Have you stored a little extra fat to carry you through the cold months of Canadian winter? You aren’t alone.

While a glance at a magazine stand might persuade you otherwise, the majority of Canadians are overweight according to standard medical definitions. Born from our collective discontent over our size is a multibillion dollar weight loss industry; pervasive advertising cheers us on towards shrinking dimensions. So prevalent is the discourse regarding dieting and weight loss we have, as a society, come to believe than thinner is better.

But is it possible that as we jog towards a leaner tomorrow, we have lost track of why we want to lose weight? Any potential health benefits have, for some, melded into to more superficial concerns such as fitting into a bikini and vanquishing cellulite.

As emergency physicians, patients often ask us if weight loss would be beneficial to them. Our answer is usually based on our calculation of body-mass index, which is a person’s weight in kilograms divided by the square of their height in metres. A normal body-mass index is between 18.5 and 25. Another important consideration is whether fat tissue is carried predominantly around the waist instead of the thighs, as this has been associated with an increased risk of cardiovascular disease.

We know the body-mass index calculation is not a perfect predictor of health. It doesn’t account for muscle tissue, such that a bodybuilder and a couch potato might have similar body-mass index. Equally, a body-mass index in the healthy range is no assurance of cardiovascular fitness. But generally speaking, those at the upper extremes of body mass often suffer from health problems. Severe (morbid) obesity is associated with a variety of health problems, such as diabetes, hypertension, sleep apnea, heart and lung disease, and joint issues.

However most Canadians aren’t morbidly obese. The majority of those who are considered overweight according to the body-mass index classification still have numbers less than 35. As an example, a man who is 5 foot 10 inches who weighs 240 pounds has a body-mass index of 34.5. Does being pleasantly plump like this actually confer an increased likelihood of death?

A study published in the Journal of the American Medical Association at the beginning of the year has been widely reported as challenging conventional wisdom on this topic. The upshot of the study suggests being slightly overweight (with a body-mass index of 25 to 30) may actually increase your longevity. Let’s take a look at the science behind this conclusion.

The investigators, primarily out of the Centers for Disease Control in Atlanta, compiled the data from 143 separate studies including nearly three million subjects. The strongest results of the study confirm that being moderately or severely obese can decrease life expectancy — a fact which is not surprising. Those who are morbidly obese have a 25 per cent greater risk of dying.

The more controversial aspect relates to the mildly overweight (body-mass index between 25 and 30), where the data suggests a five per cent lower risk of death compared to those with normal weight. And even slightly heavier study subjects with mild obesity (body-mass indexes between 30 and 35) showed no convincing evidence of risk or harm either way.