Dr Robert Grenfell national clinical issues director at the Heart Foundation
No measurement is perfect in all circumstances, but for most people, BMI is a useful guide to whether you’re at a healthy weight or not.
When it comes to predicting risk of chronic diseases like heart disease, waist circumference is often seen as a better tool because carrying weight around your middle can be dangerous.
Overall, however, the differences between BMI and other indicators like waist circumference, waist-to-hip and height-to-waist ratios are rarely clinically significant. Studies show the variation is subtle and would be unlikely to make a significant difference for the individual person. BMI is a useful measurement for most people over 18, but there are limitations to be aware of as it may be influenced by age, gender and ethnicity.
If you’re pregnant, BMI doesn’t apply and you should consult your doctor on where a healthy weight sits. People who are likely to be unhappy with their BMI are athletes because their results may show as overweight – muscle weighs more than fat and BMI doesn’t take this into account. If people are unhappy with their BMI, they can complete a waist circumference assessment.
Nearly 10 million adult Australians are now overweight or obese (based on BMI calculations) and for most of those people, that’s an accurate reflection of their weight range. That means nearly 10 million adult Australians are facing a higher risk of heart disease, diabetes, some cancers and numerous other health problems.
Tackling our expanding weight problem needs to be our top priority. What’s important is that people get their measurements taken and act on the result, rather than focus too much on the measurement method.
Dr Richard Telford, associate professor at the Australian National University Medical School ACT
Average BMI gives us a very rough idea of whether our population in general is getting fatter or not, but it is rough. The problem is that it can’t distinguish between fat, muscle or bone.
As Australians have become bigger over recent decades, we have increased our lean tissue as well as fat, so BMI tends to overestimate increases in our population’s fatness. But where BMI can get it completely wrong is in estimating individual fatness. I remember measuring a champion rugby league player who was 105kg and 1.83m, which made his BMI 31.4 and classified him as obese. This was absolute nonsense as the chap was a ball of muscle, but BMI didn’t know that.
And take the case of 35-year-old Kate. At 70kg and 1.5m, her BMI was 31.1 and we measured her body fat accurately by scan at 33.1 per cent. Kate wisely decided to consult a fitness advisor and a dietitian. After six weeks of conscientious work, she was demoralised to find that her BMI and weight had not budged. However, her follow-up scan showed her fat percentage had actually dropped to 31.5 per cent. Kate had shed one full kilo of fat, but had increased her muscle mass by one kilo as well. She was relieved to find she was slimmer, more toned, and healthier without her BMI changing.
But there is another even more important fact to consider. Studies have consistently shown that it is not unusual for an overweight, but physically active person to be healthier than a normal or underweight person who is inactive.
The message I like to send to all people with high BMI is to get up and move around regularly throughout the day. Physical activity is great (in fact, vital) for your health, irrespective of any weight loss.