A body mass index (BMI) result will label you as 'underweight', 'healthy weight, 'overweight', or 'obese'.
But while this may be a simple (and cheap) way to get a quick insight into your health, does it really paint the whole picture?
Circling all the way back to the 1830s, Belgian astronomer, mathematician and statistician Adolphe Quetelet developed (what was then called) the 'Quetelet Index' not to be used for medical assessment, but for his study on the average man, based on the 'ideal'.
Fast forward to the 21st century, and BMI calculations are still a routine part of our health service. Here, we've spoken to a plethora of doctors and weight experts about whether they think it's an outdated tool or not.
What is BMI?
"BMI is a widely used measurement that assesses an individual's body weight in relation to their height. This involves calculating a person's weight in kilograms by the square of their height in metres, allowing them to be categorised into different BMI ranges, such as underweight, normal weight, overweight, and obesity," says Dr Babak Ashrafi of Superdrug Online Doctor.
"BMI serves as a quick and accessible tool for healthcare professionals to screen and categorise individuals based on their body composition. This can be a really beneficial tool to establish whether or not you would benefit from weight loss treatments."
You can calculate your BMI for free using this NHS calculator, which it says takes into account natural variations in body shape, giving a healthy weight range for a particular height.
BMIs for adults:
below 18.5 is classed as underweight
between 18.5 and 24.9 is classed as a healthy weight
between 25 and 29.9 is classed as overweight
between 30 or over is classed as obese
A higher BMI has been linked to an increased chance of developing long-term conditions like type 2 diabetes and heart disease.
"BMI doesn't account for muscle mass and healthy fats as it solely relies on weight and height, making it a limited metric in assessing overall health. For instance, individuals with higher muscle mass may register as overweight or obese, even though their body fat percentage is low," Dr Ashrafi explains.
"In the same light, someone with a lower BMI may have a higher proportion of body fat, potentially overlooking health risks associated with excess fat accumulation.
"BMI doesn't distinguish between different types of fats, such as the beneficial subcutaneous fat [located between the skin and the outer abdominal wall] and the more harmful visceral fat [hidden inside your body on and around your organs]."
BMI doesn't account for muscle mass and healthy fats as it solely relies on weight and height, making it a limited metric in assessing overall health.
"The gold-standard way to measure visceral fat is with either an MRI or DEXA scan," says Dr Alasdair Scott at Selph. "However, these are expensive, take time and there’s no way the NHS can afford them. So instead we look for other measurements that are closely correlated with visceral fat."
Other options include waist circumference, waist-to-hip ratio, or waist-to-height ratio. The NHS site may ask you to measure your waist after getting your BMI result.
Dr. Hana Patel, NHS GP, says, "The WHO advised in 2004 that BMI is not appropriate for Asian people, as Asian people may be at a higher risk of type 2 diabetes and cardiovascular disease even within a 'healthy' BMI.
"The BMI scale was initially created using data and evidence from a research study that was carried out on 'predominantly European men'." The NHS currently states the calculator will ask for information on your ethnic background to give more accurate advice.
"There are also concerns raised about the use of BMI as a barrier to treatment for eating disorders [e.g. turned away for a BMI 'not low enough'."
In 2021, the House of Commons Women and Equalities Committee said BMI can also inspire weight stigma and eating disorders and called for a 'healthy at every size' approach.
Is BMI an outdated tool?
"On a population level, we're very unhealthy, and most people in the high BMI ranges have excess body fat (as opposed to muscle) and will likely be leading an unhealthy lifestyle too. It's the combination of excess body fat and unhealthy habits (like poor diet, no exercise, smoking etc) that's the real problem," says Robbie Puddick, registered nutritionist at NHS-backed Second Nature.
"BMI can be used as a fairly reliable predictor of the Western population's risk of chronic disease as most people with a high BMI will be relatively unhealthy," he adds. "However, on an individual level, it shouldn't be used on its own to determine an individual's treatment pathway or risk of disease."
Also assessing whether BMI is outdated, Dr Patel says, "There is the argument that it should not be used in determining health outcomes for women or non-white people, as they were not part of the initial clinical study," before agreeing more generally: "It's a quick measure to get an estimate but should not be used on its own."
...it shouldn't be used on its own to determine an individual's treatment pathway or risk of disease.
After acknowledging the limitations of BMI, Dr Ashrafi says, "It is still a useful tool, and a good starting point for a discussion about an individual’s health and risk factors for other diseases.
"In that respect, it is not outdated, and should continue to be used in parallel with other assessment techniques to give a better overall picture of someone’s health."
More unwaveringly for BMI, Tam Fry, chairman of National Obesity Forum, believes, "BMI is not outdated at all. It is still recommended by the WHO as the most practical way to establish if individuals are a healthy weight or if they are over/under weight.
"An accurate waist measurement may be more meaningful if an individual's weight is of concern but it is more difficult to achieve consistently.
"People who question BMI because they believe its use causes eating disorders are wrong. It is simply a tool to describe a general picture of healthy/unhealthier weight and should be used appropriately."
To note, the NHS BMI calculator site says it should not be used by those under 18, pregnant, diagnosed with or suspect they have an eating disorder, or have a condition that affects their height.
Watch: Is it time to get rid of the BMI?
While the answer is nuanced, it seems using BMI alone is outdated as it doesn't always reflect someone's overall health, but still holds purpose when used with other tools.
Rather than a one-size-fits-all solution, Dr Alasdair Scott concludes, "It’s important to realise that we don’t just have to pick one metric and make health decisions solely based on that alone. A diligent practitioner will measure height and waist and hip circumference.
"Beyond that we’ll look at a person’s sugar-handling, lipids and inflammatory markers. We’ll also take into account family history and lifestyle. We then build up a holistic picture of a person to assess their metabolic health and future disease risk."
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