Vitamin B12 Overview

Vitamin B 12 , an essential nutrient, is also known as cobalamin. The cobal in the name refers to the metal cobalt contained in B 12 . Vitamin B 12 is required for the normal activity of nerve cells and works with folate and vitamin B 6 to lower blood levels of homocysteine , a chemical in the blood that might contribute to heart disease. B 12 also plays a role in the body's manufacture of S-adenosylmethionine (SAMe) .

Anemia is usually (but not always) the first sign of B 12 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 B 12 . In 1948, vitamin B 12 was identified as the cure for pernicious anemia. B 12 deficiency also causes nerve damage, and this may, in some cases, occur without anemia first developing.

Vitamin B 12 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.

Extraordinarily small amounts of vitamin B 12 suffice for daily nutritional needs. The official US 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 B 12 deficiency is rare in the young, but it's not unusual in older people: Probably 10% to 20% of the elderly are deficient in B 12 . ^{1 2 3} This may be because older people have lower levels of stomach acid. The vitamin B 12 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 B 12 from our food. Fortunately, vitamin B 12 supplements don't need acid for absorption and should, therefore, get around this problem. However, for reasons that are unclear, one study found that B 12 -deficient seniors need very high dosages of the supplements to normalize their levels, as high as 600 to 1,000 mcg daily. ⁴ Similarly, people who take medications that greatly reduce stomach acid, such as omeprazole (Prilosec) or ranitidine (Zantac) also may have trouble absorbing B 12 from food and could benefit from supplementation. ^{5 6 7 8 9} Stomach surgery and other conditions affecting the digestive tract can also lead to B 12 deficiency. Vitamin B 12 absorption or levels in the blood may also be impaired by colchicine (for gout), metformin and phenformin (for diabetes), and AZT (for AIDS). ¹⁰ Exposure to nitrous oxide (such as may be experienced by dentists and dental hygienists) might cause B 12 deficiency, but studies disagree. ¹¹ Slow-release potassium supplements might impair B 12 absorption as well. ¹² Vitamin B 12 is found in most animal foods; it is also found onlyin animal food. Beef, liver, clams, and lamb provide a whopping 80 to 100 mcg of B 12 per 3.5-oz 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 B 12 supplements or consume B 12 -fortified foods, or they will eventually become deficient. ¹³ Contrary to some reports, seaweed and tempeh do notprovide B 12 . (Some forms of blue-green algae, such as spirulina , contain B 12 , but it is not in an absorbable state. ¹⁴ )

Vitamin B 12 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 B 12 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 B 12 deficiency, thereby allowing permanent nerve damage to develop without any warning. More mild deficiencies of vitamin B 12 may cause elevated levels of homocysteine in the blood, potentially increasing risk of heart disease. (See the Homocysteine article for more information.) Mild B 12 deficiency (too slight to cause anemia) may also impair brain function. ¹⁵

For correcting absorption problems caused by medications, taking vitamin B 12 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.

Vitamin B 12 deficiencies in men can lead to reduced sperm counts and lowered sperm mobility. For this reason, B 12 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 B 12 produced no benefits on average in the group as a whole. ¹⁶ However, in a particular subgroup of men with sufficiently low sperm count and sperm motility, B 12 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.

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