Beyond BMI: A New, More Nuanced Way to Define Obesity

The Lancet Diabetes & Endocrinology Commission* has recently proposed a shift in how health professionals assess obesity. They recommend moving beyond BMI (Body Mass Index) to a more detailed approach that better evaluates the impact of excess body fat on health. 

What are the changes, and what do they mean for you? First we need to understand... 

*FYI: the Lancet Diabetes & Endocrinology Commission is a panel of global experts dedicated to advancing research and improving healthcare for metabolic and endocrine conditions.  

...the problem with BMI 

To re-jog your memory, BMI is calculated using your weight (kg) divided by your height squared (m²) The resulting number fits into a category to determine whether you are normal, overweight, or obese.

The thing is, relying on BMI alone is flawed. Why? 

• It doesn’t distinguish between muscle and fat: A muscular rugby player with low body fat might be classified as "overweight" or "obese" due to their higher BMI.
• It ignores fat distribution: Visceral fat (around the abdomen and organs) is much riskier than fat stored in less harmful areas like the thighs.

A new definition: Preclinical and Clinical Obesity

The new definition proposes a more nuanced way of understanding obesity by introducing two categories:

• Preclinical Obesity: Excess body fat with no immediate health problems or daily limitations. People in this category may still face higher risks of future health complications but aren’t currently experiencing ill effects.
• Clinical Obesity: Excess body fat that directly causes health problems or limits daily activities. Symptoms may include difficulty breathing, joint pain, or metabolic issues, like diabetes. In this category, obesity is viewed as a chronic illness requiring treatment. 

What this may mean for you

Health professionals may now use multiple tools and methods to assess obesity more accurately:

1. Confirm Excess Body Fat (Adiposity):

Instead of looking only at BMI, doctors can use one of the following methods:

• At least one measurement of body size (see image) plus BMI measurement.
• At least two measurements of body size, regardless of BMI.
• A direct body fat measurement like a DEXA scan.
• Excess body fat is automatically confirmed if your BMI is over 40 kg/m².

2. Assess Clinical Obesity:

• If excess fat is confirmed, doctors will review your medical history, perform a physical exam, and run blood tests to identify signs of organ dysfunction. They’ll also assess how fat impacts your daily activities—difficulty dressing, bathing, or even walking may indicate clinical obesity.

For preclinical obesity, the focus will be on reducing future risks, while for clinical obesity, treatment will aim to improve symptoms, not just weight loss.

Differences between ethnic groups

The health risks linked to obesity vary between ethnic groups. For example, people of Asian descent may face health problems at lower BMI levels compared to Caucasians. On the other hand, Māori and Pacific Islanders often have more lean muscle at the same BMI, which means their health risks might not match standard BMI categories.

This is why it’s important for doctors to use ethnicity-specific measurements instead of applying the same BMI cut-offs to everyone. For example, waist-to-hip ratios or specific waist circumference recommendations can help provide a more accurate picture of health.

I dug out the appendix from the report which includes BMI ranges and waist measurements for different ethnic groups. 

Here’s a summary:

Why does any of this matter?

This new framework brings a fairer, more individualised way to assess and manage obesity. It reduces stigma, acknowledges the complexity of obesity, and ensures treatment focuses on overall health rather than just weight.

Further resources:

• This infographic is great. 
• The original report (The Definition and Diagnostic Criteria of Clinical Obesity)
• Supplementary Appendix 2 of the report, outlining recommendations by ethnicity for BMI and waist circumference