The origins of BMI can be traced back to 1833. The standardised – and highly contentious – weight metric was devised by a Belgian statistician to describe the “average man”. The current iteration of the BMI calculation was developed in the 1970s by an American physiologist and subsequently adopted as a primary indicator of health.
There is a clear link between body fat and adverse health effects such as type 2 diabetes and heart disease. However, the absence of an easy, precise method to measure a person’s fat levels has led to an over-reliance on BMI, which is calculated by dividing an adult’s weight in kilograms by the square of their height in metres. The core issue with BMI is that it focuses solely on body weight and does not differentiate between muscle and fat, or account for more dangerous fats found in specific areas of the body.
The report urges doctors to assess a patient’s overall health rather than relying solely on BMI, which has proven to be an unreliable indicator of individual health. This dependence can lead to under-diagnosis of those seen to be lean and over-diagnosis of those who are presumed to be obese because of their BMI.
“People have recognised this problem with BMI for a long time. A person, for example a rugby player, with high muscle mass will register as obese, but to suggest that this person is at high risk of heart disease would be inaccurate,” says Farooqi. BMI remains useful when researchers examine entire populations, Farooqi notes, “but it’s not so great when giving individual health advice”.
What are the main changes in the report?
Recognising the various limitations of BMI, the Lancet report aims to collate evidence and highlight alternatives to better identify individuals whose health is adversely affected by their level of body fat. As a result, the report recommends two new categories of obesity: clinical and pre-clinical. The former applies to those whose chronic illnesses and diseases are directly caused by their weight, while the latter refers to individuals who are healthy but face an elevated risk of developing issues in the future. The intention is to reflect that people with obesity have a diverse range of life experiences and health outcomes.
The report also advocates for the use of additional indicators, such as measuring a person’s waist or waist-to-hip ratio, alongside directly asking questions about how their weight might be affecting aspects of their health, including their joints, fertility, blood pressure and mobility.
Why has change been so slow?
Though there have been significant social and cultural movements that have championed body acceptance and pushed for equal treatment of fat people, the medical conversation around obesity has remained stagnant for many years. Experts have long criticised the strict adherence to BMI, as it fails to provide a meaningful assessment of who might require medical intervention.
One clear reason for BMI’s persistence is its simplicity and convenience. Another is the inherent challenge of establishing a definition that finds broad international consensus. “I think the key was getting everybody together behind a single solution, and that’s really what this commission did. It was a very exhaustive process; you can see the long list of people from around the world – everybody got to have their say,” Farooqi says.
Why is it important to reframe obesity in this way?
For many decades, there have been very few treatments for obesity. That is no longer the case, Farooqi explains: “We now do have them, and they are effective,” referring to semaglutides (better known under brand names such as Ozempic or Wegovy). “There’s an even clearer rationale for this report because it lays out who might benefit from this treatment, which can have dramatic effects on people’s health, including reducing their risk of heart attacks.”
Defining clinical obesity in this way paves the way for more personalised, targeted care. The experts behind the report hope it will enable individuals with clinical obesity, who have often been dismissed by healthcare professionals, to access proper medical evaluations and treatments. “What often happens is people are just repeatedly told to go on a diet and exercise, but what this report recognises is that clinical obesity is a disease that should be assessed with the same rigour as conditions like asthma or high blood pressure.”
Conversely, it also ensures that those at risk but without current health issues are not over diagnosed, and avoids misdiagnosing individuals with higher weight due to muscle mass.
“I think this is a gamechanger,” Farooqi says. “It’s a really powerful piece of work that redefines one of the major health problems of our time. It allows us to see the clinical problems, recognise its prevalence in society, and most importantly, to help destigmatise obesity.”
