Obesity has long posed problems to the medical world, in terms of coverage and therapeutic care. Its genetic, behavioral and socio-cultural components make healthcare management of overweight difficult.

The WHOs observation of increasing obesity these past years in the general population, and the progress made in understanding its pathophysiological mechanisms, lead to the conclusion that obesity is a disease and that it is a public health issue.


Obesity is defined as an excess of fat that leads to health drawbacks. It is associated with increased mortality and morbidity, in particular of a cardiorespiratory or metabolic nature (diabetes, hyperlipidemia).

To these somatic consequences, one can add the psychological and social factors that can influence quality of life.

Obesity is a heterogeneous clinical entity that results from the interaction of biological, psychological, and behavioral determiners, as well as environmental factors. In recent years, the knowledge on the regulation of energy reserves has progressed while, in clinics, the therapeutic objectives have been better defined.

Strictly speaking, the diagnostic of obesity should be based on a measure of the fat mass, such as the BMI (body mass index). The BMI is the ratio of weight (kg) divided by the height (m), squared, of the subject. This number gives an approximate but sufficient evaluation of the fat mass. Devised according to the level of risk associated with that BMI, a classification of obesities has been established by the World Health Organization (WHO). A BMI under 18.5 kg/m2 characterizes thinness; between 25 and 30 kg/m2, simple overweight; a BMI superior to 20 kg/m2 is considered to have excessive fat mass, because the individual presents seriously higher risks of morbidity and mortality.

Moderate obesity is defined by a BMI between 30 and 35 kg/m2, whereas obesities higher than 35 kg/m2 are severe up to 40 kg/m2, and recognized as “morbid” when higher than this value.

These definitions do not take into account age, sex, ethnicity, age of onset of condition, duration of condition, or the distribution of fat tissue; all these are elements that can increase the risks linked to overweight. At an individual level, and without waiting to reach the 30 kg/m2 threshold, overweight can be considered a medical problem if it increases the risks of contracting an associated disease, such as diabetes.

On the contrary, in some individuals whose BMI is higher than 30 kg/m2, the excess of weight can be perfectly tolerated. If we add to that that everyone has his or her own perception of “desirable weight”, influenced by sociocultural stereotypes, and that this will determine his or her possible desire to change, we can understand that a purely statistical definition of obesity has its limits when confronted to certain individual situations. It is important in each case to synthesize by taking into account not only epidemiological data, but also those that are specific to each subject, whether objective (state of health, age, fat tissue distribution, family antecedents, blood pressure, etc.) or subjective (perception of state of health, self image, etc.).


The prevalence of obesity varies by country, region, socio-economic category and age category (the clearest increase in weight affects the 35-44 and 45-54 year old categories). In North America, it is twice as prevalent as in Europe. Obesity cannot be considered a rich country´s problem, however. Developing countries and so-called “emerging countries” have seen a spectacular increase in recent years.
In France, obesity affects about 8% of adults; it is close to the levels in Sweden and the Netherlands, and by far inferior to those of Great Britain, Central Europe, and the United States (25% to 30%).

The increase of obesity in France during the last decade of the 20th century seems more limited than in Anglo-Saxon countries. However, its increase among children gives reason to worry and is a probable cause for an increase in adult obesity in the future. There are regional differences: obesity is twice as frequent in the north of France than it is in the south. In industrialized nations, obesity is far more common among the poorer classes.


Obesity is the manifestation of a dysfunction in the energy balance, which leads to an increase in the stocking of energy in the form of fat in adipose tissue. This imbalance results in the interaction of factors of genetic predisposition with behavioral and environmental factors.


Whatever the level of genetic factors, it is important to note that the determining factors are environmental and behavioral. The frequency of massive obesity has doubled in the last fifteen years in Great Britain, during which the hereditary characteristics (the genome) of the British people did not have time to change. Western lifestyles (in industrialized countries) facilitate the emergence of obesity.

The increase in the caloric density of foods, the reduction of consumption of complex carbohydrates (fibres, starch), the disintegration of eating rhythms, the diversity and the availability of foods are so many factors that can defeat the physiological regulatory mechanisms of the energy balance. The reduction of energy expenditure (exertion) linked to means of transportation, heating, decrease of physical labor, changes in the means of acquiring food, changes in leisure (more sedentary), all this plays a role in the development of many cases of obesity. Finally, one must take into account psychological factors that influence eating behaviors. For example, anxiety and depression can cause impulsive eating that increase the daily caloric intake. One must finally also mention the influence of the family environment (eating habits and conditioning), the social environment, and the economic environment. Everything in a consumer society contributes to favor obesity in genetically predisposed individuals.


Obesity evolves in different ways: a build-up phase, called “dynamic” phase, during which the individual goes from a “normal” weight to a weight excess; a static phase, in which the individual maintains his weight; phases of weight loss linked to therapeutic interventions; and phases of regaining lost weight (successive failed diets).

The build-up phase indicates a positive energy balance, whatever its origin (increase in intake and/or decrease in exertion). The static phase comes from a new balance: weight is stable, energy intake and expenditures balance out. During periods of weight loss, the body adapts by decreasing its energy expenditure in order to avoid undernourishment: every diet therefore encounters a limit of “effectiveness” which manifests through a new stability in weight. This adaptation mechanism kicks in at different levels of weight loss and food restriction from one individual to the next. The consequence is that weight loss capacity also varies from one individual to the next, particularly in the case of age. This must be taken into account when establishing therapeutic goals: a subject that is predisposed to overweight must have realistic weight loss goals. These goals can and should be determined according to overweight history, evolution of the weight gain process, food intake, and family antecedents.

Another particularity of obesity is its tendency to spontaneous aggravation: it is a chronic situation that relapses as soon as the treatment is suspended. Excessively restrictive diets, followed repeatedly, can facilitate the development of “resistance” to weight loss, which makes the problem worse and makes further successive attempts less and less effective (also called the yo-yo effect or cycle dieting).


The major complications linked to obesity are diabetes, insulin resistance, hyperlipidemia, high blood pressure, and sleep apnea are all diseases for which overweight increases the risk. More than 80% of non insulin-dependant diabetics are obese, and 30% of subjects with serious obesity are diabetic. Abdominal obesity, also called Android obesity (characterized by an increase in fat in the abdominal cavity) facilitates the development of metabolic and cardio-vascular diseases.