Anyone worried about their weight has been given an extra thing to worry about this week.
The way that doctors judge how overweight we are, a measure called body mass index or BMI, is unreliable and needs to change, according to a group of 58 experts. So what can and can’t BMI tell us about our health?
While most people gauge their weight just by the reading on their bathroom scales, that’s less useful for doctors.
For over five decades, doctors have judged how overweight people are by their BMI, a measure of weight in proportion to height. It is calculated by dividing your weight in kilograms by the square of your height in metres.
If your BMI is 24.9 or more, you are overweight, and if it is 30 or over, you are obese. If BMI is under 18.5, on the other hand, this suggests you are too thin. The cut-offs are nudged slightly lower in people from certain ethnic minorities.
But, while generally useful, BMI can fail to identify some people who are unhealthy due to their excess weight, and misclassify others as being unhealthy when they are not. This is the conclusion of an expert panel called the Commission on Clinical Obesity, just published in the medical journal, The Lancet Diabetes & Endocrinology.
One problem is that BMI fails to take into account where in the body most of the fat is distributed, and that can affect health.
If body fat is stored mainly in the belly and inside abdominal organs, this is linked with higher rates of conditions such as heart attacks and diabetes than if fat is stored more evenly around the body, including under the skin.
One way of assessing such “abdominal obesity” is to measure waist circumference, waist-to-height ratio or waist-to-hip ratio, and the Commission on Clinical Obesity proposes that doctors use such measures when assessing people’s health.
Health conditions
BMI also fails to consider if someone has health conditions caused or worsened by obesity, such as joint pain or breathlessness on exertion. “Some individuals with obesity can maintain normal organ function and overall health, whereas others display signs and symptoms of severe illness,” said Commission chair Professor Francesco Rubino, a weight loss surgeon at King’s College London.
The Commission has called for doctors and health systems to split obesity into two types: clinical obesity, when people do have associated health conditions, and pre-clinical obesity, when they don’t have current illness.
People with clinical obesity should be prioritised for access to obesity treatments, like weight loss injections, said Professor Rubino.
A third drawback of the BMI system is that it doesn’t distinguish between a heavier body weight from fat and muscle mass. This means that people who are muscular, for instance very fit atheletes or gym goers, can be classed as obese when medically speaking, they are a picture of health.
“It can be misleading in the range of BMI between 25 and 34.9, said Professor Tom Sanders, a professor of nutrition and dietetics at King’s College London.
In practice, though, this isn’t usually a problem, as athletes and body builders who could have this misclassification may well be tracking their health using other methods.
Real world
The Commission’s recommendations have been backed by many obesity experts globally. But it remains to be seen if they will be used in the real world, said Professor Simon Griffin, a professor of general practice at the University of Cambridge, who wasn’t a member of the Commission.
GPs can easily record someone’s height and weight, but it takes longer to start measuring waists or calculating waist-to-hip ratios, he said. “The anticipated improved prediction of disease through measurement of waist is unlikely to be realised as waist will be imprecisely measured,” he said. “If someone tells me their weight and height the BMI calculation is usually not too far from the truth.”
On the other hand, the arrival of weight loss injections has been a game changer for obesity treatments. Over the next few years the medicines are likely to be rolled out on the NHS to increasing numbers of people, in order of clinical priority. So a more accurate way of working out how much weight affects people’s health could be helpful.
“Our reframing acknowledges the nuanced reality of obesity,” said Professor Rubino. “This will facilitate a prioritisation of treatment options.”
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