Soft drinks, fast food, and hours of computer games and television viewing are common features in today's youth. Unhealthy diets and lack of physical activity are the leading causes of avoidable illness and premature death in Europe . Of particular concern is the increasingly unhealthy diet and physical inactivity of children and adolescents. The great Public Health burden of overweight and obesity requires widespread dissemination of effective prevention strategies aimed at improving energy balance related behaviours (EBRBs) (e.g. television viewing, active transport, soft drink and snack consumption) .
School-based interventions are promising because of their potential to reach almost all children in the population. To improve the effectiveness of interventions that aim at improving EBRBs, we need to identify how these interventions can lead to improvement of EBRB. Mediation analysis is a method to assess the processes by which an intervention achieves its intended effects . Mediation analysis identifies which intermediate variables are responsible for an intervention's effects.
Most interventions are designed to change intermediate – or 'mediating'- variables that are hypothesized to be causally related to the outcome of interest. These variables are called mediators when they explain the relationship between exposure to the intervention and the outcome variable. Mediation analysis is useful, because it can be used to separate elements of an intervention that are critical to its success from those that are not. If ineffective and effective intervention elements can be identified and eliminated or expanded, respectively, an enhanced intervention program can be developed that provides greater benefit and costs less .
If an intervention is effective, mediation analysis can identify which mediating mechanisms are responsible for this effect. When an intervention is not effective mediation analysis can help to find possible causes of this lack of effect . Maybe the intervention was not effective in influencing the mediating variables. Other competing processes may have a suppression effect – i.e. accomplish a negative intervention effect – diminishing the intervention effect caused through the mediating variables. Another possibility would be that the hypothesised mediator does not mediate behaviour change.
Improving dietary behaviour and physical activity patterns may be achieved by inducing changes in personal and environmental mediators of such EBRBs. The school-based Dutch Obesity Intervention in Teenagers (DOiT) aimed at improving the following EBRBs:
(1) consumption of sugar containing beverages, i.e. consumption of soft drinks and fruit juices;
(2) consumption of high caloric snacks, i.e. consumption of savoury and sweet snacks;
(3) screen-viewing behaviour, i.e. time spent on television viewing and computer use; and
(4) active commuting to school.
To improve these behaviours the DOiT-program tried to influence the following potentially mediating variables: attitude, subjective norm, perceived behavioural control and habit-strength. These determinants were chosen based on a combination of an analysis of systematic reviews on determinants of energy balance-related behaviours [4–6], and personal interviews with teachers, parents, experts in the field of physical activity, dietary behaviour, and behavioural change . The Theory of Planned Behaviour suggests that the most proximal determinant of behaviour is the intention to perform this specific behaviour, and that three additional determinants predict the intention: attitudes, perceived subjective norms, and perceived behavioural control, or self-efficacy . However, EBRBs are typically a natural part of adolescents' everyday lives that do not require much intentional effort to be set in motion [9, 10]. Habitual behaviour is considered to be "automatic," triggered by environmental cues instead of conscious evaluations of possible outcomes, the opinion of other people, and confidence about being able to perform the behaviour. The Habit Strength Theory posits that when habits are formed, subsequent behaviour is automatically triggered by specific environmental cues that normally precede the action . Therefore, we also included habit strength as a possible mediator of behaviour change. Earlier reports on the DOiT-study indicate that the intervention resulted in lower skin fold thickness among girls and lower consumption of sugar containing drinks among both boys and girls .
There have been few formal mediational analyses conducted for school-based physical activity and nutrition intervention programs. To our knowledge, no studies are available on mediating variables in interventions specifically aimed at reducing time spent in screen-viewing behaviour, or consumption of soft drinks or high-energy snacks.
The purpose of this study was to examine whether the DOiT-program was effective in improving the targeted mediators and to identify the mediating mechanisms targeted by the DOiT-program.