Only between a quarter and a third of people currently diagnosed with obesity should be considered as in poor health, according to a major new study that joins calls to overhaul how the condition is diagnosed. The research, published in the journal Nature on Wednesday, argues that relying on body mass index (BMI) alone gives a misleading picture of who is actually ill.
The study estimates that, on average, only around 27 per cent of people classified as having obesity across the four countries examined would meet the criteria for clinical obesity under a new framework — equivalent to approximately 8.5 per cent of all adults. The rest would be classed as having pre-clinical obesity, where the condition is largely a risk factor rather than an active cause of ill health.
The findings are based on data from almost 850,000 adults from China, the United Kingdom, the United States and Spain. The researchers applied a proposed new definition of obesity to the data to show what it would mean for diagnosis levels, prevalence and ultimately treatment, for a condition the World Health Organization estimates affects more than one billion people globally.
The new framework, which follows a proposal last year from experts and medical organisations, divides obesity into two categories. Pre-clinical obesity is when excess body fat is present but is not yet causing illness, while clinical obesity is when it is causing ill health, such as heart, liver or kidney problems. The distinction matters because the two groups need very different care.
Under the proposed system, diagnosis would move beyond BMI — a measure based only on a person's weight and height — to include body fat, measured for example by waist circumference, as well as checking whether the patient has any organ dysfunction linked to weight. The study estimates that 1 to 3 per cent of those defined as obese using BMI alone would no longer meet the criteria, while another 5 to 9 per cent, with higher body fat for example, would newly be diagnosed.
The study's senior author, Francesco Rubino, professor and chairman of metabolic and bariatric surgery at King's College London, said the inaccuracy of BMI measurements was well known. BMI cannot distinguish between fat and muscle, and it says nothing about where fat is stored in the body or whether it is damaging organs — the factors that actually determine health.
The implications are far-reaching. Obesity is associated with an increased risk of type 2 diabetes, cardiovascular disease, liver and kidney disease and certain cancers, and health systems around the world use BMI thresholds to decide who qualifies for treatment, including weight-loss drugs and surgery. A more precise definition could mean treatment is directed at those who need it most, rather than at everyone above an arbitrary BMI cut-off.
At the same time, the researchers stress that pre-clinical obesity is not a clean bill of health. People in that group still carry an elevated risk of developing illness, and they need monitoring and support to prevent progression — but not necessarily the same intensive treatment as those whose organs are already affected.
The study is the latest step in a growing movement among clinicians to rethink obesity. Last year's proposal from international experts argued that BMI should be only one part of the picture, and the new research puts numbers on what that change would mean in practice, country by country.
For patients, the message is that a diagnosis of obesity based on BMI alone does not automatically mean poor health — and that a proper assessment should look at body fat and organ function as well. For doctors and policymakers, the challenge now is to turn the new framework into practice, so that the millions of people affected get care matched to their actual health, not just to a number on a scale.
