Objective understanding of front of pack warning labels among Mexican children of public elementary schools. A randomized experiment

BACKGROUND: Warning Labels (WL) highlight excessive amounts of critical nutrients in order to discourage consumption of unhealthful packaged food products. This study aimed to evaluate among Mexican school children, the objective understanding of traditional and numeric WL (aimed at small products) considered by the Mexican regulation, and whether cartoon characters influenced the understanding of WL. We also tested some communication strategies to facilitate the correct use of the WL. METHODS: We carried out a randomized experiment in July 2019 in public elementary schools from Morelos, Mexico. Participants aged 6–13 years, were randomly assigned to one of four groups: 1) Nutrient Facts Panel (NF) (n = 120), 2) Nutrient Facts Panel with cartoon characters (NF + C) (n = 83), considered the control groups, 3) Warning Labels (WL) (n = 109), and 4) Warning Labels with cartoon characters (WL + C) (n = 96). After allocation, children assigned to both WL groups (WL or WL + C), were randomly required to watch two posters simultaneously or a video explaining how to correctly interpret WLs. Logistic regression models adjusted by sex, age and cluster (school) were fitted. RESULTS: The percentage of children correctly choosing the healthiest or the unhealthiest option was higher for WL groups (56.8, 95%CI; 40.8–72.8) compared to NF groups (24.3, 95%CI; 20.4–28.3, p < 0.05). The understanding of traditional WL was higher (28.7, 95%CI: 22.8–35.4) than the numeric WL (19, 95%CI: 14.2–25.0, p < 0.05). But, correct answers for identifying healthy and unhealthy products were higher for numeric WL than for NF groups. Cartoon characters reduced the percentage of correct answers for choosing unhealthiest products (WL + C: 48.9, 95%CI: 25.6–72.4 vs WL: 58.7, 95%CI: 36.4–81.1, p < 0.05). The video was 2.23 times more helpful than the posters to the correct interpretation of the WL (p < 0.05). CONCLUSIONS: In scholar Mexican children, traditional and numeric WL were useful to identify healthier and unhealthier packaged products in comparison to NF, suggesting that both WL formats may effectively communicate the excessive content of nutrients of concern among children. Cartoon characters may reduce the objective understanding of the WL, underscoring the need to regulate advertising directed to children along with the implementation of front-of-pack labeling. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12937-022-00791-z.