Do you like to blame your weight on being "big-boned?" If you've ever thought that the body mass index (BMI) — a ratio of weight to height often used as a yardstick of obesity — doesn't tell the true story of your relative weight and health, you may be onto something if you're elderly, UCLA researchers say.

A fundamental question is "whether or not BMI is the appropriate measure of obesity in older adults," Dr. Preethi Srikanthan and colleagues at David Geffen School of Medicine at UCLA, Los Angeles, California write in the Annals of Epidemiology.

In these individuals, the authors write, "changes in body size and composition that commonly occur with aging may limit the usefulness of BMI" for determining how much fat a person is carrying around - and also their risk of death in a given period.

The researchers looked at data from 1189 male and female subjects in their 70s over more than a decade, starting in 1988. By 2000, 492 subjects had died.

The investigators found no link between the risk of death during the study and BMI or waist size. However, the risk of death during the 12-year study was tied to the ratio of waist to hips.

Typically, that ratio is less than one, because the waist is smaller than the hips. As it grew closer to one, or increased above one, the risk of death in the study period increased.

Why was one yardstick a better predictor of death than another? "Waist-to-hip circumference ratio is different (from) waist circumference as it...also takes into account hip circumference, which includes muscle in the upper thigh, Srikanthan told Reuters Health. And a higher ratio of muscle, as opposed to fat, may suggest better health.

The link between a higher waist-to-hip ratio seemed to be stronger for women than for men.

The study may mean that "improving muscle bulk in the lower extremities may be important in moderating the health risk of abdominal fat in older individuals."