The prevalence of individuals with an elevated body weight has markedly increased since the 80’s, up to the point that a new diagnosis category has emerged “severe obesity” defined by a body mass index (BMI) of 40 kg/m2 and more. Indeed, whereas more than 40% of the US population now reach the criterium for the diagnosis of obesity, more than 9% are considered as being characterized by severe obesity (1). This subgroup of individuals represents the fastest growing relative weight category in the US, a statistic highlighting the inability of the scientific community to curb this “lifestyle” epidemic. This phenomenon with major socio-economic consequences has been observed despite the fact that many countries have launched Obesity societies over the last 40 years, producing a still growing biomedical research community trying to address this public health issue. With more and more sophisticated techniques to study energy metabolism, energy balance, including better imaging techniques to precisely assess body composition, a tremendous amount of knowledge relevant to the development and management of excess adiposity has been produced. However, all this knowledge has failed to influence populational body weight trajectories of most countries around the world. Thus, the question remains: what type of science do we need in order to effectively address this challenging problem from the perspective of patients, clinicians and public health agencies?

We would like to propose that, as it was the case for the way we used to communicate on the link between diet and health (the culprit in the 80’s being dietary fat), we have a problem when comes the time to explain to the lay public and to health care practitioners what is excess adiposity as a health issue.

For instance, a simple question such as “what is obesity” has not been properly answered yet. The WHO defines it as an excess amount of body fat that may have negative consequences on health. Although there is nothing fundamentally wrong with this definition, the tool used to make its diagnosis (BMI≥30 kg/m2) in clinical practice has major limitations. Although the BMI may be an adequate tool to crudely monitor the evolving prevalence of excess adiposity at the population level, it only provides a crude assessment of the amount of body fat, particularly in the normal weight, overweight and class 1 obesity ranges where substantial individual variation in body composition can be found. Commonly used examples to illustrate the limitation of the BMI are the heavy and muscular American football or male and female rugby players. Furthermore, cardiometabolic imaging studies that have used sophisticated imaging techniques have revealed that this anthropometric index is of no use to assess individual variation in the regional distribution of body fat (2).

In order to address this issue, a Lancet Commission of international experts is currently working to come up with a definition of clinical obesity based on signs and symptoms rather than on weight and height (3). In the present narrative review, evidence from studies showing the remarkable individual differences in the way humans store energy as body fat will be reviewed, with a discussion of their consequences on cardiometabolic and mental health. Clinical and public health implications of these findings will also be discussed.