Why are we still using the BMI to measure health?

The BMI scale is outdated and has a few shortcomings. Photo / 123RF

By Emile Donovan of RNZ

It might come as a surprise to some that the basis of one of the most important and widely used health metrics is a 190-year-old index invented by a Belgian statistician who had absolutely no expert medical knowledge.

Adolphe Quetelet sought to determine the measurements of "the average man".

In the course of his work, Quetelet developed a mathematical formula: take an individual's weight in kilograms and divide this by their height in metres squared.

Today, we know this calculation as the body mass index – or BMI.

The BMI, ostensibly, is a rudimentary tool which is used to tell us about how much excess body fat a person, or a population of people, has.

But it has many shortcomings: it may be inaccurate when applied to particularly tall or short people; it also isn't especially useful when applied to people who have a lot of muscle, like professional athletes. According to the BMI, Richie McCaw was extremely overweight when he led the All Blacks to World Cup glory in 2015.

So why are we still using this metric?

Otago University Professor Sir Jim Mann says BMI is a reasonably good indicator of body fatness at a population level.

"And body fatness at a population level is one of the major determinants of health in a country, or internationally," he says.

"But there are limitations, and I think it's important to be aware of those limitations."

"It is certainly useful when the BMI is markedly raised. If you're talking about a BMI over 30, it is almost always very relevant and very useful and should be an important start to the conversation when a GP or indeed a hospital doctor sees a patient.

"The real difficulty is when the BMI is in the slightly elevated range, that 25 to 29 sort of area. That is when it is particularly important to look at it in the context of other clinical measurements."

One criticism of BMI is that because it was invented by a European man in a then-largely monocultural country in the 1800s, it isn't applicable to people from different ethnic groups.

Sir Jim says this is indeed backed up by data.

"There is one situation which I think is particularly important to mention in the New Zealand context, and that is that Māori and Pacific do tend to have more muscle than the average non-Māori, non-Pacific individual, and that means at any given BMI, they will have a bit less body fat and a bit more muscle.

"Interestingly, there are a group of people who differ at the other end, which is to say at a given BMI, they have more body fat and less muscle than European populations, and I'm talking about in general people from the Indian subcontinent, and perhaps for them there should be a lower cut off as to when people are regarded as being overweight."

Mann says a delicate issue in this discussion relates to the very real discrimination or fatphobia many people face, which Mann says must be balanced with the reality that there is such a thing as 'excess body fat', which is linked with greatly increased risk of negative health outcomes.

"I have said unequivocally that I think stigmatisation is totally inappropriate.

"It is equally difficult for the medical professional, whether they're doctors or nurses, to know how to tackle it. And in the past, I believe my own profession, the medical profession tended to go in for stigmatisation. They either ignored the issue of obesity, or they went over the top and people were accused of being gluttons or something, which is completely inappropriate.

"The vast majority of people who have got excess body fat are fully aware of it, and sometimes they welcome the discussion, but it should be done in a sensitive and appropriate way that will be different for every individual. Telling someone that they're too fat and they should lose 25kgs is almost always totally inappropriate.

"But at the same time, when I know that a patient with diabetes, for whom I have some degree of clinical responsibility, can potentially achieve remission of type 2 diabetes, with all the complication that goes with type 2 diabetes, there's clearly an obligation to make them aware of that. But there are ways of doing it."