Vitamin B12
VITAMIN B12 (COBALAMIN)
Vitamin B12 (cobalamin), along with its various active forms or coenzymes (hydroxocobalamin, methylcobalamin and adenosylcobalamin), is one of the main water-soluble vitamins. Vitamin B12 is involved in many metabolic processes in the body. It also promotes the normal nervous system and brain function, the formation of epithelial cells in the intestine, and the normal development and function of red and white blood cells. Vitamin B12 is also involved in the synthesis of DNA and nucleotides. It is needed in folate activation and the building process of phospholipids in cell membranes. In addition, vitamin B12 is involved in synthesizing succinyl-CoA, an intermediate in the citric acid cycle. It is therefore needed for energy metabolism.
B12 deficiency is relatively common among vegans. Vitamin B12 deficiency may also occur in people with chronic gastritis (long-term stomach lining inflammation). The stomach secretes the intrinsic factor necessary to absorb vitamin B12 in the small intestine. Chronic gastritis (or atrophic gastritis) reduces intrinsic factor secretion. Atrophic gastritis is a relatively common illness in individuals over 60 (10–30 %). Left untreated, long-term vitamin B12 deficiency may lead to pernicious anemia or various nervous system disorders (hand numbness, memory problems, problems walking, etc.).
Vitamin B12 deficiency may also be caused by various inflammatory intestinal diseases and parasites. Usually, the deficiency accrues slowly over time; the liver can store several years' worth of vitamin B12 depending on the previous intake. Some estimates show deficiency symptoms can become apparent after 20–30 years. Such symptoms may include nervous system disorders, fatigue, and exhaustion, breathlessness, or symptoms involving mucous membranes. Vitamin B12 deficiency is common particularly in the aged population (in the Lieto aged population study, 11 % of those aged 65+ were deficient). Some of these symptoms are subclinical or the individuals are only found to have mild, non-specific neurological disorders. A study by the University of Turku found that up to 32 % of Finns over 65 were barely over the vitamin B12 deficiency limit.
Like folate, vitamin B12 is involved in homocysteine metabolism (see section Homocysteine). High homocysteine level is a known risk factor for cardiovascular diseases as well as Alzheimer’s disease and dementia. The risk of Alzheimer’s disease increases significantly if the homocysteine level in the blood exceeds 15 μmol/L. Gray matter atrophy associated with Alzheimer's disease can be prevented in the elderly by taking a supplement of 800 mcg of folate, 20 mg of vitamin B6, and 500 mcg of vitamin B12.
Some studies suggest that vitamin B12 deficiency may be a predisposing factor for breast cancer. Vitamin B12 deficiency in pregnant women may cause developmental issues in the fetal neural tube. Low levels of vitamin B12 in the body are also linked to depression and the onset of osteoporosis.
Vitamin B12 occurs in dietary supplements mainly as cyanocobalamin or methylcobalamin. Some products contain a slightly rarer form, adenosylcobalamin, which is the form vitamin B12 takes inside cells. Vitamin B12 injections, available by prescription, contain cyanocobalamin. Methylcobalamin, on the other hand, comes in a bioactive form – it is significantly more easily absorbed via the digestive tract compared to cyanocobalamin. Methylcobalamin also has a more comprehensive effect on the body compared to cyanocobalamin.
VITAMIN B12 FROM FOOD
The recommended intake of cobalamin (vitamin B12) for adults in Finland is as follows:
- Men: 2.0 mcg per day
- Women: 2.0 mcg per day
- Breastfeeding women: 2.6 mcg per day
The average vitamin B12 intake for adult Finnish men (aged 25–64) is 7.1 mcg per day, and for adult women (aged 25–64) it is 5.0 mcg per day. 14.1% of Finnish men and 22.7 % of women take a dietary supplement containing vitamin B12. The average vitamin B12 intake from supplements across all population groups is 25 mcg daily.
No toxic adverse effects have been found in association with vitamin B12, whether ingested as food or as a dietary supplement. One-time doses of 1–2 mg have been used to treat vitamin B12 deficiency and pernicious anemia. No adverse effects have been detected with such doses. Because of this, no maximum daily intake limit has been established for vitamin B12.