Aim: Cubilin receptors, which are responsible for the absorption of vitamin B12 from the intestine, are also found in the kidney and regulate protein reuptake from urine. We claimed that vitamin B12 deficiency may lead to less proteinuria as a result of increased Cubilin receptors, or higher vitamin B12 levels may cause proteinuria by down regulating the receptors. To support this hypothesis, the relationship between vitamin B12 and urine protein levels was investigated. Methods: The last two-year hospital records of patients between the ages of 18-65 were retrospectively scanned. Patients who were tested for the vitamin B12, plasma creatinine, and urine protein analyses and whose eGFR value was calculated as >60mL/min were included. Patients diagnosed with diseases that may cause proteinuria such as diabetes, malignancies, or kidney diseases were excluded. Vitamin B12 level <200 pg/ml was accepted as a deficiency, 200-1000 pg/ml as normal, and >1000 pg/ml as hypervitaminosis. Results: Of the 31,776 patients who met the criteria, vitamin B12 deficiency was found to be 38.6% in men; 33.1% in women, and 35% in total. Rates of hypervitaminosis B12 were 1.4% in both men and women, and overall. Urinary protein was observed in 13% of all patients. A statistical significance was found between vitamin B12 levels and proteinuria. In post-hoc analysis, this statistical difference was detected between the hypervitaminosis group and the other levels. Conclusion: Vitamin B12 hypervitaminosis was found to be statistically associated with proteinuria, indicating that caution should be exercised in vitamin B12 treatment.