The Healthy Supermarket Coach: effects of a nutrition peer-education intervention in Dutch supermarkets on adolescents’ food purchases | BMC Medicine

Context and design

This study was conducted as part of a collaboration between the Amsterdam Healthy Weight Programme [21], the Amsterdam Health and Technology Institute (AHTI), Albert Heijn supermarket (the supermarket chain with the largest market share in the Netherlands) and the Vrije Universiteit Amsterdam in the Netherlands. The overall aim of this collaboration is to create a healthier food environment for children and their parents in neighbourhoods in Amsterdam where the prevalence of childhood overweight and obesity are the highest and to study the effects of these efforts.

We conducted a quasi-experimental study including adolescents from two intervention schools and one comparison school (located in close proximity to supermarkets), in which intervention group participants received an in-store nutrition peer education and comparison group participants did not. We included a baseline (T0, 2 weeks prior to intervention), a post-intervention (T1, 2 weeks after the intervention) and a follow-up measurement (T2, 3 months after the intervention). Data were collected from March 2018 through June 2018 using supermarket cash receipts and questionnaires. The timing of the data collection was the same for both the control and intervention groups in order to mitigate seasonal effects.

This research is performed in accordance with the guidelines in the Declaration of Helsinki. The study was approved by the Medical Ethical Committee of the Amsterdam UMC and registered in the Dutch Trial Register (6531). Passive written informed consent was obtained using an opt-out method, where caregivers were informed about the study via a letter and could express their objection by returning a signed form or by sending an email to the researchers. In addition, active oral informed consent (in relation to cash receipts) and written informed consent (in relation to the questionnaires) to participate were given by all adolescents.

Supermarket, school and participant recruitment

From the head office of the supermarket chain we received a list of supermarkets in Amsterdam that were willing to participate in our study. The researchers selected all secondary schools that were located within walking distance (≤ 500 m) of these supermarkets. The following criteria were used to select schools: (1) students were allowed to leave the school grounds during school hours, such that they were able to visit the supermarket; (2) they should be first-year and second-year secondary school students (12–14 years of age), because the eating habits of adolescents who have recently undergone the transition from primary to secondary school are more likely to be changing and becoming less healthy [22] and (3) students should be following either a pre-vocational track (‘low education level’) or pre-university track (‘high education level’). To clarify, the secondary school system in the Netherlands offers three possible education tracks aimed at different levels of intellectual ability: pre-vocational, senior general and pre-university.

In total, seven schools met the criteria and were approached by the researcher via an email with an invitation to participate. Two schools agreed to participate; the other schools declined due to a lack of time. To recruit a comparison school, we selected a school that matched the intervention schools according to the inclusion criteria. We only included one comparison school due to resource constraints. We included a comparison school that was located in a different part of the city (and that was located within walking distance (≤ 500 m) of a supermarket from the participating chain) than the intervention schools, to prevent cross-contamination. After the comparison and intervention schools agreed to participate, the supermarkets in the vicinity of these schools were informed about their participation and the store managers of the supermarkets were visited by a head-office employee and the researcher. The selected supermarkets were comparable in size, pricing and product offerings.

Description of the intervention

The nutrition peer-education intervention, the ‘Healthy Supermarket Coach’ (HSC), was developed in 2016 (for detailed information, see [13]). The overall aim of the HSC was to improve adolescent purchasing behaviour and promote healthier food choices during school hours. The HSC consists of a 45-min workshop for a group of approximately 15 adolescents, led by two young supermarket employees who are trained in advance. The HSC intervention is based on principles from the social cognitive theory, the theory of planned behaviour [23, 24] and prior successful nutrition peer-education interventions [16]. Compared to our earlier study and the insights that we collected, we adjusted some elements of the workshop including (1) improvements to the interactive quiz and (2) removal of the exercise in which adolescents had to create an affordable, healthy lunch since it turned out that adolescents did not purchase lunch in the supermarkets but primarily purchased snacks. This was therefore replaced by a 2-week ‘healthy snacking challenge’. In this new challenge, adolescents formulated action plans [25] about when and how they would replace the purchase of an unhealthy snack with a healthy snack during school breaks for a 2-week period (e.g. When I go to the supermarket during school time, I will purchase an apple instead of a chocolate bar.). To increase the likelihood that the adolescents completed the ‘healthy snacking challenge’, each group of participants signed a ‘contract’ and committed to the challenge [26]. The supermarkets’ HSCs conducted 24 workshops across the two participating stores, each involving a new group of approximately 15 adolescents who participated during school hours. The workshops were conducted in the supermarkets.

Study procedure

Cash receipts were used to measure purchase behaviour of the adolescents and were collected at T0, T1 and T2. They were collected for 3 days (Monday, Wednesday, Friday) during the two main school breaks within a 1-week period. Adolescents were approached by one of the researchers at the checkout counters, after they had paid for their purchases and were asked which school and class they were attending to ensure that they were from the participating schools. If the adolescents were from the participating schools, they were asked to provide their cash receipt. The researcher recorded the name of their school, their class and sex on the cash receipts. During T1 and T2, adolescents from the intervention schools were also asked if they had received the HSC intervention (also recorded). Adolescents from the intervention schools who did not receive the HSC intervention were not included in this study (n = 22). The main reason adolescents did not provide cash receipts or answer questions was ‘lack of time’. Because of the large number of adolescents at the checkouts during the measurements, we were unable to keep track of those unwilling to participate, although we aimed to keep this number as low as possible.

Determinants of food purchase behaviour and the appraisal of the HSC intervention were measured with questionnaires at T0, T1 and T2. Adolescents completed the questionnaires on paper in their classroom under the supervision of a teacher and accompanied by a researcher. Only the adolescents who had completed questionnaires at T0 and T1 were included in the analyses (n = 355 adolescents from the intervention schools, n = 108 adolescents from the comparison school) (Additional File A). Reasons for not filling out the questionnaire included absence due to illness or logistical reasons (e.g. unexpected changes in class schedules).

Measures

Cash receipts

All food and drink items on the cash receipts were scored for their degree of healthiness based on the Dutch dietary guidelines using the Wheel of Five criteria of the Netherlands Nutrition Centre [27]. The Wheel of Five contains products from the five core food groups (e.g. ‘fruit and vegetables’, ‘bread, grain or cereal products and potatoes’, ‘dairy, nuts, fish, legumes, meat and eggs’, ‘drinks’ and ‘spreading and cooking fats’) that either provide essential nutrients or have a beneficial effect on health [27]. Products were categorized as ‘healthy food’ (Wheel of Five) and ‘unhealthy food’ (Not Wheel of Five).

Questionnaires

At the baseline measure (T0), we collected data on the following demographic characteristics: sex (boy, girl), level of education (low for pre-vocational track and high for pre-university track), school year (first, second), age (in years). The food purchase characteristics included frequency of supermarket visits during a regular school week (less than once a week to more than 5 days a week) and the average amount of money spent on food in the supermarket during school time per day (in euros, open-ended answers).

During T0, T1 and T2, we also collected data on determinants of food purchase behaviour (nutritional knowledge, intention, attitude, self-efficacy, social norm, social support for healthy/unhealthy food purchases). These were assessed with validated questions used in previous studies among adolescents [24, 28,29,30]. Nutritional knowledge was measured according to 11 multiple-choice items about various aspects that have been shown to influence the food choices of adolescents. The follow is one example: “On average, how many calories should girls consume per day?” (response categories: “1 = 2000,” “2 = 2500,” “3 = 3000”). All questions included 3 answer options and there was one correct answer to each of the questions, equivalent to one point. The scores on the 11 items were summed and averaged to create a total score (0–11), with higher scores indicating more nutritional knowledge. The following determinants were measured using a 5-point Likert scale (strongly disagree [− 2] to strongly agree [+ 2]). Attitude towards healthy food purchases (T0 α = 0.64) and attitude towards unhealthy food purchases (T0 α = 0.61) were assessed with four questions each, and an average total score was calculated. Intention to make healthy food purchases was assessed with eight items, of which an average total score was calculated (T0 α = 0.85). Self-efficacy, social norms and social support regarding healthy and unhealthy food purchases were assessed with one question each. Higher scores indicated better attitude and greater intention, self-efficacy, social norms and social support for healthy/unhealthy food purchases. Additional File B includes all the questions on the determinants of food purchase behaviour as well as Cronbach’s Alphas for the scales and the response options.

The post-intervention questionnaire for the intervention schools (T2) included also ten questions assessing the appraisal of the HSC intervention. Unless mentioned otherwise, adolescents were asked to use 5-point Likert scales (not at all [− 2] to very much [+ 2]). First, they were asked to indicate whether they had enjoyed the workshop, and then whether the workshop was difficult, informative or childish. Subsequently, they were asked to grade the HSC intervention (on a scale from 1 to 10) and to explain their grade in an open question. They were then asked to indicate whether they perceived the HSC as a role model and whether they would like to appear similar to the HSC. Finally, they were asked to indicate whether they were interested in following the HSC’s recommendations about healthy food purchases in the supermarket and whether they had learned how to make healthy food choices in the supermarket. Mean scores were calculated, with higher scores indicating more positive evaluations.

Statistical analyses

For the cash receipt data, we calculated the number and percentage of unhealthy and healthy food purchases at T0, T1 and T2. Because we were not able to measure food purchases at an individual level, standard Fisher exact tests were used to examine differences in the percentage of food purchases between the intervention schools and the control school at T0, T1 and T2 and to investigate the difference in the change in food purchases between the control school and the intervention schools at T1 and T2 (compared to T0).

A series of linear mixed model analyses, including a random intercept at the participant level, were conducted to investigate the effect of the HSC intervention on determinants of adolescent food purchase behaviour. In all mixed model analyses, the condition (intervention [yes = 1, no = 0]) was used as an independent variable and the scores on the determinants of food purchase behaviour at T1 and T2, adjusted separately for the score at baseline (T0), as the dependent variables. Both a crude model (adjusted for baseline score) and an adjusted model (additionally adjusted for age, gender and school year) were analysed. All analyses were stratified by level of education to investigate the effects of low and high education separately. In addition, we conducted independent sample t-tests to investigate differences in the appraisal of the HSC intervention between the two education groups. Statistical analyses were performed using IBM SPSS Statistics for Windows version 25.0. We used a two-tailed p-value of less than 0.05 statistical significance.