Spotlight on vitamin B12

Vitamin B12 Basics

Vitamin B12 is the largest and most complex of all the vitamins. It is also unique among vitamins because it is the only one that contains a metal - cobalt - as part of its structure. For this reason cobalamin is the term used to refer to compounds having B12 activity. The type of cobalamin used in most dietary supplements, cyanocobalamin, is readily converted to active forms of vitamin B12 in the body. B12 is needed to help maintain healthy nerve and red blood cells, and to make DNA, the genetic material in all cells.

Another unusual feature of this vitamin is the multi-step process required for its absorption. Vitamin B12 is bound to the protein in food. Hydrochloric acid in the stomach is needed to release B12 from its protein bonds during digestion. Once released, B12 must combine with a substance called gastric intrinsic factor, which is secreted by specialized stomach cells. This B12-intrinsic factor complex can then be absorbed by the intestinal tract with the help of calcium.

Who Is at Risk for Vitamin B12 Shortfalls?

A B12 deficiency can occur as a result of an inability to absorb B12 from food. Most people are familiar with the autoimmune condition pernicious anemia, which results from an inability of stomach cells to secrete intrinsic factor. People with this condition usually receive B12 by injection and large oral doses. Aside from pernicious anemia, older people in general can have difficulty absorbing B12 due to lower levels of stomach acid. Sometimes the only symptom of this limited absorption is subtly reduced mental function resulting from early B12 deficiency. Anemia, dementia or nerve problems may follow.

Up to 40% of older adults may have atrophic gastritis, an overgrowth of intestinal flora, and be unable to normally absorb vitamin B12 in food. Atrophic gastritis, an inflammation of the stomach, not only decreases gastric acid secretion but can also result in overgrowth of normal bacterial flora in the small intestines. The bacteria may take up vitamin B12 for their own use, further contributing to B12 deficiency. Older individuals with atrophic gastritis are, however, able to absorb the B12 added to fortified foods and supplements, and these may be the best sources of vitamin B12 for people over 50.

Strict vegetarians and vegans are another group at risk for developing B12 deficiency because natural sources of vitamin B12 are limited to animal foods such as meat, poultry, fish, eggs and dairy. Fortified cereals are one of the few non-animal sources of B12. The use of certain medications such as the diabetic drug Metformin, can also interfere with B12 absorption.

B12, Brain Function and Bone Health

Researchers have long been intrigued by the connection between vitamin B12 deficiency and dementia. One review examined the relationships between cognitive skills, homocysteine levels, and blood levels of folate, B12 and B6. The authors suggest that vitamin B12 deficiency may decrease levels of substances needed for the metabolism of neurotransmitters, the chemicals that transmit nerve signals (1). Reduced levels of neurotransmitters may result in cognitive impairment.

Another area rapidly gaining attention is the observed link between B12 and osteoporosis. B12 has been associated with bone cell activity and bone formation. Recently, researchers from Tufts measured bone mineral density (BMD) and B12 levels in more than 2,500 men and women participating in the Framingham Osteoporosis Study (2). They found that those with low B12 had significantly lower average BMD, a measure of bone quality used to assess fracture risk. These results follow two earlier studies, which reported similar findings in women (3,4). The Tufts authors conclude that low B12 may be a risk factor for osteoporosis - one that could be remedied by consuming more B12 from supplements and fortified foods.

References

1. Hutto BR. Folate and cobalamin in psychiatric illness. Comp Psychiatry 38:305-14, 1997.
2. Tucker K, et al. Low plasma vitamin B12 is associated with lower BMD: The Framingham Osteoporosis Study. J of Bone and Min Res 20:152-8, 2005.
3. Dhonukshe-Rutten RA, et al. Vitamin B-12 status is associated with bone mineral content and bone mineral density in frail elderly women but not in men. J Nutr 133:801-807, 2003.
4. Stone KL, et al. Low serum vitamin B12
5. levels are associated with increased hip bone loss in older women: A prospective study. J Clin Endocrinol Metab 89:1217-21, 2004.