Conventional wisdom about body weight used to be simple: If you carried around extra bulk, you'd be more likely to develop Type 2 diabetes, high blood pressure, heart disease, joint problems and a host of other ailments. It was the kind of thinking that pushed overweight Canadians to jump on the treadmill and inspired weight-loss reality shows featuring the morbidly obese.
But a growing number of leading experts in obesity research say that that logic is in drastic need of a makeover, and may need to be thrown out altogether.
The problem, they say, is that the body mass index – the most common method of defining "ideal weight" – is flawed.
Although BMI is widely used by medical professionals to determine whether people are normal, overweight or obese, it is criticized because it can't differentiate between muscle mass and fat, and can't easily be applied to different ethic groups with varying body types. BMI also doesn't account for genetic variations or other differences that help determine a person's risk of health problems.
In other words, an individual can be overweight or obese on the BMI scale for decades, but never develop cardiovascular disease, sleep apnea or other ailments linked to excess weight. On the other hand, a person who is normal weight and gains a couple pounds could develop Type 2 diabetes.
As awareness grows that the one-size-fits-all approach to weight and health isn't working, obesity researchers are looking for ways to identify those most at risk for serious problems. But so far there is no consensus on how that should be accomplished.
Some say it will depend on improving BMI so it can be used as a health predictor rather than simply dictating which weight category a person is in.
A study published last week in the journal Obesity argues for a new measurement, the body adiposity index, which is based on hip circumference and height. The researchers, led by Richard Bergman at the University of Southern California, Los Angeles, say it can measure body fat more accurately and be applied across ethnicities.
The study has received widespread attention, partly because BAI seems to be better than BMI at determining how much body fat a person has. A BAI measurement is also more easily obtained, because it doesn't require exact weight as part of the calculation. The study follows previous research that suggested using hip and waist measurements to determine a person's body fat.
But Robert Ross, a professor at the School of Kinesiology and Health Studies at Queen's University, noted it's unclear whether the new measure trumps BMI in the most important area: "The question is, does it predict [health] risk factors better than BMI?" he said. "Time will tell."
Combining BMI with waist measurement is a better way to predict weight-related health problems, says Prof. Ross, who helped create the BMI and waist-circumference tool that's well recognized by the medical community.
Evidence is growing that people with larger waists are at greater risk of heart disease, stroke and other serious problems. (The general rule for white, sub-Saharan African, Middle Eastern and eastern Mediterranean populations is men whose waists measure more than 102 centimetres, or 40 inches, face increased health risks, while for women the healthy cutoff is 88 centimetres, or 35 inches. Among Asian, south Asian, Malaysian, Chinese, Japanese, South and Central American ethnicities, the cutoff is 90 centimetres, or 35 inches, for adult males and 80 centimetres, or 32 inches, for adult females).
"If that waistline is expanding, and more fat is in the waist, you're at a greater health risk. It's not that complicated," said Prof. Ross, who is also director of the Centre for Obesity Research and Education at Queen's.
But the BMI and waist-circumference model still doesn't reconcile the fact that some people who are overweight won't develop related health problems, while some who are considered to have a healthy weight will.
What seems to unite many researchers is that it's no longer accurate to say a high BMI score translates into future health problems.
"I think amongst medical clinical experts, this discussion is getting louder and louder," said medical professor Arya Sharma, chair of obesity research and management at the University of Alberta. "You can find people with BMIs below what is medically defined as obesity who become diabetic. … There's not just the weight that's making you sick. It's also, very often, genetic factors."
Instead of simply relying on BMI or other measurements, Dr. Sharma said, people who are heavier than average should be monitored for ailments they have or are at risk of developing. That's an effective way to see who may need medical intervention without telling everyone who is overweight they need to shed the excess pounds, Dr. Sharma said.
It might also be safer. Dr. Sharma noted that many people who lose weight gain it all back, and that the rapidchanges put stress on the body.
To some, the idea that overweight or obese people should be left alone unless they are at clear risk of health issues may sound drastic. But Dr. Sharma's position reflects the extent to which common thinking about defining "healthy weight" is shifting.
"If you're healthy, it doesn't really matter what your weight is," Dr. Sharma said.