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FRIDAY, March 28, 2014 (HealthDay News) -- It's said you can never be too rich or too thin, but new research suggests otherwise. People who are clinically underweight face an even higher risk for dying than obese individuals, the study shows.

Compared to normal-weight folks, the excessively thin have nearly twice the risk of death, researchers concluded after reviewing more than 50 prior studies.

Obesity has occupied center stage under the public health spotlight, but "we have [an] obligation to ensure that we avoid creating an epidemic of underweight adults and fetuses who are otherwise at the correct weight," said study leader Dr. Joel Ray, a physician-researcher at St. Michael's Hospital in Toronto.

The findings appear in the March 28 issue of the Journal of Epidemiology and Public Health.

Studies included in the analysis followed people for five years or more and focused on associations between BMI (body-mass index, a key indicator of healthy weight) and fatalities related to any cause.

Ray's team also looked at how death rates related to weight patterns among newborns and stillborns.

Underweight patients of all ages (those with a BMI of 18.5 or under) were found to face a 1.8 times greater risk for dying than patients with a normal BMI (between 18.5 and 25.9), the study found.

By contrast, obese patients (those with a BMI between 30 and 34.9) face a 1.2 greater risk for dying than normal-size patients. Severely obese patients -- those with a BMI of 35 or more -- faced a 1.3 times greater risk.

Ray said it's important to keep a healthy body size in mind when attempting to tackle the obesity epidemic.

"BMI reflects not only body fat, but also muscle mass. If we want to continue to use BMI in health care and public health initiatives, we must realize that a robust and healthy individual is someone who has a reasonable amount of body fat and also sufficient bone and muscle," Ray said in a hospital news release. "If our focus is more on the ills of excess body fat, then we need to replace BMI with a proper measure, like waist circumference."

Typical factors linked to a higher risk for being underweight included malnourishment, drug or alcohol use, smoking, poverty and mental health issues.

More information

For more on weight and health risks, visit the U.S. National Heart, Lung, and Blood Institute.