What’s less commonly understood is that both definitions - and the assumptions behind them - are regarded by many critics as highly suspect.
Chewing The Facts explores this debate in depth. The 10-part podcast series talks to a range of experts, including some who argue that our attempt to link fatness to poor health outcomes is scientifically unsound and serves only to stigmatise fat people, without solving the underlying problems which cause the diseases, such as poor access to nutritious food and good healthcare.
The BMI, for instance, dates back to the 19th century when Belgian mathematician Lambert Adolphe Jacques Quetelet created a metric to identify the average man, by dividing a person’s weight in kilograms by their height in metres squared.
Otago University primary health care and general practice senior lecturer Dr Lesley Gray said the measure was never intended to be used in science today.
“It was the ‘ideal’ population mean. Now bear in mind that a Belgian mathematician 200 years ago meant a white person. And the people that were measured were principally European (men) and so that becomes problematic straight away, in that many women were not even included in that measure and certainly people of colour were not represented.”
Since the 1970s, BMI has been used to track population-level statistics, but it is used to extrapolate individuals’ health risks without making the appropriate additional diagnostic tests such as high blood pressure and insulin levels, she said
Gray said obesity was associated with diabetes, but not proven to be the cause.
She pointed to a 2016 study that addressed historical links between coffee and cancer. It found previous studies failed to adjust for people who smoked cigarettes, leading to confounding results.
“Chances are it wasn’t actually the coffee that was causing the cancer, it was more likely to be the cigarette.”
Grey said multiple studies reported higher risk and higher severity of H1Ni bird flu among fat people in 2009, but a systematic review and meta-analysis of the outbreak and BMI showed that after adjustment for medical bias against fat people receiving antiviral treatment, there was no increased risk of death for fat people.
This year, the American Medical Association released a policy criticising the use of body mass index.
World Health Organisation nutrition, health, and development director Francesco Branca told Chewing the Facts that BMI was the most cost-effective measure, but it was not an accurate proxy of health, given its variability in populations.
“It’s a good starting point for a primary care doctor [but] we can do better, and that is actually the circumference of the waist.”
It was one of the reasons why the World Health Commission established a Lancet Commission to look at the diagnostics of obesity.
General Practice NZ Chair, Dr Bryan Betty said poor health outcomes associated with obesity pointed to subcutaneous fat - or fat accumulation around vital organs or the waist.
People with a lower body mass index were equally at risk of having excess subcutaneous fat.
“There’s real questions about how we measure this thing we call obesity and certainly BMI is acknowledged to be a very rough guide in a lot of cases, predominantly because we can’t measure intra-abdominal fat without a CT scan.”
The WHO classes people with a body mass index of more than 25 as overweight, and people with a BMI of more than 35 as obese. Overweight and obesity are defined as abnormal or excessive fat accumulation that may impair health, the WHO website says.
The latest New Zealand Health Survey says one in three adults aged 15 and over are classified as obese using the body mass index.
Chewing the Facts - new episodes out every Sunday. Produced with the NZ Herald, with support from NZ On Air.
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