The Truth about BMI

Written by Lucy Walton

Body Mass Index (BMI) is used to measure an individual’s health in many health care settings and in national guidance. But what is the history behind BMI and is it a helpful and accurate measurement of health?


What is BMI?

BMI is a measure that uses height and weight to generate a score, which places individuals into different weight categories.


If your BMI score falls out of the ‘normal’ range, it is considered to be ‘unhealthy’ in our society. We are told that ‘health’ depends on an ideal body size with little regard for other factors including behaviours and measures such as blood pressure and heart rate, physical activity levels or lifestyle behaviours.


The History of BMI

In 1832, Adolphe Quetelet, a mathematician, created “the Quetelet-Index” (later named BMI) as a way to measure body weight in different populations (1). Until the 1970s height and weight charts were used to measure body composition with different charts for men and women. However, researchers started to notice that BMI appeared to be linked to overweight related problems so began to use it in population based studies, and then on an individual level.


BMI has been widely adopted in the medical world ever since as a way to categorise individuals into ‘healthy’ and ‘unhealthy’ categories based on their weight for height. Interestingly, Quetelet stated that the Quetelet-Index (BMI) was never meant to be used on an individual level.


How BMI has changed?

BMI as we know it today is not how it has always been. Originally, the ‘normal’ range for BMI was 20-27, the category is now set at 18.5 – 24.9 with the addition of stigmatising labels added too. A BMI of 25-29.9 is categorised as ‘overweight’ and 30+ as ‘obese’. The change, made in the 1990s, resulted in individuals becoming ‘overweight’ overnight and increased fear of weight gain and the “obesity epidemic.” The BMI was increasingly being used worldwide, but with conflicting research to support the intended use of the measure.


So why the change? It could be argued that pharmaceutical giants looking for profit are at the centre of the change. Two of the brands behind the biggest weight loss medications available at the time of the change, began to fund research studies. These studies would conclude positive findings in favour of weight loss for health, in turn funding the companies medications. The issue regarding conflict of interest due to the involvement of these brands were never considered.


To this day, some doctors and healthcare professionals are trained in using BMI incorrectly. Assumptions are made that if an individual is in the overweight category they will have health problems and if they are in the obese category more significant health concerns will arise.


This is not the truth however. Studies have found that individuals in higher BMI categories who make positive lifestyle choices i.e don’t smoke, drink within the recommended guidelines, eat five portions of fruit and vegetables a day a day and engage in regular physical activity have been shown to have a similar mortality risk as those who are of a ‘normal’ weight and also engage in those health-promoting behaviours (2). Not only this but it is estimated the risk of mortality for individuals in the 'overweight' category is 94% less than for those in the 'normal' weight category (3).

Is BMI a good measure of health?

BMI uses just two points of data, weight and height. It BMI doesn’t account for the body composition of an individual for example body fat percentage compared to lean body mass or physical activity status. This means that muscular people, including athletes, are often classified in the ‘overweight’ or ‘obese’ BMI categories despite being metabolically fit and physically well. Studies have shown that you can be metabolically fit and fat which means individuals in the ‘overweight’ or ‘obese’ BMI categories may also be wrongly stereotyped as ‘unhealthy’ [4]


Additionally, age, genetics or sex are not accounted for. Females naturally have a higher fat mass than males for many reasons, including hormone production and evolution in order to child bear. This is normal, yet when using BMI on an individual level, they are treated exactly the same.


Another thing BMI fails to measure, is an individual’s relationship with food. This may include what they are eating and how much/often as well as how flexible they are with their intake and variety. Without this, health professionals who do not have the appropriate knowledge about BMI might assume answers to these questions or to ignore them which could lead to medical needs being missed or undiagnosed. An example of this could be an individual in a “normal” BMI range having a medical condition overlooked because they are assumed ‘healthy’. Equally, individuals in larger body’s may be told to lose weight without proper medical investigations for their symptoms. Unfortunately, in the case of eating disorders and for this reason, many people do not receive the treatment they deserve.

How should we use BMI?

When BMI is used as a stand-alone outcome measure, it can take the focus away from important lifestyle behaviours such as good quality sleep, nutritional intake, managing stress, movement and physical activity, smoking, alcohol intake and social interaction [5].


Like any measure, it is not perfect and is known to both overestimate and underestimate adiposity (fat) levels in individuals [1,6]. While BMI can be a helpful measure at a population level [1], using it at an individual level without considering a person’s history and other lifestyle behaviours is not accurate and can cause more harm to a person’s physical and mental health.


What about in eating disorders? 

BMI is a tool that is used in eating disorder treatment, both for diagnosis and to guide recovery. There is a debate among health professionals, people in recovery and those that have recovered about the suitability of using BMI. As mentioned, when used as a stand alone measure, there will always be negative consequences of using BMI, especially in eating disorder recovery. Some examples of this include:

  • Some treatment services have BMI cut-offs at a very low weight which results in many individuals being turned away from treatment (remembering that the majority of people with eating disorders are not underweight)

  • Using BMI to diagnose anorexia nervosa can result in people who have the diagnosis of "atypical" anorexia nervosa feel that they are not sick enough to deserve or access treatment

  • While BMI can indicate severe malnutrition, you can be malnourished at any weight or shape (i.e. weight above the "underweight" range) with significant weight loss or severe energy deficiency

  • Not many people naturally sit below a BMI of 20 and unfortunately, many health professionals (and the NHS) use outdated BMI criteria listing the "normal" weight range as 18.5-25, instead of the 20-25 used by most eating disorder services. This can lead to many people in recovery stopping weight restoration prematurely out of fear of gaining too much weight

  • In recovery due to fear of weight gain, many people will become stuck, controlling their weight to hug the lower end of the "normal" weight range which inhibits full weight restoration and recovery

  • Using BMI as the only measure to guide weight restoration, without considering a person's weight history, medical