Background: Genome-wide association studies found common variants in the fat mass and obesity-associated (FTO) gene associated with adiposity in Caucasians and Asians but the association was not confirmed in African populations. Association of FTO variants with insulin resistance and energy intake showed inconsistent results in previous studies. This study aimed to assess the influence of FTO variant rs9939609 on adiposity, insulin resistance, energy intake and physical activity in European - (EA) and African-American (AA) youth.Methods: We conducted a cross-sectional study in EA and AA youths. One thousand, nine hundred and seventy-eight youths (48.2% EAs, 47.1% male, mean age 16.5 years) had measures of anthropometry. Percent body fat (%BF) was measured by dual-energy X-ray absorptiometry, visceral adipose tissue (VAT) and subcutaneous abdominal adipose tissue (SAAT) by magnetic resonance imaging. Energy intake and physical activity were based on self report from up to 7 24-hour recalls. Physical activity was also measured by accelerometry.Results: FTO rs9939609 was significantly associated with body mass index (BMI) (P = 0.01), weight (P = 0.03) and waist circumference (P = 0.04), with per-allele effects of 0.4 kg/m2, 1.3 kg and 0.8 cm, respectively. No significant association was found between rs9939609 and %BF, VAT, SAAT or insulin resistance (P > 0.05), or between rs9939609 and energy intake or vigorous physical activity (P > 0.05). No significant interactions of rs9939609 with ethnicity, gender, energy intake or physical activity were observed (P > 0.05).Conclusions: The FTO variant rs9939609 is modestly associated with BMI and waist circumference, but not with energy intake or physical activity. Moreover, these effects were similar for EAs and AAs. Improved understanding of the effect of the FTO variant will offer new insights into the etiology of excess adiposity.
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