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This week, doctors confirmed what many people have felt for a long time: the way we think about obesity is not helpful, BMI (body mass index) is unreliable, and this needs to change.
The Lancet Commission has urged a “radical overhaul” of how obesity is diagnosed, warning that reliance on BMI has resulted in both under- and over-diagnosis of the condition affecting approximately 1 billion people globally.
But if you cannot rely on BMI’s simple calculation of height-to-weight ratio, how can you tell if your weight is a problem?
Prof Francesco Rubino, the chair of the Lancet Commission that produced the report, said BMI should be used as a “screening tool” to understand risk, not to diagnose illness.
BMI cannot establish how much body fat someone has, since weight is connected to how heavy your bones are, how muscular you are naturally and what you have built up in the gym. Only a BMI of 40 and upwards definitely reflected a problem with excess fat, Rubino said.
Clinicians can instead determine whether someone has “clinical obesity” through blood tests which look for high cholesterol or if they are pre-diabetic, and a physical exam, or they may diagnose “pre-clinical obesity” if there is risk of disease due to high levels of fat.
“Obesity is a spectrum: it’s not always a disease, it can be something that people can live with in relatively good health. At the same time, others can have full-blown illness due to obesity alone,” Rubino said.
The new approach paves the way for more research to be gathered on how obesity can act as a disease by directly damaging organs, as opposed to contributing to heart disease, cancer and diabetes. It will also be used by health systems to manage who gets access to weight reduction treatments such as Ozempic and bariatric surgery.
But while Rubino noted that media coverage had focused on the idea of over-diagnosis of obesity, there is also a problem with under-diagnosis, especially among individuals with low muscle mass.
The BMI measure is based on average European height and weight, and is less effective for some ethnicities, in particular for people of Asian heritage, for whom it tends to underestimate obesity. “Some ethnic groups find it more difficult to get surgery, I’ve seen this over time. This creates inequalities that obesity unfortunately is already associated with and compounds the problem,” said Rubino.
One particular risk factor is where your fat accumulates – specifically, the belly. You can measure waist circumference, though Rubino said this should be done by a clinician. People with a higher BMI should not be overly concerned if they have strong limbs rather than belly fat, he said.
The report has spawned discussion of whether you can be “fat but fit”, though Dr Andrew Jenkinson, a bariatric surgeon and the author of Why We Eat (Too Much) views this as an unhelpful term.
“You can be a high BMI and be fit, but it’s unlikely if you have a high fat percentage you’re going to be very fit,” he said. “I wouldn’t say anyone with significant central obesity is metabolically fit – their risk in future is much higher than someone without belly fat.”
He said you could have a high BMI due to muscle mass without being a regular gym-goer, as some people are ‘“genetically big” – but suggested visiting a GP if your BMI was above 25 and you were of normal to slim build.
Jenkinson said it was important to take a non-judgmental approach to obesity, given that two-thirds of the risk is genetic, and the western food environment is a major contributing factor – including snacking, processed food, a lack of sleep, high stress and a sedentary lifestyle. He regularly sees patients struggling with their weight who have received unhelpful advice from dieticians to calorie-restrict, which trains the metabolism to hoard fat.
Dr Oli Williams, who researches weight stigma at King’s College London, said the Lancet’s work would “give people far more certainty about the limits of using BMI and weight alone to measure health”, though he feared that the terminology of “pre-clinical obesity” unhelpfully implied that illness was inevitable.
Williams has researched the relationship between obesity and identity and found that people whose BMI defined them as obese felt stigmatised, and that the term failed to capture the difference between those making an effort to manage their weight through diet and exercise and those who did not.
“There’s an assumption if you’re classified as obese you’re lazy or stupid or irresponsible,” he said. “For a lot of medics it’s neutral terminology, but that isn’t how the public engage with that term. It’s highly emotive, and often derogatory.”
He added that people with a healthy BMI or who appeared slim might not be in good physical or mental health, using smoking or severe calorie restriction to lower their weight. The cultural association between weight, health and morality also contributed to the high incidence of eating disorders, he said, while BMI could deter people, especially women, from strength training.
Williams noted that in Japan, where obesity levels are low, five measurements were used to determine the impact of excess fat on ill health instead of one – with responsibility for managing weight understood on a “structural, systematic level”, rather than the emphasis in the UK on “individual willpower and behaviour change”.
