**5. Conclusions**

The results of the present study sugges<sup>t</sup> that the *FTO* gene, when tested individually, and all selected genes (*FTO*, *FABP2*, *LEP*, *LEPR*, and *MC4R*), when tested in gene–gene interactions, can modify body weight and composition parameters (BMI and FMI). We showed that harboring the *FTO* A allele might be associated with an over two times increased FMI. This observation constitutes the first important finding of the study, implying that harboring this specific allele is unfavorable for some individuals. The carriers of the A allele might have an increased accumulation of body fat and a higher risk of many obesity-related disorders. The results suggested that the *FTO* (rs9939609) polymorphism is a candidate marker for affecting body mass and body composition parameters in the Caucasian population. From the gene–gene interaction analysis, we established the second

novel finding. Namely, all five studied polymorphisms are involved in the formation of obesity-related traits in the Caucasian population. The results imply that some individuals might benefit from carrying some combinations of genotypes (e.g., for BMI: AT × AA, TT × AA, TT × AG; for FMI: AT × GG, TT × AA, and TT × AG-*FTO* × *LEPR*, respectively), as regards the BMI and FMI. Additionally, the gene–gene interaction analysis confirmed that harboring the *FTO* T allele might be favorable for some individuals. This observation confirmed that the genetic risk of obesity is connected with the accumulation of numerous variants; thus, methods based on numerous SNPs are more informative than methods based on a single polymorphism. Understanding the genetics of obesity can extend our knowledge of diet individualization and exercise programs. It is crucial for the prevention of obesity-related diseases, supplementation and medical care.

**Author Contributions:** Conceptualization, E.M. and A.L.-D.; methodology, E.M.; software, A.G. and A.M.; formal analysis, E.M. and A.G.; investigation, E.M.; resources, E.M.; data curation, E.M., E.S., T.L., R.Ł. and A.A.; writing—original draft preparation, E.M.; writing—review and editing, A.L-D. and A.M.; visualization, E.M., A.L.-D. and A.G.; supervision, E.M.; project administration, E.M.; funding acquisition, J.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** The study was financed by the Ministry of Science and Higher Education in 2020/2022 as part of the Scientific School of the Academy of Physical Education in Warsaw—SN No. 5 "Bio-medical determinants of physical fitness and sports training in adult population" and Ministry of Health in 2016–2020 as part of the National Health Program (agreement 518/2016/DA).

**Institutional Review Board Statement:** The procedures used in this research were conducted in accordance with the World Medical Association's Declaration of Helsinki and the research protocol was approved by the Ethics Committee of the Military Institute of Hygiene and Epidemiology— resolution number 1/XXI /2016.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy/ethical restrictions.

**Acknowledgments:** The authors sincerely thank all patients who participated in the study.

**Conflicts of Interest:** The authors declare no conflict of interest.
