Vitamin (water-soluble)

• UK RNI 1.5 mcg
• US RDA 3 mcg

Functions

• Detoxifies cyanide (found in tobacco smoke and some foods)
• DNA synthesis
• Growth and development
• Healthy nerves

Good food sources

• Cheese
• Eggs
• Fish
• Liver
• Meat
• Yoghurt

Found only in animal foods, although some vegan products are fortified with extra B12 by the manufacturers.

Deficiency symptoms

• Agitation
• Anaemia
• Disorientation and confusion
• Fatigue
• Hallucinations
• Increased risk of heart disease (by promoting raised homocysteine levels)
• Loss of sensation in feet and legs
• Nerve and spinal cord degeneration (with unsteadiness and mental deterioration)
• Sore, smooth tongue

Low levels of several B vitamins have been found in psychiatric patients and (for B12 in particular) in senile dementia.

Preventing deficiency

Although vitamin B12 is only found in animal foods, vegans (who do not eat any animal foods) survive because our requirements for this vitamin are very small. Yeast extracts used as food flavourings are often fortified with vitamin B12, and vegans should ensure that they consume such foods regularly, or some other product with a guaranteed vitamin B12 content, such as a B12 supplement. Algae and seaweeds are sometimes promoted as plant sources of this vitamin, but it is now known that they contain only vitamin B12 analogues - substances which are quite similar to vitamin B12 but may actually block the bioavailability of the real vitamin.

Between 16 and 75 per cent of vitamin B12 in a meal is absorbed. The more B12 is ingested, the lower the percentage of absorption. The bioavailability of Vitamin B12 is reduced by a lack of intrinsic factor (see below), by parasitic infections and by bacterial overgrowth of the small intestine. Chronic diarrhoea, tapework and other intestinal disorders can also inhibit B12 absorption.

Comments

Vitamin B12 contains the trace element cobalt, and provides its only known function in the human body.

Pernicious anaemia, which is the vitamin B12 deficiency disease, is not usually caused by a poor intake of this vitamin, but by a lack of 'intrinsic factor', a substance found in the stomach which combines with vitamin B12 and allows it to be absorbed by the lower part of the small intestine. Elderly people are particularly susceptible to a lack of intrinsic factor. Without it, only about 1 per cent of dietary vitamin B12 can be absorbed. Intestinal parasites can also cause vitamin B12 malabsorption.

Vitamin B12 deficiency is common even in the presence of normal blood levels, particularly in the elderly. In a study carried out on 548 surviving members of the original Framingham study, serum B12 and folate levels were compared with levels of metabolites (biochemical markers) which are raised if the nutrients are not available in sufficient quantities to enable them to be broken down. The rate of B12 deficiency was found to be 12 per cent. (Lindenbaum J et al: Prevalence of cobalamin deficiency in the Framingham elderly population. Am J Clin Nutr 60(1):2-11, 1994.)

SUPPLEMENTATION

In research studies, Vitamin B12 supplements (administered by injection) have been found to help:

• Chronic pain
• Fatigue
• Mental confusion
• Multiple sclerosis
• Numbness of the extremities
• Some cases of mental illness
• Some cases of tinnitus

Preferred form and suggested intake

Vitamin B12 is best taken by injection (administered by a doctor) when there is an absorption problem. Other forms of supplementation are vitamin B12 tablets or capsules, or B Complex supplements (typically providing up to 100 mcg each), and vitamin B12 nasal gel. There is some evidence that the nasal gel is better absorbed than oral supplements.

Cautions

No cautions are required with vitamin B12 supplementation as there is no known unsafe dose.