Vitamin B12 in Cancer Patients: Clinical Insights into Deficiency, Excess, Diagnosis, and Management

Vitamin B12 (cobalamin) is a critical micronutrient involved in hematopoiesis and neurological function. Its deficiency, commonly presenting with anemia and neurological symptoms, is particularly relevant in oncology. While anemia affects up to 60% of cancer patients, the contribution of vitamin B12 deficiency to cancer-related anemia remains underexplored. Additionally, cobalamin-related neuropathy manifests or exacerbates existing chemotherapy-induced peripheral neuropathy (CIPN), a serious side effect of chemotherapy. Prevalence estimates in cancer populations range widely (6-48%), with higher rates in elderly and gastrointestinal cancer patients. This review summarizes current evidence on the prevalence and implications of both vitamin B12 deficiency and excess in patients with solid tumors. It discusses laboratory markers (such as serum vitamin B12, holotranscobalamin, methylmalonic acid, and homocysteine) that could improve diagnostic accuracy in oncology settings. Additionally, it evaluates supplementation strategies and discusses its role in mitigating CIPN. Additionally, it addresses B12's emerging immunological role in cancer therapy.