Weighing up BMI: is your body mass a reliable assessment tool for health?

Rose Stokes

22 Sep 2023

Between the 1970s and the 1990s, body mass index (BMI) became an important tool to measure obesity – and thus health – in the UK. Initially developed between 1830 and 1850 by a Belgian astronomer and statistician called Adolphe Quetelet, BMI was never intended to be used within the realms of medical assessment, but rather as a tool of sociological analysis.

However, as the prevalence of obesity increased in the western world towards the end of the 20th century, so too did interest in developing a tool to measure body fat. BMI was therefore repurposed to identify relative obesity – first in the US, and later the UK. According to Quetelet’s formula, BMI is calculated by dividing your mass (in kilos) by the square of your height (in metres).

Dr Helen Hartley, associate medical director at Aviva Health, considers BMI to be a useful tool because it removes the confounding factor of height when comparing weights across a population. Given that taller people are generally heavier, BMI allows for rapid diagnosis of obesity [BMI greater than 30 for adults, or 27.5 for those from Asian, Chinese, Middle Eastern, Black African and Afro-Caribbean backgrounds*], which, she says, is linked to many health conditions. “Obesity is associated with higher risks of disease, particularly type 2 diabetes, coronary artery disease, stroke, some cancers – eg breast and bowel – and depression.”

But it is not without its limitations and has been cast into the spotlight in recent years because of its lack of nuance, causing many people to question whether it is still fit for purpose. “BMI doesn’t take account of body composition,” says Hartley. “As muscle is denser than fat, some very muscular people (eg boxers, rugby players and athletes) have high BMIs that would be considered technically obese, even though they are very fit.” Ironically, this means that some of the world’s healthiest people – with a high muscle, low body fat ratio – are categorised by their BMI as at high-risk for developing certain conditions when assessed within this framework.

BMI is also not an indicator of overall health, she says. “It is more that an unhealthy BMI can be a cause for concern in relation to the risks of obesity and associated diseases.”

To counter these limitations when it comes to body composition, she says that the waist to hip ratio has gained ground in recent years as an important measure to be used alongside BMI. “A ratio of 0.5 or higher indicates health risk from obesity.”

Health clearly cannot be measured by a single data point, rather it is a constellation of different numbers and ratios that create an overall picture. Because of this, focusing purely on a person’s BMI would risk underplaying the impact of other lifestyle factors when assessing their general health. “Obesity is just one factor that is worth getting under control to reduce the risk of ill health,” Hartley says.

“Other lifestyle factors that affect health include exercise, diet, sleep hygiene, stress and mental wellbeing, UV exposure, smoking, alcohol and recreational drug use.” Taken without this vital context in mind, there is potential for an overfocus on the importance of BMI to veer into unhealthy territory, she says.

Still, Hartley believes that keeping an eye on BMI is important. “Monitoring our BMI is essentially watching our weight,” she says. “It is recommended to keep within the ‘healthy range’, and to take steps to reduce our weight if we are becoming overweight or obese.”

For most people, this means following a healthy diet and increasing levels of activity, whether that is brisk walking, cycling or taking other forms of exercise. “If that doesn’t work, it is worth consulting with a GP to rule out medical conditions contributing to weight gain and for further advice.”

*Because of ethnic differences in propensity to some health conditions, such as diabetes and heart disease, lower thresholds need to be used in diagnosing obesity in some ethnicities

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