Vitamin B-12
Background
Vitamin B-12 plays a critical role in the metabolism of fatty acids that are essential for the maintenance of nerve myelin. It is well known that prolonged B-12 deficiency can lead to nerve degeneration and irreversible neurological damage. Vitamin B-12 is available in three forms: cyanocobalamin, hydrocobalamin, and methylcobal-
amin. Cyanocobalamin is the most widely available and least expensive form, found in most over-the-counter multivitamins. However, cyanocobalamin is an inactive precursor that must be converted into one of two active metabolites: methylcobalamin and adenosylcobalamin.
Methylcobalamin is essential for folate metabolism and for the formation of choline-containing phospholipids, which are the building blocks of myelin. Adenosylcobalamin is required for the formation of succinyl coenzyme A, the lack of which causes impairment in the formation of neural lipids. Some evidence suggests methylcobalamin is better utilized and retained in the tissues than cyanocobalamin. In addition, many clinicians also claim methylcobalamin is preferred due to improved clinical outcomes.
Clinical Studies
Diabetic Neuropathy
The clinical effectiveness of vitamin B-12 and its active coenzyme for the treatment of diabetic neuropathy was assessed in a meta-analysis of clinical trials to date. Outcomes were measured in these trials based on symptoms or signs, vibration meter detected thresholds of vibration perception, and electrophysiologic measures, such as nerve conduction velocities. Three studies involved the use of vitamin B complex (including B-12) as the active drug and four used methylcobalamin. Both the vitamin B-12 combination and pure methylcobalamin had beneficial effects on somatic symptoms, such as pain and tingling. In three studies, methylcobalamin therapy improved symptoms.
In one study, methylcobalamin was given orally at a dose of 500 mcg, three times per day, to patients with diabetic neuropathy. In this double-blind study, the treatment group showed statistical improvement in the somatic and autonomic symptoms with regression of the signs of diabetic neuropathy. Improvements were noted in burning sensation, pain, numbness, and muscle cramping. The treatment was well tolerated by the patients, and no side effects were observed.
Doseage
The recommended dosage for clinical effect is 5 to 15 mg per day of methylcobalamin, administered orally (sublingually), intramuscularly, or intravenously. Positive clinical results have been reported irrespective of the method of administration. Vitamin B-12 therapy should only be undertaken with the guidance of a healthcare practitioner familiar with its use.
Safety and Side Effects
Methylcobalamin has excellent tolerability and no known toxicity. Alcohol, antibiotics, oral hypoglycemic agents, beta blockers, anti-acid drugs (H2 blockers), oral contraceptives, nicotine, and HIV drugs can all cause vitamin B-12 depletion.
