Vitamin B-12, or cobalamin, participates in the synthesis of DNA within your cells; it is essential for the production of myelin, which is the insulating sheath that protects your nerves. Cobalamin deficiency leads to the production of enlarged, poorly-functioning red blood cells. This condition, called megaloblastic anemia, is reversible with vitamin B-12 supplementation. Prolonged cobalamin deficiency also causes nerve damage, manifested by sensory deficits, muscle spasticity, walking difficulties, weakness, confusion and paranoia. Neurological damage due to vitamin B-12 deficiency can be irreversible.
Vitamin B-12 Absorption
When your stomach produces acid, it also secretes a specialized protein called intrinsic factor. Intrinsic factor binds to the vitamin B-12 from your diet, and the intrinsic factor-B12 complex is absorbed from your intestine. If your stomach does not make enough intrinsic factor, you cannot efficiently absorb vitamin B-12. However, your liver reserves of vitamin B-12 can sustain your physiologic needs for up to six years.
Vitamin B-12 Requirements
Although your daily needs for vitamin B-12 are small -- measured in micrograms, rather than milligrams like other vitamins -- it is still an essential nutrient. In 2002, the National Academy of Sciences updated its Dietary Reference Intakes, or DRIs, for vitamin B-12, which range from around .5 mcg to nearly 3 mcg daily, based upon your age and gender. Dr. Elson Haas, author of "Staying Healthy with Nutrition," recommends 10 to 20 mcg daily as "a good insurance level."
Vitamin B-12 Supplementation
Due to the unique absorption of vitamin B-12 -- no other vitamin requires intrinsic factor for its assimilation -- physicians assumed for years that only injectable forms of cobalamin would successfully reverse deficiencies in individuals without intrinsic factor. These patients, many of them elderly, reported to their doctors every month to receive their vitamin B-12 shots. However, scientists eventually proved that oral supplementation with large doses of vitamin B-12 also corrected deficiencies, an observation that was reaffirmed in a December 2010 "Presse Médicale" study.
Sublingual Vitamin B-12
The discovery that vitamin B-12 deficiency was corrected by supplementing with large doses of oral cobalamin prompted the development of sublingual, or under-the-tongue, formulations of the vitamin. The mucous membrane beneath your tongue is capable of absorbing certain medications, such as nitroglycerin and nifedipine, and vitamin B-12 also crosses this barrier fairly readily. A study published in December 2003 in the "British Journal of Clinical Pharmacology" indicated that sublingual doses of vitamin B-12 successfully reversed cobalamin deficiency. The researchers concluded that 500 mcg of cobalamin administered orally or sublingually is effective for correcting vitamin B-12 deficiency.
Vitamin B-12 is an essential nutrient that is only needed in small amounts by humans, but deficiencies are potentially devastating. Although vitamin B-12 is inefficiently absorbed by people who have low levels of intrinsic factor, this inefficiency can be overcome by supplementing with relatively large doses of cobalamin -- 500 to 1,000 mcg daily -- either orally or sublingually. If you are taking vitamin B-12 for a deficiency, check with your doctor to see which form is best for you.