Overhauling obesity: Is this the end of the BMI as we know it?

For a condition that affects one in three Australian adults and an estimated one billion people worldwide, it may come as a surprise that obesity is not always considered a disease.

This can have far-reaching effects. When health insurers and governments, for instance, regard obesity as a risk factor for other diseases rather than a disease in its own right, they’ll often insist on a coexisting condition such as diabetes to be present before funding therapies for it.

The disease/risk factor spectrum is just one of the issues addressed by the newly-published Lancet Global Commission on Clinical Obesity, published in the The Lancet Diabetes and Endocrinology.

Lancet commissions tend to be two to three years in the making, marshalling experts and advocates worldwide to chart the latest knowledge, and implement sweeping recommendations all the way down to how we train medical students.

And the consensus among this commission’s 56 expert contributors, ranging across endocrinology, internal medicine, surgery, biology, nutrition and public health, and people with lived experience of obesity, is that current definitions and clinical practice regarding obesity are ineffective.

We’re not getting any healthier. Many people are unintentionally misled about their true risk, and either miss out on the right care, or are over-diagnosed.

BMI is not enough: Comparing apples to pears

“I think obesity is still a disease that is under-recognised and under-treated,” says commission contributor and bariatric surgeon Professor Wendy Brown.

A major culprit, she says, is the most commonly-used diagnostic tool, the BMI. Calculated as weight divided by height, the BMI is quick and easy to obtain, but it doesn’t tell the whole story.

“The classic scenario is that if your BMI is 30, and that’s largely made up of fat, particularly around the central area or around your heart, then that’s an unhealthy situation,” Professor Brown says.

“But if you're an athlete and you weigh 100 kilos and you’re solid muscle, your BMI might flick up to 30, but you're more likely to be a really healthy person because you're lean, fit and active. It doesn’t account for what the weight is made up of, or where the weight is carried.”

And, she says, we need to consider the presence of disease independently of weight. Someone with a BMI of 25 or 26, well within the healthy range, but carrying dangerous fat deposits around the middle – the classic “apple shape” – can have heart disease or diabetes.

When it comes to Brown’s own specialisation, the threshold for bariatric surgery is a BMI of 35 or 40. She’d like this to change to make it easier for people with a lower BMI to qualify, or be eligible for other less invasive treatments, such as GLP agonist drugs, where BMI might determine cost of access.

Laying waist to longstanding practice

Even a casual observer visiting any reputable health website will know waist measurement is a widely accepted indicator of unhealthy deposits of visceral fat around organs such as the liver. Yet this hasn’t made its way into clinical guidelines or practice.

Professor Brown says this is where things start to get complicated.

“Every doctor has a height measure in their surgery, and every doctor has a set of scales,” she says. “That’s fairly straightforward. But the more weight a person is carrying, the more difficult it can be to measure the waist accurately. The narrowest part of the torso is then often not where the true or natural waist sits.”

Not to mention the impact on the doctor-patient relationship of whipping out a tape measure.

“Many healthcare practitioners still don’t feel comfortable raising the issue of weight with their patients, and I think this is because our training in it has been inadequate over time, and also because clinicians don’t raise the issue of weight with their patients because they're worried about offending them or making them feel shamed.”

Beyond risk factors: Recognising obesity as a disease

Co-commissioner and endocrinologist Associate Professor Priya Sumithran says part of the reason there’s disagreement about whether obesity should be considered a disease is because it’s regarded as only a risk factor for other diseases, such as type 2 diabetes.

“The impact of obesity is not the same for every individual. The commission’s proposed definitions help us determine whether or not a person’s excess weight is affecting their health or daily functioning,” she says.

What’s new

The Lancet guidelines broaden the diagnostic criteria through body size measurements – waist circumference, waist to hip ratio, or waist to height ratio. More complex, expensive tools, such as scans to assess fat mass, are also an option, with the caveat that these are expensive and not widely available, especially in low to middle-income countries.

They also introduce two new definitions:

• Pre-clinical obesity: Where a person meets the criteria for obesity, but is not experiencing any ill effects.
• Clinical obesity: Where excess fat has become an illness, rather than just predicting future illness.

“So, we're not getting rid of the BMI,” Professor Brown says. “We’re supplementing the BMI to help us understand what the weight is made up of, and where it's carried. And then we really want to start having more nuanced conversations with our patients.

“Preclinical obesity is something we’d monitor – similar to when someone’s blood pressure is borderline, or someone who’s on the edge of diabetes. We just keep track of things and help prevent them from tipping into a point where that excess fat is actually impacting their life.”

Conversely, people with clinical obesity are those whose excess fat is affecting their health, in the form of diseases such as diabetes, heart disease, liver disease, infertility or cancer.

“There's also a group that, in the past, we would have diagnosed with obesity, with a BMI of 30 – for instance, elite athletes. But they don’t have excess fat, so they no longer will be labelled with that disease.”

Where to from here?

About 74 professional societies worldwide have signed up to the new guidelines, which Professor Brown says augurs well for their uptake around the world, from the most affluent to the most resource-constrained settings.

She and other commissioners hope to put the stigma of obesity, tied to a widespread belief that it’s a failure of discipline, to bed. This will mean we need to re-train and re-educate healthcare workers and policymakers.

“Diet and exercise are part of any weight-loss regime, and the way you lose weight is to use up more energy than you're taking in.

“However, it’s really hard to lose a significant amount of weight and keep it off in the long term. When you lose weight, every mechanism in your body that makes you hungry is up-regulated, and every mechanism in the body that burns energy is down-regulated.

“So that means only about 3% of people can keep off a significant amount of weight in the long term. So therefore, for the 97% of people who aren’t able to achieve that, it’s really important that they have conversations with their healthcare professionals about how they can help them to keep that weight off into the long term.”

This article was co-authored with Liz Lopez, communications manager in the school of translational medicine.