Being underweight puts people at highest risk of dying, just as obesity does, new research has found. The connection between being underweight and the higher risk of dying is true for both adults and fetuses（胎儿）. This is so even when factors such as smoking, alcohol use or lung disease are considered, or adults with a chronic or terminal illness are excluded, the study found.

The study, led by Dr. Joel Ray, a physician-researcher at St. Michael's Hospital and the hospital's Li Ka Shing Knowledge Institute, was published today in the Journal of Epidemiology and Community Health.

Dr. Ray's meta-analysis looked at 51 studies on the links between BMI and deaths from any cause, plus data on newborn weight and stillbirths in Ontario.

He found that adults who are underweight -- with a BMI under 18.5 or less -- have a 1.8 times higher risk of dying than those with a "normal" BMI of 18.5 to 24.9.

The risk of dying is 1.2 times higher for people who are obese (BMI of 30-34.9) and 1.3 times higher for those who are severely obese (a BMI of 35 or higher).

The researchers required that studies follow people for five years or longer, to weed out those who were underweight simply because of cancer or chronic lung disease or heart failure. Common causes of being underweight include malnourishment, heavy alcohol or drug use, smoking, low-income status, mental health or poor self-care.

"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," Dr. Ray said. "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."

Dr. Ray also said that as society aims to curb the obesity epidemic, "we have obligation to ensure that we avoid creating an epidemic of underweight adults and fetuses who are otherwise at the correct weight. We are, therefore, obliged to use the right measurement tool."