Background: Bariatric surgery (BS) is considered the intervention most effective for patients with severe obesity and to maintain weight loss and glycemic control for the long-term.

Objective: To analyze the effects of genotypes and haplotypes of the FTO and MC4R genes on total body weight loss (TBWL), post-surgery weight, and post- BMI after bariatric surgery.

Methods: We retrospectively selected 101 patients from Bajío High Specialty Regional Hospital, León Guanajuato, México who underwent Roux-en-Y gastric bypass (RYGB) to register their body mass index (BMI), blood pressure, biochemistry characteristics, and comorbidities. Post-surgery, patients were quoted for registered anthropometry and blood pressure. Glucose, lipid and hepatic profiles, insulin, leptin, and ghrelin were measured, and rs9939609, rs9930506, and rs1421085 FTO were genotyped.

Results: After 6 (4 - 8) years post-surgery, TBWL was 34.7 kg, and post-surgery BMI was 33.8 kg/m². BS induced a remission of T2D in 82% and a remission of hypertension in 78% of patients. Post-surgery weight was higher in carriers of the risk genotypes for the rs9939609 and rs1421085 polymorphisms. TBWL was lower in those with risk genotypes for the rs993056 and rs1421085 polymorphisms. Insulin and HOMA-IR were higher in the risk genotype of three FTO polymorphisms. There were significant interaction effects of rs9930506 and rs1421085 FTO risk genotypes on weight and BMI in response to BS.

Conclusions: Genotypes and haplotypes of FTO influence TBWL, insulin levels, and HOMA-IR. We found an interaction effect of rs9930506 and rs1421085 risk genotypes with response to BS on weight and BMI.

Obesity, Bariatric surgery, Interaction of FTO, TBWL, Post-surgery weight