Why we need to bin BMI

The body mass index (BMI) has repeatedly come under fire as an unreliable health measurement, but it’s still used universally. Here’s why we need to stop using it altogether…

Boris Johnson’s new anti-obesity drive plans to restrict deals on junk food in supermarkets and enforce calorie counts on restaurant and takeaway menus. There’s no doubt that obesity is a problem in the UK and much of the western world, but what about the way we measure it?

BMI only tells you if you’re too heavy – not if you’re too fat. And those can be two very different things.

What is BMI?

The body mass index (BMI) is a universally recognised method of how healthy you are, based on your height and weight. It’s endorsed by the WHO (World Health Organisation, the US Centers for Disease Control and Prevention, and the NHS – to name a few.

It’s pretty simple to calculate your BMI. You just divide your weight in kg by the square of your height in metres. Your BMI number then puts you into one of four health categories:

18.5 or below = Underweight

18.5-24.9 = Healthy

25-29.9 = Overweight

30-39.9 = Obese

This all sounds legitimate – a quick and easy way to measure how healthy a person is without any special equipment. However, BMI only tells you if you’re too heavy – not if you’re too fat. And those can be two very different things.

super-fit people with a high muscle density can be classed as ‘obese’

BMI limitations

The wording that goes with the NHS’ online BMI calculator includes two revealing statements:

• ‘The BMI cannot tell the difference between excess fat, muscle or bone.’
• ‘The adult BMI does not take into account age, gender or muscle mass.’

In real terms, this basically means that super-fit people with a high muscle density can be classed as ‘obese’, while at the other end of the spectrum, older people who lose muscle as they age can be ‘healthy’, even though they’re still carrying too much fat.

A study in the International Journal of Obesity compared patient BMI measurements to data from the US National Health and Nutrition Examination Survey, specifically looking at cardiovascular health indicators such as blood pressure, cholesterol, and glucose levels.

According to the results, nearly half of people classed as ‘overweight’, 29% of ‘obese’ people, and 16% of ‘morbidly obese’ people were, in fact, ‘metabolically healthy’ – that is, they didn’t show any of the signs of poor cardiovascular health that are usually associated with being an unhealthy weight. Not only that, but some 30% of ‘healthy’ individuals did on this research show poor cardiovascular health.

A BMI-informed misdiagnosis can negatively affect mental health, impact health insurance premiums and influence what medication we’re prescribed

Why does it matter and what are the alternatives?

This issue matters because the BMI promotes an ‘ideal’ bodyweight that might work for a small number of people, but more generally promotes an illusion of health based on some very old and very generic maths. A BMI-informed misdiagnosis can negatively affect mental health, impact health insurance premiums and influence what medication we’re prescribed, to name but a few adverse consequences.

Measuring your body fat percentage is a much better gauge of how healthy or unhealthy someone is, and there are plenty of ways to go about it:

• Good old-fashioned skinfold callipers – measures subcutaneous fat
• Hydrostatic weighing – measures your body density while you’re underwater
• Air Displacement Plethysmography – similar to underwater weighing, but uses air volume and pressure instead
• 3D body scanning – use infrared sensors to estimate your body fat percentage based on your body shape
• Dual-Energy X-ray Absorptiometry (DXA) – uses two different x-ray energies to estimate body fat
• Bioelectrical Impedance Analysis (BIA)/Electrical Impedance Myography (EIM) – measures the response of small electrical currents through your body
• Multi-compartment models – collate individual measurements of body mass, volume, water and bone content

Generally, of course, the more accurate a method, the more expensive and unavailable it is to the public. But in a world where AI can diagnose skin cancer and artificial organs can be grown in labs, you’d think at least one of these would emerge as a prevalent measure of obesity.

We need to bin BMI and take the time to deliver personalised healthcare

Let’s bin BMI!

If you’re still on the fence, just think: Should we really be making vital health-related decisions based on a 200-year-old method that was derived from data on a couple of hundred Belgian men in the 1800s, was deemed ‘flawed’ as far back as the 1970s, and is considered unfit for purpose today by a wide range of health and fitness experts?  

We need to bin BMI and take the time to deliver personalised healthcare, especially to people that do have a higher percentage of body fat, and ensure that people are given the treatment that’s right for them.