Context: A common variant in the FTO gene, rs9939609, associates with body mass index (BMI) in adults and in children aged 7 yr or older.
Objective: Our aim was to examine the associations of the FTO genotype with BMI, cardiovascular risk factors, energy intake, and leisure-time physical activity in children followed up since infancy.
Methods: Healthy participants of the STRIP Study, genotyped for rs9939609, were followed from age 7 months (n = 640) to 15 yr (n = 438). The children were randomly assigned to lifestyle intervention and control groups. Height, weight, blood pressure, and serum lipids were measured annually. Food records and physical activity index were obtained at age 15 yr.
Results: The FTO genotype did not associate with BMI in children younger than 7 yr of age. From age 7 yr onward, the children homozygous for the A allele had progressively higher BMI than the children with one or two T alleles (P = 0.029 for FTO by age interaction). Furthermore, in longitudinal, BMI Z-score-adjusted analysis, the AA genotype associated with higher systolic and diastolic blood pressure and with elevated serum total and low-density lipoprotein-cholesterol (P = 0.01, P < 0.001, P = 0.05, and P = 0.04 for main effect, respectively). The FTO genotype did not associate with energy intake or physical activity index at age 15. The FTO *Study group interactions were not significant.
Conclusions: Our results suggest that the effect of the FTO genotype on BMI becomes evident only after age 7 yr. These results further suggest that the FTO gene is not directly associated with energy intake or physical activity.
Trial registration: ClinicalTrials.gov NCT00223600.