Maximal Fat Oxidation During Exercise in Healthy Individuals: Lack of Genetic Association with the FTO rs9939609 Polymorphism

Background/Objectives: Previous studies suggest that there is a genetically determined component of fat oxidation at rest and during exercise. To date, the FTO gene has been proposed as a candidate gene to affect fat oxidation during exercise because of the association of the “at-risk” A allele with different obesity-related factors such as increased body fat, higher appetite and elevated insulin and triglyceride levels. The A allele of the FTO gene may also be linked to obesity through a reduced capacity for fat oxidation during exercise, a topic that remains largely underexplored in the current literature. The aim of this study was to analyze the association between the FTO rs9939609 polymorphism with the rate of fat oxidation during exercise and metabolic syndrome criteria in healthy participants. Methods: A total of 80 healthy participants (41 men and 39 women) underwent comprehensive assessments, including measurements of anthropometric variables, blood pressure and blood measures of fasting glucose, triglycerides, low- and high-density lipoprotein cholesterol (LDL-c and HDL-c), insulin, interleukin-6 (IL-6) and C-reactive protein (CRP) concentrations. Additionally, the Homeostatic Model Assessment (HOMA-IR) was used to evaluate insulin resistance. Peak oxygen uptake (VO2peak) and maximal fat oxidation rate (MFO) were also measured during an incremental cycling test. FTO rs9939609 genotyping (TT, AT, AA) was performed using genomic DNA samples obtained from a buccal swab and measured with PCR. Results: There were 32 participants (40.0%) with the TT genotype; 31 (38.8%) with the AT genotype; and 17 (21.2%) with the AA genotype. Age, body characteristics, VO2peak, blood pressure and blood variables were similar across all three genotypes. However, serum insulin concentration and HOMA-IR were associated with the FTO rs9939609 genotype with higher values in AA with respect to AT and TT participants (p < 0.050). Still, MFO was similar in TT, AT and AA participants (0.35 ± 0.13, 0.37 ± 0.11, 0.33 ± 0.11 g/min, p = 0.702). In the dominant model, there was no statistical difference between TT and A allele carriers. However, the recessive model revealed that AA participants had higher values of body mass, body mass index, blood insulin concentration and HOMA-IR than T allele carriers (p < 0.050), with no differences in MFO. Conclusions: In our sample of healthy individuals, the FTO rs9939609 polymorphism was associated with several phenotypes associated with obesity and insulin resistance, particularly under the AA vs. T allele/recessive model. However, the FTO rs9939609 polymorphism was not associated with MFO during exercise as fat oxidation was similar across genotypes. This suggests that reduced fat oxidation during exercise is unlikely to be a cause of the obesogenic influence of the FTO AA genotype. Clinically, these findings suggest that the obesogenic effects of the FTO AA genotype are unlikely driven by impaired fat oxidation during exercise. Instead, attention should focus on mechanisms like appetite regulation and energy intake. Moreover, exercise interventions may still effectively mitigate obesity risk, as AA individuals retain normal fat oxidation capacity during exercise.