There is considerable evidence that a constitutional susceptibility to fat gain is necessary to become obese under the pressure of an obesogenic environment. Only children highly sensitive to abundant palatable food and conditions of decreased physical activity are at risk of developing obesity. A high-risk environment favouring obesity has no effect on non-predisposed children. The obesogenic environment is therefore a trigger but not the primal cause of obesity. It just allows the phenotypic expression of the genetic predisposition to obesity.
As the majority of children are not constitutionally predisposed to obesity, prevention strategies should not be addressed to the whole paediatric population but targeted to children at risk. The disappointing results of the large majority of previous collective interventions to prevent obesity, particularly at school, support this opinion. Moreover, this preventive approach may increase discrimination of obese children and induce eating behaviour disorders in a large number of non-predisposed children. Children at risk of becoming obese are those with at least one obese parent and those who present an adiposity rebound before 6 years of age. These at-risk children must be detected early in order to benefit rapidly by personalised intervention to prevent further fat gain. Limitation of food intake and recommendation of a less sedentary way of life are currently the two sole solutions. However, their efficacy, even very early in life, remains to be demonstrated. In the future, the improvement of obesity pathophysiology knowledge will probably allow the development of more promising preventive approaches.