Beyond the FTO Gene: Environmental and Behavioural Factors Associated with BMI and Overweight in Spanish Adolescents

Background/objectives: Obesity is a multifactorial condition influenced by interactions between genetic susceptibility and environmental factors. The fat mass and obesity-associated (FTO) gene has been widely linked to obesity risk, particularly the rs9939609 polymorphism, which is associated with higher body mass index (BMI) and adiposity. However, evidence in adolescents remains inconsistent, and lifestyle factors such as physical activity and diet may modify genetic risk. The objectives of this study were: (i) to examine the influence of environmental, genetic, physical activity, and dietary factors on the BMI and overweight-related variables of adolescents, and (ii) to assess the impact of the rs9939609 polymorphism in the FTO gene on these variables. Methods: A cross-sectional study was conducted involving 206 adolescents aged 12 to 16 years. Body mass index (BMI), physical fitness, physical activity levels, adherence to the Mediterranean diet, mobile phone usage, and FTO rs9939609 genotyping from buccal swabs were collected. Results: No significant associations were found between the FTO genotype and BMI, or with physical activity, mobile phone usage and dietary habits. Boys showed higher physical fitness and physical activity levels than girls (p < 0.05). The only factor significantly associated with BMI was regular breakfast consumption: adolescents who habitually ate breakfast had a lower prevalence of overweight (χ² = 7.98, p = 0.005). Conclusions: The rs9939609 polymorphism in the FTO gene was not associated with overweight in this adolescent cohort. The findings underscore the relevance of healthy behaviours, particularly regular breakfast consumption and physical activity, especially among boys, as factors potentially associated with lower prevalence of overweight during adolescence.