can, and stop worrying so much about dropping calories." Which could take a little weight off your mind, too.

—Christopher Intagliata

[The above text is a transcript of this podcast.]