The BMI is being abandoned as a clinical measurement tool. Here’s how to know if your weight puts you at risk

Evidence that the single measure of BMI is a poor indicator of disease and mortality risk is now overwhelming, and it’s prompting a radical rethink.

Not only is the BMI scale a poor indicator, the index itself is fundamentally flawed. Picture: istock

Evidence that the single measure of BMI is a poor indicator of disease and mortality risk is now overwhelming and it’s prompting a radical rethink about the association between weight and health at the highest of levels of medicine.

A US study published this month that found being overweight according to the BMI scale is not linked to an increased risk of death has attracted attention, but it’s only one in a string of studies in recent years that have questioned the use of the body mass index calculator in healthcare.

Not only is the BMI scale a poor indicator, the index itself is fundamentally flawed and ignores that the fact that for older people in particularly, carrying a couple of extra kilos can have a protective effect in the case of sudden disease, if other metabolic markers are normal.

In fact for everyone, the upper limit of healthy weight according to the BMI, which is 25, is pitched low and in decades past was 27.8 for men and 27.3 for women.

Evidence that the single measure of BMI is a poor indicator of disease and mortality risk is now overwhelming. Picture: istock

The reality is that those with BMIs between 25 and say 28 can sometimes be healthier on all markers than those with a BMI under 25.

“I think we need to understand the historical nature of BMI, the historical nature of unhealthy weight ranges – there’s only ever really been correlations with regards to health conditions and mortality,” says Terri-Lynne South, chair of the Royal Australian College of GPs’ specific interests obesity management group. “Correlation doesn’t necessarily mean causation. There’s multiple levels of why BMI has been used in research and in population studies, but it really does fall down when applied to an individual in regards to a diagnosis.

“You could have a low BMI or a healthy BMI and still be very, very unhealthy. The BMI is really losing its clinical value for an individual. This is actually well recognised internationally, that we need a new definition of this complex clinical condition, obesity.”

Body Mass Index Infographic Chart

In a policy statement that will have reverberations around the world, the American Medical Association at its annual meeting of delegates last month acknowledged “significant limitations associated with the widespread use of BMI in clinical settings”. The AMA’s new policy “noted that BMI is significantly correlated with the amount of fat mass in the general population but loses predictability when applied on the individual level”. It was particularly problematic in that it was based primarily on caucasian data and body type.

The US’s top doctors now suggest BMI be used in conjunction with other valid measures of risk such as measurements of visceral fat, body adiposity index, body composition, relative fat mass, waist circumference and genetic/metabolic factors.

Dr South suggests that if people fall into the overweight BMI category, before becoming concerned or attempting to lose weight, they should assess other factors including waist circumference and taking note of where on the body their body fat is deposited.

A waist circumference associated with low risk of disease is under 80cm for women and

94cm for men.

If fat is deposited primarily around the trunk region, that is riskier than carrying more weight around the hips and backside. Blood pressure readings, metabolic markers such as blood sugar and insulin levels, and cholesterol levels can also give a much better indication of health than BMI and are likely to be markers of whether a patient has higher levels of the most dangerous type of fat, visceral fat around the organs.

Dietitian Fiona Willer, who has a doctorate in weight-neutral approaches to dietetics, says it is extremely pleasing to see acknowledgment in the medical community that larger-sized people can be metabolically healthy despite weight sometimes being an indicator of risk of disease.

She says the correlation between exercise levels and dietary quality and risk of premature mortality is much more firmly established than the correlation between weight and risk of death, except at the extreme ends of body mass.

“Presuming that people with a lower BMI are ‘healthier’ is a mistake that will lead to missed opportunities for supporting the health and wellbeing of Australians,” Dr Willer says.

“For health professionals, these findings should give us cause to put our concerns about weight on the backburner and focus on the health, wellbeing and preferences of the person in front of us. For those in public health, weight-focused messaging should be reconsidered because we now know it’s inaccurate, ineffective and perpetuates weight stigma in both the health sector and the general public.

“Trying to change the body shape to affect health risk is like painting a house without considering the state of the foundation and internal structure,” she says. “Using the same analogy, walking every day or eating well could be seen as slowly replacing the structural beams - they’re not going to necessarily make a difference to the appearance of the house but will make it less likely to fall apart in a cyclone.”