A new Canadian review of research on the relationship between weight and risk of premature death finds that having a body mass index in the underweight range is linked to an even higher risk of death than having a body mass index in the obese range.

Led by Dr. Joel Ray, a physician-researcher at St. Michael's Hospital, University of Toronto, the review is published in the Journal of Epidemiology and Community Health.

The investigators found that for both adults and fetuses, being underweight is linked to higher risk of death from any cause.

For their meta-analysis, they reviewed and pooled data from 51 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.

BMI is a measure of body fat based on weight and height that applies to adult men and women. It is commonly used to classify underweight, overweight and obesity in adults. It is defined as the weight in kilograms divided by the square of the height in metres (kg/m2).

According to the World Health Organization BMI classification, a BMI under 18.50 is classed as "underweight," 18.50 to 24.99 is classed as "normal," 25.00 to 29.99 is classed as "overweight," and a BMI over 30.00 is classed as "obese."

For example, an adult who weighs 70 kg (154 lbs) and whose height is 1.75 m (5 ft 9 ins) will have a BMI of 22.9 which is in the "normal" weight range.

Being underweight linked to higher risk of death than being obese

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Having a BMI in the underweight range is linked to an even higher risk of death than having a BMI in the obese range.

In their review, Dr. Ray and colleagues found that adults with a BMI classed as underweight (under 18.50 or less) had a 1.8 times higher risk of dying from any cause than adults with a BMI classed as normal. This was even higher than for people classed as obese.

For people with a BMI in the range 30.00 to 34.99 (obese), the risk of dying was 1.2 times higher than that of people with BMI classed as normal, and for those with a BMI higher than 35.00 (severely obese), the risk was 1.3 times higher.

The analysis only included studies that had followed people for 5 years or more, to rule out those who were underweight simply because of illnesses like cancer, heart failure and lung disease.

The researchers note that common causes of being underweight include malnutrition, heavy use of drugs or alcohol, low-income background, smoking, poor self-care and poor mental health.

BMI reflects muscle mass, not only body fat

Dr. Ray says it is important that we realize:

"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."

He suggests if we are trying to look at the problems of excess body fat, then perhaps we should replace BMI with a proper measure of body fat, such as waist size.

As we fight to curb the obesity epidemic, he warns that we also need to "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."

In 2011, Medical News Today reported a study that found being underweight was linked to raised risk of death following surgery. Writing in the Archives of Surgery journal, a team from the US found that people under or on the slightly lighter side of normal weight appeared to have a higher risk of death in the 30 days following surgery than people on the heavier side.