Health
Bye bye, BMI: new guidelines overhaul the way we look at obesity
We’ve been measuring obesity with a rudimentary tool developed in the ‘70s for far too long. Now, a new definition of obesity proposed by a commission of global health leaders is set to revolutionise the way we approach and treat this condition.
By Sabrina Rogers-Anderson
Most Australians have had their body mass index (BMI) measured by a doctor or have calculated it themselves with an online tool at some point in their lives.
But in recent years, a growing number of medical professionals have criticised BMI as an inaccurate measure of body fat content and overall health.
Now, after 5 years of consultation, the Global Commission on Clinical Obesity has proposed new guidelines that dramatically overhaul the way we define obesity.
So, what is the new definition all about and how will it improve the way obesity is treated? Here’s everything you need to know.
Why BMI is inaccurate
BMI has been around for close to 2 centuries, so the fact that it’s incomplete and outdated isn’t surprising.
The simple equation – weight divided by height squared – was originally developed by Belgian statistician Adolphe Quetelet in 1832 to determine the weight of the “normal man”.
In 1972, physiologist Ancel Keys dubbed it “body mass index” and it has been the standard tool for measuring overweight and obesity ever since.
Here’s a snapshot of BMI classification in adults:
BMI (kg/m²)
Classification
Less than 18.5
Underweight
18.5 to less than 25
Normal weight
25 to less than 30
Overweight but not obese
30 to less than 35
Obese class I
35 to less than 40
Obese class II
40 or more
Obese class III
Some of the issues that have been raised about BMI include:
- It doesn’t distinguish between body fat and muscle mass, so people with a high amount of lean muscle can mistakenly be categorised as obese.
- BMI doesn’t tell us anything about how fat is distributed on the body, which is critical because abdominal fat is associated with a wide range of health risks, including insulin resistance, type 2 diabetes, heart disease and metabolic disease.
- It doesn't take sex and racial differences into account because participants in the original study conducted by Keys were mainly white males (with not a single woman included in the study).
“BMI by itself measures nothing,” says Tim Gill, Hon Professor at University of Sydney’s Charles Perkins Centre and Director of The Obesity Collective.
“It doesn’t distinguish between muscle and fat or even between water and fat. It doesn't measure anything about adipose tissue distribution and it doesn't determine when excess fat is a health problem.
“Having said that, a lot of the criticism of BMI is unwarranted. It's like any other screening tool. It has its limitations, but it’s only meant to be used to start the process.”
When BMI goes wrong
Amy has been battling her weight and 3 debilitating autoimmune conditions for years. Despite her best efforts to get her BMI down to a “healthy range” through diet and exercise, it always seems to creep back up.
“I wanted to get back to the gym after I had my second child 5 years ago, but my knee was causing me too much pain,” she says. “My doctor referred me to a knee surgeon who told me point blank that I didn’t need a knee surgeon. He said I needed a weight-loss surgeon for gastric band surgery because my BMI was too high.
“He didn’t ask me a single question about my medical history and didn’t even realise I’d just had a baby. He just judged a book by its cover and sent me on my way.
“It turns out I had psoriatic arthritis and he missed it. I recently saw a much nicer knee surgeon who told me the arthritis has eaten my knee alive since then. If I’d been properly assessed and diagnosed 5 years ago, it would have saved me a lot of intense pain and confusion.”
Beyond BMI: the new definition of obesity
The new guidelines published this month in The Lancet urge doctors to go “beyond BMI” when diagnosing obesity.
They recommend using a range of diagnostic measures – including waist circumference and waist-to-hip ratio – to gain a more complete picture of a person’s health.
In an effort to resolve the long-running debate over whether obesity is a disease or simply a risk factor, the new definition places obesity on a spectrum that includes 2 categories:
- Clinical obesity: This is diagnosed when organ function is impacted by weight gain. There are 18 diagnostic criteria for clinical obesity in adults, including breathlessness, knee or hip pain, sleep apnoea, obesity-induced heart failure and liver disease.
- Pre-clinical obesity: Patients who are overweight or obese but otherwise healthy fall into this category.
“People with clinical obesity have an existing problem,” says Prof Gill. “It’s a disease that’s already caused damage to their body. Those with pre-clinical obesity may have excess weight, but at the moment their health hasn't been directly impacted.
“It’s a positive change overall because there was a lot of denial about the seriousness of obesity within the community. While this new definition doesn't address all the issues, it's a step towards helping people understand that this is a real condition with exactly the same drivers and concerns as other chronic conditions.
“And if it gives people more confidence that they're not going to be misdiagnosed purely on the basis of their weight, that's also important."
BMI a screening tool for obesity, not a definition
While the approach has received widespread support in medical circles, it’s also attracted its fair share of criticism.
“It’s a little complicated, which detracts from its clinical utility,” notes Prof Gill. “With the average GP visit lasting around 4 minutes, I can’t see too many GPs going through this whole process.
“Some clinicians have also raised that the new definition could be used to avoid providing services to those with pre-clinical obesity, who perhaps have the most to gain in the long term.
“But one of the most important things the new definition will accomplish is to clarify to clinicians and the general public that BMI is not the way we define obesity. BMI is a tool for screening and it’s only one step in the process."
What to do if you’re struggling with your weight
There are so many fad diets and weight-loss programs out there that aren’t supported by evidence. Before you spend any time or money on them, talk to your GP or an accredited practicing dietitian about the best option for you.
This may help: Thinking of trying a new diet? 4 questions to ask yourself before you do
You can also find more information about how to choose a reputable weight management service on the Better Health Channel website.
Feature image: iStock/PixelVista
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