Thinking Nutrition

How useful is body mass index (BMI)?

September 15, 2020 Dr Tim Crowe Episode 36
Thinking Nutrition
How useful is body mass index (BMI)?
Show Notes Transcript

Carrying too much weight is linked to poorer health. But just how much weight is too much? Body fat can be measured in several different ways, with each method having its own pros and cons. Body mass index or BMI is by far the most common one as it only relies on knowing your height and weight. But with that simplicity comes flaws as those easy results can sometimes be misleading, which is why the widespread use of BMI is troubling. In this podcast, I’ll look at the different ways that body fat assessment can be made and explain why you should take any BMI label it may give you with a grain of salt. 

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Carrying too much weight is linked to poorer health. But just how much weight is too much? Body fat can be measured in several different ways, with each method having its own pros and cons. Body mass index or BMI is by far the most common one as it only relies on knowing your height and weight. But with that simplicity comes flaws as those easy results can sometimes be misleading, which is why the widespread use of BMI is troubling. In today’s podcast, I’ll look at the different ways that body fat assessment can be made and explain why you should take any BMI label it may give you with a grain of salt. 

The most common measure of body fat and associated health risks is body mass index (BMI). BMI was developed as a simple way to compare different groups of people, based on the correlation between height and weight as an indicator of excess body fat.

BMI is calculated by dividing weight in kilograms by the square of height in metres. See the show notes for a link to an online BMI calculator https://www.betterhealth.vic.gov.au/tools/body-mass-index-calculator-for-adult

A healthy BMI for an adult is between 18.5 and 25 kg/m². Between 25 and 30 kg/m² is considered overweight and 30 kg/m² and above obese. At a population level, high rates of body fat above 30 kg/m² indicate an increased risk of death and disease.

Labelling a person as ‘obese’ may not always be helpful in affecting positive behaviour changes, especially when a person already acknowledges that they’re carrying a bit of extra weight. “Unhealthy BMI”, “above the healthy weight range”, and “excess weight” can all carry the same message about the need to shed excess weight for better health and reduced risk of disease.

BMI is the main measure used for international obesity guidelines and is recommended by the World Health Organization. But BMI isn’t perfect. People with the same body weight and height can have different proportions of body fat to lean muscle mass. BMI does not take into account the person’s body fat versus muscle and since muscle weighs more than fat because it is denser athletes with high muscle mass, for instance, can have a lower proportion of body fat than less muscular people, so a BMI calculation can put them into an overweight or obese category, even though their risk of obesity related disease is very low.

This is a frequently cited criticism of BMI, but it needs to be put into perspective. Such people are in the minority and a quick visual inspection will clearly show that it’s muscle, not fat, that such people are carrying the most of.

Striking differences in body composition are also apparent among people of various ethnic and racial groups, making standard BMI guidelines inappropriate for some populations. For example, people of Polynesian ancestry tend to have a greater bone density and lean muscle than Caucasians; consequently, using BMI as the standard may overestimate the prevalence of obesity among the Polynesian population so a higher BMI healthy-weight-range is often recommended. People from an Asian background tend to have more body fat on a leaner frame, so a lower BMI healthy-weight-range can be used.

So, with so many flaws to BMI, why does it continue to be used? Simple. It is easy to measure, and what gets measured, gets counted. BMI tends to be used in large-scale population research and surveys as it can be calculated from height and weight, either self-reported or taken quickly and non-invasively by a researcher. If the BMI of a population goes up over time, then it safe to attribute that to excess weight gain, not that people got shorter or suddenly large proportions of the people started smashing out a bodybuilding program.

If anything though, BMI tends to underreport people with excess body fat. One meta-analysis on the subject found that BMI fails to classify half of the people with excess body fat, reporting them as normal or overweight despite having a body fat percentage classifying them as obese. And I’ll link to this study in the show notes https://pubmed.ncbi.nlm.nih.gov/20125098

Alternatives to BMI

While BMI is a useful measure of overall health risks, it fails to take into account the distribution of fat throughout the body. I often say that when it comes to linking weight to health, it is much more important where the fat is than how fat you are. For this reason, waist circumference was developed as a simpler and potentially more accurate measure of disease risk. Waist circumference is not only a gauge of body fat, but it specifically targets the most dangerous type of fat: visceral fat.

There are two important types of belly fat – subcutaneous and visceral. Subcutaneous fat is the fat that lies directly under the skin, while visceral fat lies deep within the abdomen surrounding the body’s organs. Visceral fat is found between the organs of the abdomen and contributes to belly fat. There’s a strong correlation between central obesity and cardiovascular disease, insulin resistance, type 2 diabetes, inflammatory diseases, high blood pressure and other obesity related diseases.

For men, the aim is to have a waist circumference below 94cm; for women it’s 80cm. Measures above 102cm for men and 88cm for women carry a very high risk of developing type 2 diabetes, high blood pressure, cardiovascular disease and even some forms of cancer. For people of an Asian background, slightly lower waist circumference goals apply: under 90cm for men and 80cm for women.

More recently, estimates of body fat percentage and health risks have looked at waist-to-hip ratio and even waist-to-height ratio. Both these measures take into account central fat stores so can give a better health risk estimate than BMI. For men, a waist-to-hip ratio below 0.9 and 0.8 for women correspond to a healthy weight BMI.

An advantage of using waist measures for body fat estimates is that it takes away the stigma of needing to step on the scales. It also allows for the use of cut-off values that avoid terms of overweight and obesity, and instead focus on the risk of metabolic disease.

Another technique to measure body fat is by bioelectrical impedance. This method involves passing a small electrical current through the body, normally by a specialised set of scales that a person stands on. The scales measure water volume and, by the use of special algorithms, arrive at a body fat percentage estimate. The accuracy of such machines can vary dramatically, especially around the cheaper end of the price range.

By far the most accurate way to measure body fat is by magnetic resonance imaging, computed tomography or X-ray scanning, but such methods are not realistic for the public to use and belong firmly in the world of research.

So long as the limitations of a weight assessment method are understood, methods such as BMI and waist circumference are quick and simple validated ways to assess weight and disease risk that can be used by health professionals and the public alike. But 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. 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. 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 not a measure of ‘health’ or a physiological state such as when you measure resting blood pressure or fasting blood glucose which are both certainly linked to the presence or absence of disease. BMI is simply a measure of your size. Plenty of people have a high or low BMI and are healthy and, conversely, lots of people with a normal BMI are unhealthy. In fact, a person with a normal BMI who smokes and has a strong family history of cardiovascular disease may have a higher risk of early cardiovascular death than someone who has a high BMI but is a physically fit non-smoker. So, BMI is just a number which should never be given too much importance as a primary goal for maintaining good health. 

So that’s it for today’s show. You can find the show notes either in the app you’re listening to this podcast on if it supports it, or else head over to my webpage www.thinkingnutrition.com.au and click on the podcast section to find this episode to read the show notes.

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I’m Tim Crowe and you’ve been listening to Thinking Nutrition.