In older women, it’s not the excess weight that’s deadly, but where those extra pounds collect that can shorten life, a new study reports.

Among women, 70 to 79, being overweight or obese didn’t appear to cut years off life — unless the weight was centered around the waist. But being underweight also appeared to shorten life span, researchers found.

“Abdominal fat is more deadly than carrying excess weight,” said lead researcher Zhao Chen. She’s chair of the University of Arizona’s department of epidemiology and biostatistics in the College of Public Health.

While the study found that a large waist circumference is detrimental, Hispanic women were somewhat protected — they had lower mortality rates at any waist measurement or BMI level than white or black women.

Chen added, “An older woman should be concerned when her body weight is below normal for her height, and less concerned when she is slightly heavier than normal.”

The researchers found that the risk of mortality increased when waist circumference measured more than 31.5 inches (80 centimeters), and they classified anything above nearly 35 inches (88 centimeters) as an “extreme risk.”

The study looked at weight by using body mass index (BMI) measurements. BMI is a rough estimate of a person’s body fat based on height and weight measurements. A BMI of 18.5 to 24.9 is considered normal weight. Below 18.5 is underweight, while 25 to 29.9 is considered overweight.

Obesity is a BMI of 30 or more. But obesity can also be broken into three classes, as was done in this study. Class I or “slight” obesity is a BMI of 30 to 34.9. Class II is 35 to 39.9, and class III is a BMI of 40 or above.

Although being overweight is often considered generally bad for your health, how bad may depend on your age, race and ethnic background, Chen said.

“Some of these differences may be related to the fact that body weight does not capture mortality risk, as do measures such as waist circumference, which we find is consistently associated with higher mortality,” Chen said.

In general, these findings suggest that being underweight is more detrimental in older women, and being slightly heavier in later life could be beneficial, she said.

Body weight can reflect several different aspects of body composition, each reflecting health and disease in its own way, Chen said.

“One’s weight should be interpreted with caution with respect to age, ethnicity, race and other measures of health, such as waist circumference,” she said.

For the study, Chen and her colleagues reviewed data on nearly 162,000 women aged 50 to 79 who took part in the Woman’s Health Initiative, a major study on postmenopausal women by the U.S. National Institutes of Health.

At the start of the study, the researchers measured the height, weight and waist size of all the participants and recorded other lifestyle data.

During 11 years of follow-up, more than 18,000 women died.

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