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Vitamin B-12 (Cyanocobalamin)

Vitamin B12, also known as cobalamin. The "cobal" in the name refers to the metal cobalt contained in B12. Vitamin B12 is required for the normal activity of nerve cells, and works with folate and vitamin B6 to lower blood levels of homocysteine, a chemical in the blood that might contribute to heart disease.

Anemia is usually (but not always) the first sign of B12 deficiency. Earlier in this century, doctors coined the name "pernicious anemia" for a stubborn form of anemia that didn't improve even when the patient was given iron supplements. Today we know that pernicious anemia comes about when the stomach fails to excrete a special substance called intrinsic factor. The body needs the intrinsic factor for efficient absorption of vitamin B12. In 1948, vitamin B12 was identified as the cure for pernicious anemia. B12 deficiency also causes nerve damage, and this may in some cases occur without anemia first developing.

Vitamin B12 has also been proposed as a treatment for numerous other conditions, but as yet there is no definitive evidence that it is effective for any purpose other than correcting deficiency.

Requirements/Sources

Extraordinarily small amounts of vitamin B12 suffice for daily nutritional needs. The official U.S. and Canadian recommendations for daily intake are as follows:

Infants 0–6 months, 0.4 mcg
7–12 months, 0.5 mcg
Children 1–3 years, 0.9 mcg
4–8 years, 1.2 mcg
9–13 years, 1.8 mcg
Males and females 14 years and older, 2.4 mcg
Pregnant women, 2.6 mcg
Nursing women, 2.8 mcg
Vitamin B12 deficiency is rare in the young, but it's not unusual in older people: Probably 10 to 20% of the elderly are deficient in B12.1–4 This may be because older people have lower levels of stomach acid. The vitamin B12 in our food comes attached to proteins, and must be released by acid in the stomach in order to be absorbed. When stomach acid levels are low, we don't absorb as much vitamin B12 from our food. Fortunately, vitamin B12 supplements don't need acid for absorption, and should therefore get around this problem. However, for reasons that are unclear, one study found that B12 deficient seniors need very high dosages of B12 supplements to normalize their levels, as high as 600-1000mcg daily.65

Simiarly, people who take medications that greatly reduce stomach acid, such as Prilosec (omeprazole) or Zantac (ranitidine), also may have trouble absorbing B12 from food, and could benefit from B12 supplements.5–10

Stomach surgery and other conditions affecting the digestive tract can also lead to B12 deficiency. Vitamin B12 absorption or levels in the blood may also be impaired by colchicine (for gout), metformin and phenformin (for diabetes), and AZT (for AIDS).11,12 Exposure to nitrous oxide (such as may be experienced by dentists and dental hygienists) might cause B12 deficiency, but studies disagree.14,15 Slow-release potassium supplements might impair B12 absorption as well.17

Vitamin B12 is found in most animal foods; it is also found only in animal food. Beef, liver, clams, and lamb provide a whopping 80 to 100 mcg of B12 per 3.5-ounce serving, at least 40 times the dietary requirement. Sardines, chicken liver, beef kidney, and calf liver are also good sources, providing between 25 and 60 mcg per serving. Trout, salmon, tuna, eggs, whey, and many cheeses provide at least the recommended daily intake.

Note: Total vegetarians (vegans) must take vitamin B12 supplements or consume B12-fortified foods, or they will eventually become deficient.59,60 Contrary to some reports, seaweed and tempe do not provide B12. (Some forms of blue-green algae, such asspirulina, contain B12, but it is not in an absorbable state.61)

Vitamin B12 is available in three forms: cyanocobalamin, hydrocobalamin, and methylcobalamin. The first is the most widely available and least expensive, but some experts think that the other two forms are preferable.

Severe B12 deficiency can cause anemia and, potentially, nerve damage. The latter may become permanent if the deficiency is not corrected in time. Anemia most often develops first, leading to treatment before permanent nerve damage develops. However, folate supplements can get in the way of this "early warning system." This is why people are cautioned against taking high doses of folate without medical supervision. When taken at a dosage higher than 400 mcg daily, folate can prevent anemia caused by B12 deficiency, thereby allowing permanent nerve damage to develop without any warning. More mild deficiencies of vitamin B12 may cause elevated levels of homocysteine in the blood, potentially increasing risk of heart disease. (See the homocysteine article for more information.) Mild B12 deficiency (too slight to cause anemia) may also impair brain function.

Therapeutic Dosages

For correcting absorption problems caused by medications, taking vitamin B12 at the level of dietary requirements should suffice.

For other purposes, enormously higher daily doses—ranging from 100 to 2,000 mcg—are sometimes recommended.

Therapeutic Uses

It appears that individuals who take medications that dramatically lower stomach acid, such as H2 blockers or proton pump inhibitors, would benefit by taking B12 supplements.18–23 Other individuals likely to be deficient in B12, such as the elderly, or those taking the medications listed in Requirements/Sources, might well benefit from a daily B12 supplement to prevent B12 deficiency.

For pernicious anemia, B12 injections are traditionally used but research has shown that oral B12 works just as well, provided you take enough of it (between 300 and 1,000 mcg daily).

Weak evidence suggests that B12 supplements may improve sperm activity and sperm count; on this basis, they could be useful for male infertility.30,31 Some cases of recurrent miscarriage might be due to vitamin B12 deficiency.

One placebo-controlled, double-blind study, enrolling 49 people with eczema, found benefit with a cream containing vitamin B12 at a concentration of 0.07%.66 Topical B12 is hypothesized to work for eczema by reducing local levels of the substance nitric oxide (not related to nitrous oxide).

On the basis of weak and sometimes contradictory evidence, vitamin B12 has been suggested for HIV,33–37amyotrophic lateral sclerosis,38carpal tunnel syndrome,67diabetic neuropathy,39,40multiple sclerosis (MS),41-45restless legs syndrome,46,47 and tinnitus.48

Some evidence suggests that people with vitiligo (splotchy loss of skin pigmentation) might be deficient in vitamin B12 and supplementation along with folate may be helpful. However, the evidence is very weak and not all studies agree.

Some alternative practitioners recommend the use of injected vitamin B12 for Bell’s palsy. However, the only scientific support for this approach comes from one study that was not double-blind. (For information on the importance of a double-blind design, see Why Does This Database Rely on Double-blind Studies?)

Vitamin B12 is also sometimes recommended for numerous other problems, including asthma, depression, osteoporosis, and periodontal disease, but there is essentially no evidence as yet that it really works.

A double-blind trial of vitamin B12 for seasonal affective disorder (SAD—a type of depression related to lack to lack of light during the winter) failed to find evidence of benefit.

One double-blind, placebo-controlled study of 140 people with mildly low B12 levels failed to find B12 helfpul for improving mental function and mood.

Another study failed to find evidence that vitamin B12 improved general sense of wellbeing among seniors with signs of mild B12 deficiency.

Although vitamin B12 has been proposed as a treatment for Alzheimer's disease, this recommendation is based solely on the results of one small, poorly designed study. More recent and better-designed studies found little to no benefit.

What Is the Scientific Evidence for Vitamin B12?

Male Infertility

Vitamin B12 deficiencies in men can lead to reduced sperm counts and lowered sperm mobility. For this reason, B12 supplements have been tried for improving fertility in men with abnormal sperm production. In one double-blind study of 375 infertile men, supplementation with vitamin B12 produced no benefits on average in the group as a whole.55 However, in a particular subgroup of men with sufficiently low sperm count and sperm motility, B12 appeared to be helpful. Such "dredging" of the data is suspect from a scientific point of view, however, and this study cannot be taken as proof of effectiveness.

Safety Issues

Vitamin B12 appears to be extremely safe. However, in some cases very high doses of vitamin B12 can cause or worsen acne symptoms.

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