Five experts on whether BMI is a useful indicator of health

Staying a healthy weight can be a challenge, and knowing what weight is healthy for you can be too. Most people rely on the body mass index, or BMI, which is a measure of our weight in relation to our height.

Many experts have criticised this fairly limited measure of the health of our weight, yet it still remains the most popular way for most people to judge a healthy weight.

We asked five experts if the BMI is a good indicator of a healthy weight.

Five out of five experts said no.

Is BMI of any use for measuring health?

Is BMI of any use for measuring health?

Alessandro Demaio: Medical doctor

There are two questions here – whether BMI is a good indicator of weight, and whether weight is an accurate reflection of health. BMI, the tool most often used to determine “healthy weight ranges”, was designed primarily to track the weight of populations. While it’s a simple and useful screening tool when looking at groups of people, it’s not an accurate marker of individual health. This is because BMI is a measure of our height and our weight, and the ratios of their combination. But weight alone doesn’t discriminate between a kilogram of fat versus a kilogram of muscle, nor does it account for body shape and fat distribution differences relating to, say, ethnicity or gender.

Then, just as not all overweight individuals have heart disease risk factors or unhealthy metabolisms (the conversion of food into energy), not all lean people have healthy ones. As a rough rule of thumb, both BMI and weight are still helpful for estimating healthiness – particularly when combined with a measurement of waist circumference – and excess weight or significant weight gain are associated with a range of disease outcomes. But BMI or weight alone do not replace the need for a proper checkup with your GP, nor do they provide a guarantee of well-being.

Emma Gearon: Epidemiologist

BMI is a simple indicator of weight for height and can't differentiate between muscle mass and fat mass. So BMI tends to overestimate the health risk for adults with a high muscle mass, such as some athletes, and underestimate the risk for adults with a low muscle mass, as can occur with sedentary lifestyles. Despite this limitation, BMI is generally thought to adequately identify risk across the whole population.

But we have recently found BMI is increasingly underestimating the level of risk in the Australian population compared to waist circumference. In the 2011-12 health survey, 10 per cent of women classified as normal weight by BMI, and 50 per cent of women and 25 per cent of men classified as overweight by BMI were obese according to their waist circumference. Consequently, BMI underestimated the prevalence of obesity by almost 50 per cent for women and 30 per cent for men. BMI can no longer be considered a reliable indicator of healthy weight, and further research is needed to identify a suitable alternative.

Evelyn Parr: Exercise scientist

Indicators of strength, fitness and central fat tissue are far more indicative of health than BMI. BMI does not tell us how much muscle someone has, or where their body fat is distributed such as the arms and legs vs around the middle. BMI indicates one’s weight when taking into consideration how tall they are. As such, BMI is a great measure for large studies or the time-poor doctor’s office.

Many studies, normally with thousands of participants, use BMI as a mortality predictor. As individuals, our focus should be on our fitness, as it’s our muscle that helps us stay healthy while we age. If we don’t maintain muscle, we face a generation of people with low muscle mass and too much fat. Let’s look to more important measures of fitness to indicate how healthy an individual is – irrespective of their body weight, and keep BMI for the large studies.

Steve Stannard: Exercise physiologist and nutritionist

For an individual, BMI alone is not a good indicator of whether someone is a healthy body weight. The body mass index roughly describes a person’s shape; a higher BMI representing someone with a large volume to surface area (wide for their height, ball-shaped), while a low BMI describes the opposite (thin for their height, stick shaped). Most often, someone who is wide is probably so because they carry a lot of body fat, but not always. Sometimes they are just short and muscular, and plenty of fit muscle is healthy!

However, at a population level, we do know that on average a person with a high BMI has a greater chance of suffering from many non-communicable diseases, including diabetes and heart disease, which are associated with carrying too much body fat. BMI would be better described as a good indicator of the health of a population.

Tim Crowe: Dietitian

BMI only gives a very approximate guide to health related to weight and height and gives no information on body fat content or location. BMI ranges can also vary depending on ethnicity. It is also now acknowledged that as we enter into older age, a higher BMI is linked to improved nutritional status, protection against falls and lower disease risk.

Simple measurements such as waist circumference are more useful for an individual as they look directly at body fat around the abdomen which is more directly related to disease risk. BMI is much better for looking at the health of whole populations and how this changes over time rather than as a diagnostic tool for an individual.

Disclosures: Emma Gearon has received an Australian Government Research Training Program Scholarship.

This article was originally published on The Conversation. Read the original article.

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