Vitamin B12 is a label for a group of compounds known as cobalamins. The most common form of vitamin B12 is called cyanocobalamin.

Cobalamin functions as a source of a methyl group (a carbon molecule) for a very important biochemical cycle known as the methylation cycle. In order to do this, cobalamins must be converted to the methyl donating form called methylcobalamin. Methylcobalamin is the “active” form of cobalamins.

Signs, symptoms and health problems associated with vitamin B-12 deficiency
Deficiency characteristics include anemia, fatigue, weakness, constipation, loss of appetite, and weight loss. Deficiency also can lead to neurological changes such as numbness and tingling in the hands and feet. Additional symptoms of vitamin B-12 deficiency are difficulty in maintaining balance, depression, confusion, dementia, poor memory, and soreness of the mouth or tongue.

Signs of vitamin B-12 deficiency in infancy include failure to thrive, movement disorders, delayed development, and megaloblastic anemia. (1)

Adults over 50
Up to 30 percent of adults aged 50 years and older may have atrophic gastritis, an increased growth of intestinal bacteria, and be unable to normally absorb vitamin B-12 in food. They are, however, able to absorb the synthetic vitamin B-12 added to fortified foods and dietary supplements. Vitamin supplements and fortified foods may be the best sources of vitamin B-12 for adults older than age 50 years. (1)

Young adults
One study suggests that the prevalence of vitamin B-12 deficiency in young adults may be greater than previously thought. This study found that the percentage of subjects in three age groups (26-49 years, 50-64 years, and 65 years and older) with deficient blood levels of vitamin B-12 was similar across all age groups but that symptoms of vitamin B-12 deficiency were not as apparent in younger adults. This study also suggested that those who did not take a supplement containing vitamin B-12 were twice as likely to be vitamin B-12 deficient as supplement users, regardless of age group. (1)

Mental function
Methyl B-12 can be very effective in improving mental function. In a 1995 double-blind study of elderly people with low levels of B12, over 60% improved significantly with vitamin B-12 supplementation. It was suggested that those who did not improve had long-term (longer than 6 months) signs of mental impairment. (2)

Autism
Children with autism have been shown to be deficient in cysteine and glutathione and it appears that methylcobalamin can help restore the formation of the nutrients. (3)

Can I overdose on vitamin B-12?
The Institute of Medicine (an independent body that advises the government on science issues) did not establish a UL (upper intake level) for vitamin B-12 because it has a very low potential for toxicity. The IOM states that "no adverse effects have been associated with excess vitamin B-12 intake from food and supplements in healthy individuals". In fact, the IOM recommends that adults older than 50 years get most of their vitamin B-12 from vitamin supplements or fortified food because of the high incidence of impaired absorption in this age group of vitamin B12 from foods that come from animals. (1)


(1) Office of Dietary Supplements, National Institutes of Health, Bethesda, Maryland.
(2) van Goor L, et al. Review: Cobalamin deficiency and mental impairment in elderly people. Age Ageing (1995) 24:536-42.
(3) James SJ, et al. Metabolic biomarkers of increased oxidative stress and impaired methylation capacity in children with autism. Am J Clin Nutr (2004) 80:1611–7.