Weighing the Value of BMI

The body mass index measures fat, but does it speak to health and fitness? The experts weigh in.

Weighing the Value of BMIPhoto: iStock

We’ve come to think of body mass index (BMI) as the gold standard in measuring our fat, but there is a growing consensus among scientists that it may not be the best tool to do so. According to a 2013 editorial in the journal Science, BMI does not accurately measure the amount and location of body fat (belly fat is more hazardous than evenly distributed fat because it surrounds the internal organs) or the proportion of muscle to fat (muscular types may score in the overweight or obese range).

The researchers point to estimates that 24 per cent of adults with a “normal” BMI actually have symptoms of insulin resistance and a higher risk of heart disease, which may be due to having more body fat and less muscle mass. On the flip side, about one in 10 adults with an “obese” BMI are healthy, possibly because they have a lot of muscle.

Should we say goodbye to BMI?
That depends. BMI is a good tool for policy-makers such as Health Canada when they are assessing whether a large group, like Canadians, is getting fatter, according to Dr. Arya Sharma, professor of medicine and chair of obesity research and management at the University of Alberta. “If you have enough people in a study, BMI does correlate with body-fat composition,” he says. “If you calculate BMI in 1,000 people, the group with the highest results on average is going to have more body fat. But for individuals, the index is almost meaningless. You can find two people with exactly the same BMI, but one has 40 per cent body fat and all of the problems associated with being obese, while the other has 20 per cent body fat and no health problems.”

BMI’s true value is in research, agrees exercise physiology expert Len Kravitz, advisory panel member of CanFitPro, a certification organization for fitness professionals. “With some people, it’s a weak predictor of weight-related health problems.”

Health Canada also recognizes the limitations of BMI for indi­viduals. Its official stand is that assessing an individual’s healthy weight cannot be determined by BMI alone.

Additional factors-such as the degree of lean body mass, ethnic background, lifestyle habits, fitness level and other health risks-also need to be considered, according to Health Canada.

New tools at our disposal
Measures that include waist size appear more effective. In fact, A Body Shape Index (ABSI), which measures height, weight and waist circumference, more accurately predicted death in 14,000 people than BMI in a 2012 study at the City College of New York.

And in a 2012 review of 31 studies, obesity researchers at Oxford Brooks University in England found that waist-to-height ratio (your waist in inches, measured one inch above your belly button, divided by your height in inches) was a better predictor of disease, including cancer, stroke and heart disease, than BMI or waist circumference alone. Both methods are currently used in Canada but not widely.

However, says Sharma, these tools still do not tell you whether your stomach fat is the unhealthy kind. Even a measure of your body-fat percentage-determined by a body-fat scale or with calipers at your gym-won’t do that. “Like BMI, your percentage of body fat isn’t a good measure of health,” he says.

To find out if you are truly at a healthy weight, get assessed at your doctor’s office via checks on blood pressure, cholesterol and glucose levels and a physical exam to check for health risks related to obesity, such as back pain, acid reflux disease, osteoarthritis and sleep apnea. Says Sharma, “No other tool, whether it’s a scale, measuring tape or BMI calculator, is as effective as that.”

BMI 101
Invented by a Belgian mathem­atician in the 1800s, body mass index (BMI)-weight in kilograms divided by the square of height in metres-has long been used around the world to assess if a person’s weight falls in the healthy range for their height and as a reliable indicator of body fat. If your BMI falls between 18.5 and 24.9, your weight is considered to be in the normal range. A result over 25 means you are overweight (25 to 29.9) or obese (30+).

Want to know your ABSI? Check out this calculator: absi-calc.appspot.com