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Is BMI an Accurate and Relevant Measurement?

The body mass index equation has been around for over 100 years. It is most commonly used by healthcare practitioners as a measurement of an adult individuals overall health based on their weight to height ratio. In essence, is a person’s weight healthy or unhealthy? This is then used in turn, as a predictor for developing chronic illness; heart disease, high blood pressure, diabetes etc. This method of assessment does have its critics however, so should we be so reliant on it as a measure of our wellbeing?

Jacques Quetelet, a Belgian astronomer, statistician, and sociologist, established the body mass index in the 1800s, for the original purpose of population studies (1). His formula to calculate BMI, was to divide a person’s weight by the square of their height, or BMI=kg/m2, this figure was then compared to a standardised chart. A BMI below 18.5 is considered underweight; 18.5-24.9 is healthy; 25.0-29.9 is overweight; and 30-39.9 is obese (2).

Example 
Weight = 70kg 
Height = 1.75m 
70/(1.75 ☓ 1.75) = 22.9

BMI 22.9 = Healthy

Today there are many BMI calculators available which will do the maths for you, empowering us all to monitor our numbers and reflect on our "health". What is really interesting however, is that Quetelet stated that his method was not useful in the study of individuals (3).

There are many who condemn BMI as a method of assessment, suggesting it has outlived its use and should be replaced. As an example, The Conversation, an independent source of news and views, sourced from the academic and research community in July 2018 asked five experts, including one professor, three associate professors and a research fellow whether BMI was a good way to tell if weight is healthy? All came back with an emphatic NO! (4). Similarly, in 2013 Professor Nick Trefethen, a mathematician from Oxford University wrote to The Economist, calling BMI a 'bizarre measure' (5). Being a mathematician, he devised his own calculation for assessing BMI, but his version does not seem to have caught on.

Categorising people as being over or underweight, obese or morbidly obese and at risk of chronic illness, using only a simple and centuries old equation, does appear to fly in the face of our modern world and modern-day needs. Especially when you consider that all of this can be achieved without anyone presenting for a face-to-face consultation. One of the main points of contention amongst BMI critics is that the measurement does not take into consideration body composition (the percentage of fat to lean tissue an individual may have). Equally, other significant health factors such as cholesterol, sugar levels, lifestyle habits, and physical activity are also not factored in. So are we right to question BMI as the most reliable method?

Consider two people, same height, weight, gender, and age. Both would register the same on the BMI scale. If however, one is an athlete, rugby player or even a body builder, and the other is completely sedentary, could they both be considered equal in terms of general health and potential risk factors? Alternately, if both were equal in terms of their physical activity, but one is a smoker or a heavy drinker, would BMI be enough to measure their wellbeing? Is BMI alone sufficient to establish good health?

These examples highlight how open to potential inaccuracies this method could be. Yet as our anecdotal Take Five piece, 'A Tall Tale' shows, BMI is still intrinsic to our healthcare system today.

There are other methods of assessing body mass in relation to what are deemed 'norms', so if we accept the limitations of BMI as a given, are other methods more reliable? Or could they also produce inconsistent results?

Height and Weight Charts

Simple height and weight charts offer very general guidance on a persons weight, providing a scale to indicate what is considered normal, overweight, and obese. Many such guides are easily available on the internet, simply enter your basic metrics and obtain a quick result. Continuing with the theme of BMI however these could also fall into the problematic category of being too overly simplistic.

Waist Circumference Measurements

Waist measurements are often used as indicators for the risk of diabetes, coronary heart disease and other problems associated with excess body weight. According to the British Heart Foundation, this is the preferred method to use for sports persons, who have a higher percentage of lean muscle mass than adipose (fatty) tissue, and possibly a smaller waist.

Guidance
Male 
94cm (37 inches) - low risk 
94-102 cm (37-40 inches) - high risk 
102cm (40 inches) and over - very hight risk

Female  
80cm (31.5 inches) - low risk 
80-88cm (31.5-34.6 inches) - high risk 
88cm (34.6 inches) and over - very high risk (6)

There are some very specific guidelines on how to take a waist circumference measurement. This is important to ensure the consistency of calculations over time as errors can be easily made, leading some to question its reliability.

The NHS guide to taking waist measurements

Find the bottom of your ribs and the top of your hips.
Wrap a tape measure around the midway point between these landmarks.
Breathe out naturally before taking the measurement (7).

Hip to Waist Ratio

Again there are online calculators to assist with this measurement (8), which can be an indicator of body shape. In simple terms the formula divides your waist size by your hip size. Again however accuracy with assessment is vital for the effective production of data over a given time period. Errors with consistency can again lead to unreliable results.

A guide on taking hip to waist measurements (8)

Hip measurements should be taken at the widest part of your buttocks.
Your waist should be measured just above your belly button.

It has been suggested that this method is more accurate than BMI for predicting some chronic illnesses. A study of 15,000 adults, concluded that a high waist to hip ratio was a good indicator for the increased risk factors of early death, even in people with a normal BMI (9).

Guidance
Women
0.80 or lower - low risk
0.81–0.85 - moderate risk
0.86 or higher - high risk

Men
0.95 or lower - low risk
0.96-1.0 - low risk
1.0 or higher - high risk

Modern Measurement Devices

There are many modern devices aiming to provide quick data without maths, charts and complex calculations. Body fat calculators, bio-impedance scales and body composition analysis machines can all be bought for personal use and are ever increasingly available at local gyms. They tend to provide a non-invasive measurement of overall weight, body fat, lean tissue mass, hydration and BMI. Such devices can however also produce inaccurate results, it has been suggested by as much a 21-36% (10). Taking a scan when poorly hydrated, directly after training session, or following a meal or snack can all affect the result. They also tend to be less accurate in those individuals classified as obese.

In April 2020 the World Health Organisation (WHO) announced that in 2016 the amount of people over 18 and overweight worldwide was a massive 1.9 billion; of these 650 million were classified obese. In that same year over 350 million children and adolescents aged 5-19 were considered overweight or obese. They also stated that in 2019, 38 million children under five were overweight or obese.

At present BMI is still the accepted method of assessment, the WHO consider BMI to provide "the most useful population-level measure of overweight and obesity as it is the same for both sexes and for all ages of adults" (11). Yes it is simple and easy to apply consistently and with accuracy, but it should be considered a guide, not an absolute. The WHO temper their stance saying that BMI "should be considered a rough guide because it may not correspond to the same degree of fatness in different individuals" (11), acknowledging that high weight does not always equal high levels of body fat. Although the majority of the population are not athletes or professional sports people, we would still advocate any assessment being completed in person with a trained practitioner for the most considered approach. This seems the best common sense application of BMI as a measurement. Yes it has flaws, but so do the available alternatives, so perhaps the best use could be to employ a range of methods, to enable optimum accuracy and more careful and individualised assessment.

LIFESTYLE DEVELOPMENT SCIENCE

Is BMI an Accurate and Relevant Measurement?

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