Child and adolescence overweight and obesity prevalence has been increasing worldwide during the last decades. Overweight and obesity are considered the most widespread disorders in Europe, affecting, in 2002, approximately 1 in 6 non-adults and in some parts of Europe up to 1 in 3. Adolescents with a body mass index (BMI) equal to or greater than the 85th percentile are at increased risk of obesity in adulthood 1 . Thus, overweight and obesity prevention is an international public health priority requiring the implementation of effective interventions to produce changes in dietary and physical activity patterns in individuals. Two systematic reviews with inconsistent results have been published in this field 2 3 , and a recent commentary review explained the discrepant results 4 as being the heterogeneity of the studies in terms of target population, theoretical underpinning, study design and outcome measures.

Only one study in each review targeted adolescents, which confirmed that most programs and studies involve children. However, during adolescence, children are becoming independent and self-determined enough to establish eating habits and physical activity patterns. Besides communities and families, schools have been identified as key settings for public health strategies to lower or prevent the prevalence of overweight and obesity 5 . Fifteen-year-old adolescents spend more time at school than at any other setting outside of the home. The school food offerings potentially have a large impact on adolescents' eating habits because many students, especially those who board full-time or half-time, consume a substantial proportion of their total daily intake at school 6 .

Many theoretical considerations underpin the choices, orientations, ways and means of implemented intervention strategies such as healthy eating, nutritional education, physical activity and environmental modifications. Stand-alone interventions or integrated interventions have discrepant effectiveness. The Ottawa charter provides a framework for health promotion actions around 5 means, of which 3 are particularly relevant in this field and context: develop personal skills, reorient health services and create supportive environments 7 . The contribution of each to overweight and obesity prevention alone and in combination has not been extensively explored. Such information would be of great interest for improving public health policies. In 2001 in France, the government set up a National Nutrition and Health Program ("Programme National Nutrition Santé", PNNS) to enhance the global health status of the population by improving nutrition. One of the main objectives was a 20% reduction in excess weight and obesity prevalence among adults and to stop the increase in obesity prevalence among children and adolescents 8 . Research results are awaited the plan renewal.

A powerful trial with an appropriate design - namely clustering and factorization -- and with wide outcomes from knowledge to anthropometric measurements is needed to measure the long-term impact of such health promotion strategies among adolescents in schools. The present report describes the design, implementation and baseline characteristics of clusters and participants of the PRALIMAP (Promotion de l'ALImentation et de l'Activité Physique) trial, a 2 × 2 × 2 factorial cluster, school-based randomised intervention trial testing the effectiveness of 3 overweight and obesity prevention strategies in adolescents.

The need for randomised trials of complex interventions such as health promotion are high, but such trials are a relatively new phenomenon 2 3 , and their role is still not self-evident in public health nutrition research. Clustering, multiple factorial and long-term duration are particularly suited for health promotion trials intended to provide high-quality evidence to support public health policy 30 . Such trials allow for implementing interventions in real conditions within appropriately diverse populations from heterogeneous settings and reporting on a broad range of health outcomes.

Cluster randomised trials are a common and necessary design for assessing community interventions, especially when they involve environmental actions and rely on interactions between subjects. This type of trial has methodological difficulties 31 32 and is still not well reported 33 . We paid attention to the building of clusters for representativeness at the regional level and a minimal clustering effect through stratification; to the sample size calculation, taking into account several ICC estimates for each outcome 31 ; and finally to the reporting process according to the CONSORT statement extended for cluster randomized trials 28 . A limitation of clustered randomised trials is that the interventions cannot be blinded. This potential bias is minimized since we ensured randomisation by high school, that only school nurses are responsible for anthropometric measures, and the factorial plan created a combination of interventions.

The PRALIMAP trial incorporates a rare 2 × 2 × 2 factorial cluster randomised design. The design was selected to evaluate all 3 strategies and their potential synergy. Factorial designs have been used in individual randomised trials, but combined with clustering, fewer than 10 were 2 × 2 designs, and to our knowledge, only 1 was a 2 × 2 × 2 design 34 .

The PRALIMAP trial duration is in line with the Sharma et al. recommendations to provide interventions longer than 6 months 35 36 . The PRALIMAP interventions spread out over 24 months allows for drawing conclusions that are sustainable in the long run.

High schools were included in 2 waves, of 8 and 16 schools each. We chose this format because we were unable to implement the interventions and the measurements at the same time in the 24 schools spread over a 23,547-km² area and including more than 2,000 professionals to be informed and trained. This design appears to be superior to an experimental pilot site, through the dynamics created between the 2 waves of high schools and the ability to respect the randomisation plan (balance between strategies according to stratification criteria). A wave effect will be looked for and, if needed, taken into account in the analysis.

We chose adolescents as the target for the PRALIMAP intervention. During adolescence, individuals develop responsibility for health-related behaviours and attitudes that affect their future 37 38 . Moreover, eating habits initiated during this time are long-lasting 39 . Eventually, a strategy based on fostering personal responsibility, cognitive self-regulation and competence could be effective in improving healthful eating and physical activity behaviours among middle school children 40 . We did not involve parents in the intervention because adolescents' increasing independence around food choice is described as an act of parental defiance and peer solidarity 41 . Adolescents resolve the conflict between their need for autonomy over their food choices and the needs of others in the family by making their own meals, eating out, eating what is served, and negotiating to have their own and other family members' food choices and needs met 42 . This behaviour is one of the reasons why we chose a school-based intervention as opposed to family or community interventions. Another reason was that educational skills of professional teachers should increase the effectiveness of lessons introduced into the curriculum. The school has been described as an ideal place to run prevention interventions considered an integral part of the educational mission in France 43 , as well as internationally 5 44 , and to avoid known financial barriers in prevention access.

In terms of national and regional academic programs, schools are free to choose the form in which they provide education for their students, and nutritional education programs such as that in the PRALIMAP trial are used in many French schools without the need to obtain parental consent. Moreover, schools modify their environment on their own. Parental consent is required not for the intervention per se but for the outcomes measurements. Although schools were randomly assigned without consent from adolescents and their parents, both groups received information and could decline to participate in completing questionnaires and measuring body size. Information, access to data and the right to withdraw participation is warranted by French law after approval by adequate committees.

Summerbell et al. reported that studies focusing on the combination of dietary and physical activity did not show a significant improvement in BMI but that some studies focusing on dietary or physical activity alone showed a small but positive impact on BMI status; however, nearly all studies found some improvement in diet or physical activity behaviour 3 . In contrast, Sharma et al. showed that interventions targeting both physical activity and dietary behaviour were successful in influencing adiposity indices 36 . The PRALIMAP trial aims to focus on both diet and physical activity (as part of an integrative nutritional approach) in each of the 3 strategies.

Nutritional education has been evaluated in adolescents in a few studies with varying design and effectiveness 2 35 45 . A systematic review of published and "grey" literature 35 reported a moderate effect in adolescents 13-18 years of age. However, most of these interventions were classroom-based activities with an adapted curriculum. The PRALIMAP trial involves lectures in a quantity corresponding to the median yearly number of hours devoted to this task in other countries 6 but also experiments with other kinds of education through autonomous collaborative works and nutritional parties. These activities may lead to better motivation in students, thanks to a more convivial, pleasant and positive approach to nutrition and to skills acquisition and empowerment.

The PRALIMAP screening strategy is a new concept of school-based screening combining school-based measurements of BMI associated with waist circumference to define overweight and obesity and school-based care. At the onset of the trial, this new approach aroused debates among high school professionals who considered that the high school should not be viewed as a place for health care and among health professionals who are not well trained in proactive strategies of health problems management, being more comfortable with subjects contacting them directly with a care demand. However, Kubik et al. 46 described schools as a setting for obesity prevention (primary and secondary) and particularly highlighted the school nurses' responsibilities as vital but underutilized in delivering school-based obesity prevention. Moreover, the American Institute of Medicine recommended in 2005 to measure BMI in school and to report the results to parents 5 . Some school-based screening programs, practice, and effectiveness have been evaluated, and guidance has been provided for implementing such an approach 6 12 47 .

In the PRALIMAP trial, waist circumference is associated with BMI to determine overweight and obesity because this measurement is convenient, simple to measure, and correlated with BMI, an approximate index of total body fat, and can be used for longitudinal assessment in management 44 48 . The measurement allows for avoiding false positives among athletic students in specific "sports and study" programs.

The proposed care of the PRALIMAP relies on the therapeutic education concepts in a stepwise collective approach that split up the intervention into several stages delivered by a multidisciplinary team, as recommended by the US Preventive Services Task Force 49 . Behavioural interventions were reported as probably safe in children 4-18 years of age and can be effective 50 . Barton et al. 49 showed that low-intensity interventions may be feasible for primary care but did not demonstrate a significant consistent benefit with regard to BMI. However, evidence is still insufficient because of the limited number and sample size of available studies. In the PRALIMAP trial, the intensity of the intervention is low (10-14 hours), but weight outcomes are long term and the number of subjects is high.

The environmental strategy implies reconsideration of high school nutrition policy and functioning, which can be difficult for school staff. However, since 2007, such strategies must be undertaken in French schools with regard to the new recommendations for catering 51 . Moreover, high schools implementing the environmental strategy were specially funded by the Regional Council to help them improve dietary and physical activity school facilities and offerings. Only a few studies have evaluated school nutritional environment interventions, but none showed conclusive results in terms of adolescents' body size, and one found a positive long-term effect on only dietary behaviour 2 35 35 45 52 . In the PRALIMAP environmental strategy, a new tool was used to help catering staff improve meals by use of special software that allowed for observing food consumption every day and better adapting the offerings for students, especially for fruits and vegetables 53 .

The environmental strategy features an annual nutritional party, but its objectives differ from those of the education strategy party. In the environmental strategy, the party aims to help students discover new foods and physical activities to let them diversify their energy intake and expenditure, whereas the educational strategy party is a pedagogic way to evaluate and improve knowledge.

Multicomponent interventions promoting a healthy diet have been evaluated in high school-aged adolescents in European Union countries. These interventions were of limited effectiveness for self-reported dietary behaviour, and only one included anthropometric measurements, and results were inconclusive 35 . Eating patterns are more likely to improve when changes in the school environment are integrated with classroom nutrition education 54 . "Making healthy choices easier" is a strong recommendation for combining both strategies and needs to be evidenced 55 . In fact, no study has assessed the effects of environment and education strategies and their interaction. Because of its factorial design, PRALIMAP allows for determining an interaction between strategies and thus could provide information on an expected greater effectiveness of the combination of consistent educational and environmental strategies. Moreover, it allows for investigating a higher order interaction with the screening strategy. The combination of the 3 strategies could gather all conditions that could contribute to improving the prevention of overweight and obesity.

The PRALIMAP is a pragmatic trial where interventions were implemented in the real context of high schools. Effectiveness 56 is evaluated through body composition measurements, nutritional behaviour and knowledge outcomes, as was recommended by some authors 35 45 57 . The PRALIMAP outcomes rely on anthropometric data collected by high school nurses and on self-reported measures such as knowledge and behaviours collected in the classroom setting. At baseline, we achieved a high participation rate for both types of measurements (85.7%). The availability of the Board of Education database allows for comparison of the sociodemographic differences between participants and non-participants and suggests a probable higher non-participation rate among overweight students. However, non-participation in the PRALIMAP measurements is similar across arms. The turnover with entry level and attrition at each grade is common in schools and is likely to reduce power and even introduce bias if these data differ by randomisation arm. The design offers the opportunity to analyse the data in a longitudinal cohort approach, as well as in a repeated cross sectional approach. It offers the opportunity to combine hard outcomes (body size) with declared and perceived outcomes 58 (self-administered questionnaires) to explore the hypothetical cause from knowledge to health. Questionnaires used were standardised internationally or specifically designed for PRALIMAP. The time interval between the second and last visit measurements was shorter in the vocational than general and technological high schools, but the stratification design allowed us to control for this difference.

A remarkable feature of the PRALIMAP is the extensive and comprehensive process evaluation. The trial was designed to investigate not only the outcomes relating to nutritional behaviours but also the processes involved in developing and implementing the intervention strategies, as well as the type of nutritional activities provided in the control schools for each strategy. This feature can help interpret observed relationships between the interventions and outcomes. A specific work is planned to provide an estimation of the dose of intervention, which will allow for more intensive analysis beyond a simple interpretation task by performing a per protocol statistical analysis including the dose of each strategy implemented in each high school.

In conclusion, the PRALIMAP trial aims to improve the prevention and management of overweight and obesity in adolescents by translating current evidence into public health practice. For almost 10 years, the French government has implemented a concerted nationwide strategy to reduce the prevalence of obesity at every age. Determining the most effective strategies to implement guidelines in schools is a major component of this program, which needs to be more successful in meeting the needs of subjects, particularly those from lower socioeconomic classes 59 . The results should inform how best to implement effective nutrition prevention programs in a school environment targeting adolescents at a time in their lives when they develop responsibilities and empowerment for health attitude behaviours. The initial results are expected in late 2010.

The authors declare that they have no competing interests.

SB is the principal investigator for the PRALIMAP trial. JFC is the co-investigator. NA, EB, SB and ST are outcomes evaluation and statistical managers. EB, JFC and KL are the process evaluation managers. NA, EB, SB and ST drafted the manuscript. EL is logistic head managers and EA is high school professional head managers. All authors read and approved the final manuscript. SB is the paper guarantor. PRALIMAP trial group consisted of EA, EB, SB, EL and, LB, CG, AO and RDL. It has the power to make all strategic decisions and assures the cooperation between investigator teams and between field and investigator teams.