Background: Adolescents are the age group at the highest risk of developing disordered eating behaviors (DEB) due to the physical and psychological challenges they face. Moreover, Mexican adolescents engage in less moderate or vigorous physical activity (PA) than what is considered desirable. According to the World Health Organization, levels of physical inactivity are rising worldwide, increasing the prevalence of non-communicable diseases such as obesity, one of Mexico’s main public health problems. However, when a person’s attitude towards exercise becomes extreme or obsessive, it may become in a DEB.
Objectives: To evaluate the effects of a universal prevention program for disordered eating behaviors (DEB) and sedentary lifestyle for Mexican adolescents.
Methods: This was a quasi-experimental field study with repeated measures. Participants were 527 adolescents (240 females and 287 males) aged 15 to 19 at two private high schools in Hidalgo, Mexico. One school was allocated to the experimental group (49.5%) and the other to the control group (50.5%). Both females and males were given the Mexican Brief Questionnaire for Disordered Eating Behaviors and the short form of the International Physical Activity Questionnaire (the version validated in Mexico). In addition, male respondents answered the Mexico version of the Drive for Muscularity Scale (DMS). All participants were asked to complete questionnaires on four different occasions: before the intervention (pre-test), a week after the intervention (post-test), and at 6-and 12-month follow-up. The programme (Prevención de Conductas Alimentarias No Saludables y Sedentarismo [PECANSS]) consisted of five activity-based sessions. In the first four sessions, the following topics were addressed: the culture of thinness, myths and realities about dieting and supplements, healthy eating behaviors and healthy menus. At the end of each session, students engaged in an hour of enjoyable physical activity.  In the fifth session, participants were divided into three parallel workshop groups focusing on the culture of thinness, healthy menus, and physical activity. The programme was the same for females and males, and incorporated a variety of dissonance-based interventions using interactive psychoeducational strategies
Results: After a year, repeated-measures analyses of variance (ANOVAs) showed a significant reduction in the mean DEB scores in the experimental group (F(1,350)= 4.79, p=0.029, η² = 0.014). Mean DMS scores in males in the experimental group decreased significantly over time (F (2.55, 221.91) = 11.20, p<0.001, η² = 0.114) in contrast with those in the control group (F (2.69, 231.44) = 1.79, p=0.154, η² = 0.02). The PA frequency and duration did not show a significant increase over time in the experimental group.
Conclusions: Even though the effect sizes in the analyses were low, probably as the result of the floor effect, the implementation of the program had positive effects on the adolescents.

Significance to the Field of Dietetics:

PECANSS is an important strategy in the field of dietetics, because it prevents DEB, also known as unhealthy weight control behaviors, which increase the likelihood of developing eating disorders such anorexia nervosa and bulimia nervosa. Moreover, in males, PECANSS is able to reduce behaviors and attitudes associated with the aim of gaining muscle mass (such as steroid use), which have been found to be associated with the drive for muscularity, considered a risk factor for the development of muscle dysmorphia. It is a well-known fact that all these pathologies have a major impact on the nutritional status of individuals, as a result of alterations in the quality and quantity of food intake. 
At the same time, PECANSS promotes the performance of healthy PA, which has been linked to overall health improvement. Regular PA has positive effects, for example, on the control and prevention of non-communicable diseases such obesity, hypertension and diabetes mellitus type II.
It is important to note that sessions 3 and 4 contain nutritional content to correct false beliefs about eating and nutrition as well as to provide knowledge on nutritional food content, healthy eating behaviors and sample healthy menus.

Disordered eating behaviors and sedentary lifestyle prevention program for Mexican adolescents.