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June 25, 2018

Fitness: Why we need to stop using BMIs

Body Mass Index scale
Body Mass Index scale

For decades, body mass index (BMI) has been used as the accepted method to calculate whether a person is healthy or not.

By dividing a person’s weight by the square of the person's height, you are able to tell if a person is underweight, healthy, overweight or obese.

BMI measurements have been adopted by governments and doctors around the world, but increasingly more and more research is proving it to be flawed in its ability to accurately diagnose obesity.

Recently, a new study led by UCLA psychologists found that using BMI to gauge health led to more than 54 million peopled labelled as unhealthy, even if they weren't.

Lead author of the study Janet Tomiyama said, “Many people see obesity as a death sentence. But the data show there are tens of millions of people who are overweight and obese and are perfectly healthy.”

The study analysed the link between BMI and other health markers, including blood pressure, glucose, cholesterol and triglyceride levels from data from the most recent National Health and Nutrition Examination Survey.

They found that close to half of those considered “overweight” by their BMI (47.4 per cent) were healthy. They also found that more than 30 per cent of those with BMIs in the normal range were actually unhealthy, based on their other health data.

So why is the BMI so wrong? Well, first of all, the person who came up with the BMI system explicitly said it could not and should not be used to indicate the level of “fatness” in an individual.

Introduced in the early 19th century by Belgian mathematician Lambert Quetelet, it was originally used to give a quick and easy way to measure the degree of obesity of the general population to help governments allocate resources.

When in a group, as long as it works for two thirds of the population, the errors cancel themselves out, so the trend is valid.

It was not supposed to be used for individuals for a number of reasons, the first being that it does not distinguish the difference between fat and muscle.

One pound of fat takes up a larger volume than a pound of muscle, even though they weigh the same. Because of this, athletic muscular people can be labelled “overweight by their BMI.

BMI also does not take into account where on the body fat is carried. While one person may carry their fat around their middle, another could have the same amount spread around their body. While they may both have the same BMI, the former would be at a higher health risk than the latter.

It also does not factor in height. The taller the person, the “thicker” they would have to be in order to be healthy. BMI almost assumes that we are two dimensional.

There is a real danger in using BMI to categorise someone as healthy when they are not, and vice versa. If a person is shown to be healthy according to their BMI, they may continue making lifestyle choices that may not necessarily be good for them.

Furthermore, being labelled “obese” or “overweight” can have greater implications than physical health. To a young girl or boy where body image is important, the label could lead to eating disorders and body dysmorphia, a disorder that causes a person to have a distorted view of how they look.

Jeffery Hunger, co-author of the UCLA study, concluded that the BMI is a deeply flawed measure of health, recommending that people focus on eating healthy and exercising regularly, rather than obsessing about their weight.

He said, “This should be the final nail in the coffin for BMI.”

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