This study showed that the proportion of sales of healthy food and beverages from vending machines in metropolitan bus garages was higher when the availability of healthy foods and beverages in the machines was increased and the prices lowered. Sales of healthy foods at the intervention garages were on average about double that of the control garages (55% of items sold at intervention garages compared with 19% at control garages). These findings are consistent with previous studies in worksite settings that used pricing and availability to promote healthful vending choices.
The inconsistency between the aggregate sales data and the individual survey self-reported behavior is similar to the results found in a recent school-based cafeteria environmental intervention to promote healthful food choices through increases in availability.9
In that study, sales of healthful foods increased when healthy foods were increased in availability, but student surveys of food choices did not show changes in reported food choices.
Several reasons may explain the apparent inconsistent findings between the aggregate vending sales data and the individual driver self-reported food choices. First, the vending machines are located in the bus garages, and only a small proportion of bus drivers use the vending machines frequently. Second, dietary recall data from this study (not presented here) show that only about 90 kcals per day are obtained at the vending machines, so the overall impact of changes in the vending machine choices will be small, at best. Most drivers obtain the food that they consume at work from sources other than the vending machines.
Despite limitations (described below), vending machine interventions can still be an important component of a multi-component worksite intervention package. While it is unrealistic to expect a single vending machine intervention to change overall dietary intake, vending machine interventions that are combined with a package of strategies implemented at the worksite may be effective in significantly impacting dietary intake and food choices. Vending machine interventions also may have effects on other important mediators of dietary behavior, such as perceived norms, social support, perceived environmental opportunities and knowledge and attitudes about healthful eating.15-16
Strengths of the present study include the careful implementation and monitoring of the vending machine changes during a lengthy intervention period. Research staff monitored the vending machines during the intervention with weekly site visits to garages to observe the vending machines. Accurate and complete data were collected for the entire study period by working closely with the vending machine service company drivers and managers. Thus, the vending intervention was implemented with high fidelity and the sales data quality was high.
Limitations of the study were that only aggregate sales data are available. It was not possible to know whether the vending intervention influenced individuals to change their vending food and beverage choices, or whether the intervention attracted new patrons who were self-selected in terms of their interest in healthy eating. Self-report survey data on vending machine frequency of use showed overall decreases in the vending machine frequent users. However, the objective sales data did not show such temporal declines in total vending sales (data not shown). Sales data also reflect the vending use behavior of the drivers who are physically at the garages and therefore have access to the vending machines. The survey data, by contrast, represent a much broader range of bus operators, some of whom do not spend time at the garage and thus have little exposure to the vending machines.
In conclusion, pricing and availability interventions in worksite vending machines promote sales of the targeted healthy food and beverage items as part of a multi-component environmental intervention, and may contribute to change in overall dietary intake. However, worksite nutrition interventions that target the physical food environment at the worksite may be less effective in changing overall food choices among mobile worker populations, such as bus operators, compared to worker populations that spend most of the work day at the worksite, such as in office settings.