A high Body Mass Index (BMI) does not necessarily mean that someone has obesity. Obesity is a complex, relapsing, multifactorial disease, characterised by abnormal or excessive body fat that impairs health. A diagnosis of obesity must be conducted by a trained healthcare professional in a medical setting. So, what is BMI?
BMI: A short history
BMI was invented as a way to estimate obesity at the population level in the 19th century by Adolphe Quetelet, a Belgian mathematician. Quetelet was not a physician, and he was apparently clear at the time that BMI was not a measure of individual health or body fat.
He was, however, a mathematician, one who felt that the formula of dividing weight in kilograms (kg) by height in meters squared was an easy way to estimate obesity in a population.
BMI: A screening tool for obesity
According to the standard WHO definition, obesity is equal to a BMI ≥30 kg/m2. This is important as part of a set of tools since while a high BMI may be indicative of a presence of obesity in an individual, it is not infallible. After all, BMI is a measure of size, not health. It is useful as a screening tool for obesity but has some limits as a diagnostic tool. BMI has limited predictive value for estimating body fat and lean mass at an individual level. Individuals therefore may have a high BMI due to high muscle mass rather than body fat i.e. Sports professionals.
Many have picked up on the limitations of BMI over the years and while BMI should not be immune to criticism (it is flawed after all), the fault may be more in our unrealistic expectations of how complete BMI was as a tool and less in flaws with the methodology itself. BMI cannot tell us anything about the impairment of health, it cannot distinguish between fat and muscle, and it cannot tell us anything about fat distribution. It also cannot distinguish between ethnicity, age and sex. It has its limitations, and it should have never been framed as the sole way to diagnose obesity.
BMI: The pros
But BMI is useful. It is cheap, simple, can be done at home, and can serve as a screening tool for obesity. An individual with a high BMI who thinks their weight impairs their health should see a healthcare professional who will be able to determine the presence or absence of obesity. BMI alone cannot diagnose obesity, but it can help determine who is at risk.
Additionally, given that rates of obesity are highest and growing most rapidly in low- and middle-income countries and that BMI measurement is a cost effective useful tool for describing the growing disease burden, it should continue to be used to provide a rough description of obesity within particular populations, while complementing BMI with other potential means of arriving at a more accurate diagnosis.
BMI: An indicator to estimate population health
Monitoring population health is important for resource allocation and policy planning and implementation. However, it is not feasible for everybody to be evaluated by a healthcare professional for obesity. So, we use BMI – a cheap, accessible alternative - as an indicator of obesity.
This practice is widespread, and BMI is used in most national and international epidemiological surveys. It enables us to monitor trends in obesity and it is often the backbone of evidence that advocates put to policymakers and decision-makers. BMI is not a perfect indicator (as individuals can have obesity at a range of BMI levels and individuals can experience health over a wide range of BMI levels), but it is the most widely used.
The Global Obesity Observatory
Our Global Obesity Observatory is the most comprehensive database containing obesity-related data. In it, we collate data from national and international surveys from countries around the world. Most surveys measuring adult obesity use BMI as an indicator, for all the reasons we mentioned above. It has allowed countries to estimate and monitor prevalence for years, and it is likely to continue doing so.
BMI is a powerful and useful tool but as with many surveillance tools that attempt to measure heterogeneous populations it has its limitations. We must be clear on what it can and cannot do, and what it does well is allow us to estimate health at a population level.