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What is BMI?

By Darren O'Toole | In Fitness, Health | on September 27, 2018

The rising trend in people becoming overweight and obesity isn’t solely based on visual observation. In fact, living in London, there seems to be less of a visual prevalence compared to other parts of the UK. Instead, it is through a measurement called Body Mass Index or BMI.

Body Mass Index (BMI) is calculated as weight in kilograms divided by height in meters squared. To showcase the calculation:

My weight is 71kg

My height is 1.72m.

This calculation would be 71kg divided by 1.72².

The result would be 24.0 which sits me in the ‘healthy weight’ range.

Following current recommendations, overweight is defined as a BMI of 25.0 to 29.9 and obesity as a BMI of 30.0 or higher. Obesity can be further subdivided into grade 1 (BMI 30-<35), grade 2 (BMI 35-<40), and grade 3 (BMI ≥40).

Is BMI an accurate measure of health?

When I teach on fitness courses and students undertake this measurement, we invariably discover that there are a few students who sit in the overweight to obese range. These are people training 5-6 times per week and who eat healthier than most. Are they at risk of all of the health issues of others who are overweight and obese? No. They are an anomaly. Watch any International rugby match and you will see 30 obese professional athletes competing at a physically demanding intensity for 80 minutes. Are they at the same health risks? Again, no. But, for many, this is an excuse because for them, they are at risk.

BMI and body fat

BMI has been found to be highly correlated with percentage body fat as measured by a Dexa scan – which is one of the most accurate measurements available for body fat. For men, the correlation between BMI and percentage body fat ranges from 0.72 to 0.79; for women the correlation ranges from 0.72 to 0.84. At a given BMI, black men and women tend to have higher lean mass and lower fat mass than white men and women.

So clearly, it’s not an exact science. A crude measurement will not 100% tell someone if they are at significant risk. Yet those who it doesn’t fully apply to will likely be those who are exercising lots and have developed greater musculature and bone density. For use on general populations, it has its place. People are quick to ridicule it when it goes wrong, but ignorance of its effectiveness is more dangerous.

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