BACKGROUND: Adolescence is a critical developmental period marked by significant mental and physical health changes. This longitudinal study examines the relationship between online communication and health from age 15 to 17, focusing on mental health indicators (depression, anxiety, self-esteem, and body image) and cardiorespiratory fitness (CRF). We aimed to assess whether increased time spent in online communication is associated with poorer mental health and lower CRF among adolescents, with consideration of potential moderating effects of sex and socioeconomic status (SES). METHODS: Data were collected from 315 Icelandic adolescents at age 15 (2015) and age 17 (2017
N = 236). Linear mixed-effects models were used to examine the association between online communication and health outcomes, including depression, anxiety, body image, self-esteem, and CRF. Models accounted for year, sex, and SES, with random intercepts for individual variability. RESULTS: More online communication was significantly associated with poorer mental health outcomes, including more symptoms of depression (p <
0.001, marginal R² = 0.14), anxiety (p = 0.032, marginal R² = 0.13), lower self-esteem (p = 0.006, marginal R² = 0.07), more negative body image (p = 0.010, marginal R² = 0.13), and lower CRF
p = 0.003, marginal R² = 0.35). These associations did not change between years and were consistent across sex and SES groups. CRF declined from age 15 to 17. Females reported generally worse mental health and lower CRF than males, while higher SES was linked to lower depression scores and higher self-esteem. However, no significant interactions were found between online communication, sex, or SES, suggesting that the impacts of online communication on health were broadly applicable across demographic groups. CONCLUSIONS: This study underscores the potential negative effects of online communication on mental and physical health among adolescents, regardless of sex or SES. The findings highlight the importance of balancing time spent online communicating with physical activity to support overall adolescent well-being. These insights could inform public health initiatives and preventive strategies to foster healthier digital habits in an increasingly online world, especially during this sensitive developmental period.