Comparison of TCN-2 (776C>G) Gene Polymorphism and Vitamin B12 Status with Different Body Mass Index among Saudi Adults

**Background:** Obesity and overweight are major global health problems with genetic, environmental, and lifestyle contributors. Transcobalamin II, encoded by the TCN-2 gene, transports vitamin B12 to cells. The TCN-2 (776C>G) (rs1801198) polymorphism results in a Pro259Arg substitution that may alter vitamin B12 binding and cellular uptake. Previous research has linked this polymorphism to obesity-related anthropometric traits and vitamin B12 status, but data in Saudi populations were lacking.

**Methods:** This cross-sectional study included 250 Saudi adults recruited from Umm Al-Qura University clinic: 100 healthy-weight controls (BMI 18.5 to <25 kg/m²), 100 overweight (BMI 25.0 to <30 kg/m²), and 50 obese (BMI >30 kg/m²). Participants with diabetes, chronic diseases, or malignancies were excluded. Blood pressure was measured twice in supine position. Fasting blood samples were collected for biochemical analysis (lipid profile via Cobas e411) and vitamin B12 assessment (electrochemiluminescence immunoassay; deficiency defined as <148 pmol/L). DNA was extracted from EDTA blood samples, and TCN-2 (776C>G) genotyping was performed using PCR-RFLP with MvaI enzyme digestion. The wild-type CC genotype showed one band (218 bp), heterozygous CG showed three bands (218, 128, 90 bp), and mutant GG showed two bands (128, 90 bp). Statistical analysis used Chi-square test, Fisher's exact test, Hardy-Weinberg equilibrium, odds ratios, relative risk, Kruskal-Wallis test with post hoc Dunn test, and correlation analysis. P values <0.05 were considered significant.

**Key Results:** The study population was predominantly male (81% in controls, 87% in overweight, 92% in obese). Mean ages were similar across groups (38.38±6.65, 37.61±6.74, and 39.24±6.69 years, respectively). Significant differences were observed across groups for systolic blood pressure (126.6±7.0 vs 148.1±13.6 vs 150.7±17.8 mmHg, p<0.0001), diastolic blood pressure (85.5±6.3 vs 96.2±7.5 vs 97.0±7.8 mmHg, p<0.0001), HDL (45.0±13.2 vs 39.2±11.1 vs 34.8±7.1 mg/dL, p<0.0001), LDL (181.6±40.1 vs 196.9±28.6 vs 186.6±30.7 mg/dL, p=0.04), triglycerides (165.6±18.2 vs 216.2±32.4 vs 222.7±43.5 mg/dL, p<0.0001), cholesterol (186.9±35.2 vs 233.9±25.5 vs 248.8±7.5 mg/dL, p<0.0001), and VLDL (22.9±4.2 vs 32.2±4.5 vs 36.78±4.5 mg/dL, p<0.0001). TCN-2 genotype distribution differed significantly between groups. CC genotype frequency was 59% in controls, 42% in overweight, and 32% in obese; CG was 34%, 39%, and 46%; GG was 7%, 19%, and 22% (overweight vs control p=0.01; obese vs control p=0.002). The G allele frequency was 0.24 in controls, 0.40 in overweight, and 0.45 in obese. For overweight participants, the odds ratio for CG was 1.61 (0.87-2.95, p=0.12) and for GG was 3.81 (1.47-9.88, p=0.005). For obese participants, the odds ratio for CG was 2.49 (1.16-5.36, p=0.01) and for GG was 5.79 (1.93-17.35, p=0.001). Relative risk for overweight: CG 1.25 (0.93-1.68, p=0.13), GG 2.17 (1.12-4.17, p=0.02); for obese: CG 1.31 (1.03-1.68, p=0.02), GG 2.02 (1.12-3.65, p=0.01). Vitamin B12 levels were significantly lower in overweight (305.5 pmol/L) and obese (229 pmol/L) participants compared to controls (385.5 pmol/L, both p<0.0001), though all within normal range. Smokers (12.4% of participants) had lower B12 levels (298.2 pmol/L) than non-smokers (325.6 pmol/L). Correlation analysis showed vitamin B12 was significantly associated with LDL (p=0.005), TG (p<0.0001), cholesterol (p=0.003), and VLDL (p<0.0001), with increases in B12 potentially correlating with decreases in TG, cholesterol, and VLDL.

**Clinical Implications:** This study demonstrates that the TCN-2 (776C>G) polymorphism, particularly the GG genotype, significantly increases susceptibility to overweight and obesity in Saudi adults, with the GG genotype conferring nearly 6-fold higher odds of obesity. The concurrent finding of lower vitamin B12 levels in overweight and obese individuals, along with significant correlations between B12 and lipid parameters, suggests that vitamin B12 status may play a role in obesity-related metabolic disturbances. These findings highlight the importance of considering both genetic factors and nutritional status in obesity management. Clinicians should be aware that TCN-2 genetic variation may identify individuals at higher risk for obesity and associated metabolic complications. The association between smoking and lower B12 levels adds another modifiable risk factor. Limitations include the cross-sectional design (preventing causal inference), predominantly male sample, and relatively small sample size (n=250). Future research should include larger, more diverse populations and longitudinal designs to establish causality.