This debate has been re-opened in the Lancet recently (Sharma, 2023). This is the basis for this blog.
According to the World Obesity Federation, obesity is a chronic, relapsing, progressive disease. This view of obesity, however, is not without controversy.
Pros and cons of BMI measurement
The controversy of whether obesity is a disease is in part fuelled by its rather arbitrary historical definition, which relies on cut-points for body mass index (BMI). Thus, in Caucasians, obesity has been defined as a BMI equal to or greater than 30 kg/m2. Although health risk rises proportionally with an increase in BMI, when applied to an individual, BMI is neither particularly sensitive nor specific in terms of assessing current health status.
This is not surprising, as BMI, based on height and weight, is not a reliable measure of body fat, nor does it capture body fat distribution, adipose-tissue morphology, or adipose-tissue inflammation, all of which play important roles in the health risks associated with excess body weight. Thus, for example, an elevated waist-to-hip ratio is associated with increased risk of heart disease across the entire spectrum of BMI, with BMI itself adding little to the equation.
Fit and non-fit ‘fat’ and ‘thin’ people
Indeed, it is possible to identify rather healthy individuals even amongst those with higher BMIs (the ‘fit fat’). Conversely, there are individuals with metabolic abnormalities (e.g. pre-diabetes or non-alcoholic fatty liver disease) who currently fall below the conventional BMI cut-offs for obesity (‘thin-but-metabolically-obese’), but may benefit from weight loss.
Alternative definitions of obesity
Recognition of the limitations of BMI has spawned the proposal of several disease staging systems for obesity that rely on the demonstration of health impairments rather than on anthropometric measures alone. These include the King’s System, and the Edmonton Obesity Staging System (EOSS).
The recent 2020 Canadian Clinical Practice Guidelines for adults defines obesity as ‘a prevalent, complex, progressive and relapsing chronic disease characterised by the presence of excess or abnormal body fat (adiposity) that impairs health’. The key word here is ‘impairs’, so to meet the criteria of a disease, there has to be a demonstrable impairment in health.
A person with a BMI above the accepted cutoff, who is otherwise healthy, would, by this definition, be considered to have ‘adiposity’ but not ‘obesity’. Although BMI and other anthropometric measures (e.g. waist circumference), may be used to screen for obesity, they should not be used to make the diagnosis.
Obesity vs pre-obesity vs adiposity
In practical terms, this definition requires the health professional to answer the following question: Does this patient present with a health problem that is likely to improve with weight loss? If the answer is ‘yes’, then the patient has ‘obesity’. If not, then the patient may just have ‘adiposity’, which may well at some stage progress to obesity (hence the suggestion to refer to these individuals as having ‘pre-obesity’).
