Nonlinear association between body mass index, perceived discrimination, and psychological sub-health problems among adolescents: risks of undernutrition versus overnutrition

INTRODUCTION: Previous studies have largely emphasized the adverse effects of overnutrition on mental health. However, undernourished adolescents are also likely to experience social prejudice and exclusion, which may negatively correlate to psychological well-being. However, evidence showing associations between undernutrition and mental health remains limited. In this study, we examine the relationship between nutritional status and psychological sub-health (PSH) among Chinese adolescents and investigate a mediating role for perceived discrimination. METHODS: In this cross-sectional study, a total of 2,699 adolescents from three provinces in China participated in this questionnaire-based study. Analysis of variance, logistic regression, and restricted cubic spline (RCS) analyses were conducted to examine nutritional status and perceived discrimination associations with PSH. Structural equation modeling was used to examine pathways linking body mass index (BMI), perceived discrimination, and PSH across different nutritional status subgroups, while some covariates (such as gender, sleep duration, and alcohol consumption) were adjusted. RESULTS: Overall, 461 adolescents (17.08%) exhibited PSH. Both undernutrition (Odds ratio (OR) = 1.57, 95% Confidence interval (CI) = 1.08-2.27, P = 0.017) and overnutrition (OR = 1.67, 95% CI = 1.32-2.12, P < 0.001) were associated with an increased risk of PSH, indicating a U-shaped nonlinear dose-response relationship (χ² = 15.37, P for nonlinearity < 0.001). Perceived discrimination significantly mediated the association between BMI and PSH in undernourished (β = -0.069, P = 0.012) and overnourished (β = 0.037, P = 0.010) adolescents. Additionally, overnutrition was directly associated with a higher PSH risk among male adolescents (β = 0.118, P = 0.007). CONCLUSIONS: A significant nonlinear association between BMI and PSH was observed in Chinese adolescents, with perceived discrimination playing an important mediating factor in both undernourished and overnourished groups. These findings provide clues to the causes of psychological sub-health problems among adolescents with different nutritional statuses and also point out the direction for future research on developing targeted mental health promotion strategies tailored to adolescents with different nutritional statuses.