New Canadian research, published in the Journal of Epidemiology and Community Health, suggests that people who are clinically underweight have a higher risk of premature death than obese individuals. The researchers found that for both adults and fetuses, being underweight is linked to higher risk of death from any cause.

Compared to normal-weight individuals, the excessively thin have nearly twice the likelihood of death, researchers concluded after a meta-analysis was conducted with more than 50 prior studies that examined the link between body mass index (BMI) and deaths from any cause, and also data on newborn weight and stillbirths in Ontario, Canada.

The BMI is a measure of body fat based on height and weight that applies to adult men and women. According to the World Health Organization BMI classification, a BMI under 18.5 is classed as “underweight”, 18.5 to 24.9 is classed as “normal”, 25.0 to 29.9 is classed as “overweight”, and a BMI over 30.0 is classed as “obese”.

In the meta-analysis, Dr. Joel Ray, a physician-researcher at St. Michael's Hospital, University of Toronto, and his colleagues found that adults with a BMI classified as underweight, had a 1.8 times more likelihood of dying from any cause than adults with a BMI classified as normal. Individuals with a BMI in the range 30.0 to 34.9 (obese) had a 1.2 times higher risk of dying than those classified as normal. Severely obese patients – individuals with a BMI of 35 or more -- faced a 1.3 times greater risk.

The meta-analysis only included studies that had followed people for five years or more, to rule out individuals who were underweight only because of illnesses, such as cancer, heart failure, and lung disease.

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

Dr. Ray believes keeping a healthy figure size in mind is important when attempting to address 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.”

Additional Resources:

J-shapedness: an often missed, often miscalculated relation: the example of weight and mortality