In recent years Britain has become a nation where adult overweight is the norm with Foresight modelling predicting that 60% of adult men, 50% of adult women and 25% of all children under 16 could be obese by 2050. The financial impact to society at current prices is estimated to become an additional £45.5 billion per year by 2050 with a seven-fold increase in National Health Service (NHS) costs alone . Obesity in children and adolescents is associated with a range of adverse metabolic and cardiovascular traits [2, 3]: exacerbation of asthma , poor self-esteem  and an increased likelihood of being obese in adulthood [6, 7]. Prevention, especially in children, is universally viewed as the best approach; however, evidence for effective interventions is scarce.
The most recent Health Survey for England  reports that 16.1% of boys and 15.3% of girls aged 2 to 15 were obese with 15.4% of boys and 12.9% of girls classified as overweight, and the National Child Measurement Programme  data report that more than one in five 5 to 6 year olds and one in three 10 to 11 year olds are either overweight or obese.
The relative contribution of physical activity, sedentary activity, and diet to the development of obesity in children is unclear, partly because the variables are difficult to measure and the balance of energy is complex [10, 11]. In addition, these lifestyle factors also interact with genetic factors affecting people’s propensity to gain weight, thus creating a highly individualised complex equation of factors leading to the development of obesity. However, prolonged periods of sitting (for example, TV viewing/screen-based activity) ; low levels of physical activity ; parents’ inactivity ; and high consumption of dietary fat, carbohydrates, and sweetened fizzy drinks [15–17] have been identified as common and modifiable risk factors that can be easily targeted in school-based interventions.
It is unsurprising that most childhood prevention programmes to date have been situated within the school particularly when their existing organisational, social and communication structures provide opportunities for regular health education and the possibility of a health-promoting environment. In addition, they have the potential to reach children and their families across the social spectrum, however, despite the increasing number of school-based interventions to prevent obesity in children, results continue to be inconsistent and it is still unclear what the necessary conditions are that lead to the sustained behaviour change necessary to affect weight status.
Brown and Summerbell’s  review of controlled trials of school-based interventions identified nine combined diet and physical activity interventions that showed significant improvements in mean BMI in favour of the intervention; however, only five of these studies followed up the children for longer than 12 months, considered by the National Institute for Clinical Effectiveness (NICE)  to be the minimal length of follow up to reasonably assess long-term outcomes. The authors of the review concluded that there was insufficient evidence to determine the effectiveness of dietary interventions alone, but suggested that interventions that increase activity and reduce sedentary behaviour may help children to maintain a healthy weight, although results were short term and inconsistent.
Since this review, the results of other large-scale school-based trials have been published. For example The CHILDREN study , a one-year intervention for 10 to 11 year old children, based on the Theory of Planned Behaviour  and involving parental support, showed a significant difference in BMI in favour of the intervention at one-year follow-up. The HEALTHY Study group  developed a three-year school-based intervention for 11 to 14 year olds using social marketing and building skills; however, the results did not show a significant difference between the control and intervention groups in the primary outcome (the combined prevalence of overweight and obesity) at the end of the three-year study. The Dutch Obesity Intervention in Teenagers Trial (DOiT) , which used education and environmental change showed significant differences in favour of the intervention for skinfold measures but not for BMI at 20-month follow-up.
A recent review by Khambalia and colleagues  examined the quality of evidence and findings from existing systematic reviews and meta-analyses of school-based programmes in the control and prevention of childhood obesity published between 1990 and 2010. All of the reviews recognised that studies were heterogeneous in design, participants, intervention and outcomes. Intervention components in the school setting associated with a significant reduction of weight in children included long-term interventions with combined diet and physical activity and a family component. Khambalia and colleagues concluded that, as no single intervention will fit all school populations, further high quality research needs to focus on identifying specific programme characteristics predictive of success.
Peters et al.  carried out a review of reviews of effective elements of school health promotion across behavioural domains (substance abuse, sexual behaviour and nutrition). Five effective elements were highlighted across all three domains: use of theory, addressing social influences (especially social norms), addressing cognitive behavioural skills, training of facilitators and multiple components. The authors concluded that these elements should be primary candidates to include in programmes targeting these behaviours. In addition, the Foresight review  and recent research suggest that engaging parents and offering them strategies through which they can directly (through parenting) or indirectly (through the creation of supportive environments) foster the development of healthy eating and activity behaviours among their children/family is crucial in initiating and sustaining behavioural change [27, 28]. It is also important to use delivery methods that engage the children sufficiently to be motivated to change and, crucially, to engage their parents [29, 30]. A systematic review of school-based drug-prevention programmes  showed that the most effective programmes used interactive delivery methods, used peer leaders and focussed on affecting peer norms, yet despite its potential to empower and engage children in particular, only a few school-based health promotion programmes have primarily or solely involved interactive drama as a delivery method [29, 30]. Initial results from an exploratory trial showed that schools, children and their families found the trial design and the intervention feasible and acceptable. Moreover, at 18 months follow-up, intervention children had fewer ‘negative food markers’, consumed less energy dense snacks and more healthy snacks, had more ‘positive food markers’, had lower mean TV/screen time and spent more time doing moderate to vigorous physical activity each day than children in the control schools. Intervention children had lower anthropometric measures at 18 and 24 months than control children, with larger differences at 24 months than at 18 months for all measures except percentage body fat SDS .
The aim of this cluster randomised controlled trial (RCT) is to determine the effectiveness and cost-effectiveness of the Healthy Lifestyles Programme (HeLP) in preventing overweight and obesity in children.
To assess the effectiveness of the Healthy Lifestyles Programme (HeLP), in children aged 9 to 10 years, by comparing in intervention and control schools:
BMI SDS at 18 and 24 months (primary outcome)
Waist Circumference SDS at 18 and 24 months
Percentage Body Fat SDS at 18 and 24 months
Proportion of children classified as underweight, overweight and obese at 18 and 24 months
Physical activity at 18 months
To assess the costs of HeLP and its cost-effectiveness versus usual practice
To conduct a mixed-methods process evaluation and mediational analysis to explore the way the Programme worked (that is, how it was delivered, taken up, and experienced, and what the behavioural mediators of change are).