<ce:para>From a sociological perspective, physical activity and diet are perceived as social and cultural practices, constructed and transmitted within human societies. The body is then thought of as a social construct, a sign and foundation of individual and collective identities. In this context, this article was designed to highlight some social processes underlying the obesity epidemic. Clarifying issues about a medical definition of obesity in an obesogenic society, and theoretical approaches to the meanings of the obesity epidemic are proposed.</ce:para> <ce:para>Individual stories of a gradual shift towards obesity are presented to illustrate the variety of trajectories that can lead to obesity in adulthood but also the variety of subjective experiences about the situation of obesity. In particular, the relationship to the body and experiences in terms of physical activity are investigated in order to understand how obesity is associated with non-commitment, low commitment or abandonment of physical activity. The issue of configurations in which commitment or re-commitments in regular exercise for sedentary populations can be possible are discussed. The discussion shows that although commitment to regular and sustainable physical activity requires a profound transformation of lifestyle for the persons concerned, the collective dimension of this change is rarely taken into account.</ce:para>.