People are constantly being told to watch their weight, but perhaps the advice should be watching their muscle mass. Research recently published in The American Journal of Medicine suggests that muscle mass index serves as a better predictor of long life than weight.  Most healthcare professionals are trained to focus on body mass index (BMI), which is a system for establishing weight norms based on weight and age. However, among older adults, BMI’s association with longevity (how long a person lives) has proven inconsistent. Among older adults, BMI may not be a great predictor of long life.
Dr. Preethi Srikanthan and his research team at UCLA think that measuring muscle mass may be a better way to evaluate health than just measuring weight. Using electrical devices called impedance meters, they were able to determine the amount of muscle each person had. They did this measurement on 3,659 individuals age 55+ and followed them to measure their lives. Dr. Srikanthan and team found a definite correlation between higher muscle mass index and longer life among retirees. Obesity is not associated with increased mortality in older adults. 
Some studies have found that muscle strength is related inversely to mortality risk in older adults. 
The takeaway message may be that we should do more than watch our calories. We need to stay active in ways that will keep our muscles in good shape. Get up and start moving.
 Preethi Srikanthan, Arun S. Karlamangla. Muscle mass index as a predictor of longevity in older adults. The American Journal of Medicine. Doi: 10.1016/j.amjmed.2014.02.007
 Grabowski D.C., Ellis J.E. High body mass index does not predict mortality in older people: analysis of the Longitudinal Study of Aging. Journal of American Geriatrics Soc. 2001; 49: 968-979
 Newman A.B., Kupelian V., Visser M., et al. Strength, but not muscle mass, is associated with mortality in the health, aging, and body composition study cohort. J Gerontol A Biol Sci Med Sci. 2006; 61: 72-77