Investigating Children's Exposure to Outdoor Food Marketing in 2 Swedish Cities Using a Smartphone App: Cross-Sectional Study.

Background: Childhood obesity and unhealthy dietary habits remain major public health concerns and are influenced by the surrounding food environment. Food marketing, particularly for ultraprocessed foods (UPFs), shapes children's food preferences and consumption. However, food environments are complex and constantly changing, making them difficult to map and monitor. Developing approaches that capture these dynamics is essential to understand and address children's exposure to unhealthy food marketing. Objective: This study aimed to pilot-test a novel tool consisting of a smartphone app and dashboard designed to identify areas where children are exposed to outdoor food advertising. Additionally, the study assessed the prevalence of advertisements for UPFs, health-promoting foods, and offers in the identified areas and explored differences in exposure by city size and socioeconomic status (SES). Methods: A cross-sectional study was performed in 2 Swedish counties. Initially, 46 children from 4 schools in areas with varying SES used a smartphone app to take pictures of food advertisements that they encountered in their everyday lives. The app also recorded the GPS locations of where the pictures were taken. Pictures with associated GPS data were automatically uploaded and visualized in a web-based dashboard, allowing for identification of areas where children see many food advertisements, so-called "hotspot areas." The identified hotspot areas were subsequently visited by 2 researchers (SS and PF), who systematically photographed all food advertisements in the areas. All pictures taken by the researchers were later analyzed based on their content of UPFs, health-promoting foods such as fruits, berries, vegetables, and seafood (FBVS), and price promotions. Results: Based on 1308 pictures of outdoor food advertisements taken by children using the app, 34 hotspot areas were identified through the dashboard. In these areas, researchers collected 2955 pictures of outdoor food advertisements during the mapping activity. Overall, 77.5% (2291/2955) of advertisements promoted UPFs, with no significant difference between the large and small cities. In Stockholm, a higher proportion of UPFs appeared in the high-SES area compared to the low-SES area, though the proportion was high in both areas. FBVS featured in 20.8% (616/2955) of advertisements, slightly more often in Stockholm and in the high-SES area. Price promotions appeared in 23.6% (697/2955) of advertisements, mainly featured UPFs (518/697, 74.3%) and less often FBVS (142/697, 20.4%). Price promotions for UPFs were somewhat more frequent in Stockholm, and FBVS promotions were more common in the high-SES area. Conclusions: Using a novel, child-centered, mobile health tool combining a smartphone app and dashboard, this study identified local food advertising hotspots. Most advertisements in the hotspots promoted UPFs, while a few featured FBVS, a pattern also reflected in price promotions. These trends were consistent across areas, highlighting a food marketing landscape misaligned with dietary guidelines and the need for policy action.