Obesity rates: Is the Body Mass Index a good measurement?
The Body Mass Index, a scale scientists use to determine whether someone is overweight, is not the most accurate tool to determine whether one needs to lose some pounds, but it's a useful measurement when studying global populations, experts say.
"The problem with BMI is if you’re using it for yourself, on an individual basis to guide yourself as to whether you’re obese, it’s not particularly useful," said Dr. John Millar, vice-president of the Public Health Association of B.C.
"But as a population measure, it’s very good for measuring obesity. So yes, it’s perfectly valid."
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Two recent studies have revealed some startling figures about the problems of obesity. Researchers at the Institute for Health Metrics and Evaluation at the University of Washington found that more than two billion people, or one third of the world's population, is either overweight or obese.
Both studies relied on the use of the Body Mass Index scale, which is simply a ratio of one's weight in kilograms divided by their height in metres squared. Health Canada uses four categories of BMI ranges in the Canadian weight classification system:
- Underweight (BMI less than 18.5)
- Normal weight (BMIs 18.5 to 24.9)
- Overweight (BMIs 25 to 29.9)
- Obese (BMI 30 and over)
To translate that into feet and pounds, an individual who is 5-8" and 170 pounds would have a BMI of 25.8, meaning that according to the BMI, they would be overweight.
But some argue the index too often classifies those who are healthy as being overweight or obese. For example, muscular athletes with little fat may have a high BMI because muscle weighs more than fat.
"There is no question that it is a less than ideal way to look at an individual," said Dr. Yoni Freedhoff, founder of the Ottawa-based Bariatric Medical Institute. "It’s a measure of how big a person is, not whether that person has health concerns. And it is a very simple number, which doesn't take into account things like race, age, sex, body frame, or muscle mass. And so for those reasons it is not an ideal measure for an individual.
"But looking for global trends, in looking at health-care resource utility, and costs and those sorts of things, it is useful for governments and it is useful for statisticians because it’s an easy calculable number."
In part, the scale is used as a global indicator of obesity because it's much easier to gather mass data on height and weight than other factors.
Freedhoff said when he wants to determine whether one of his patients is medically well or not, he considers the individual as a whole and not just the number that comes out of the scale.
"Scales don’t measure the presence or absence of health in individuals, but they are useful in population based studies," he said.
Millar agreed that body fat, rather than weight, is a much better measure of someone's overall health, and that electronic devices and not the BMI are better suited to examine that.
"It’s particularly the fat that accumulates inside your body and ends up around the kidney and around your liver and around your heart," he said. "That’s the really worrisome stuff."
Jennifer Kuk, associate professor at Toronto's York University School of Kinesiology and Health Science, said that people may not be aware that society is becoming more overweight, because as some studies have shown, people's goal weight has shifted upward over time.
"So that means what you think is normal and what you want to weigh has actually also drifted up," she said.
"That means people don't notice that they're overweight until much later. People don't notice that they're obese until much later and they don't consider it overweight because overweight implies you're not normal."
But Freedhoff suggested the debate about BMI distracts from what should be taken from these studies.
“I think the most important thing to take from this piece is that weights are rising whether or not you want to nitpick around what level we should call the cutoff for medical obesity. You can, but the bottom line is we are as a globe gaining weight and not slowly."