BACKGROUND:Vitamin B12 is a water-soluble vitamin, also known as cyanocobalamin, hydroxycobalamine, and by other names. It is found naturally in foods of animal origin – dairy products, meats, eggs and seafood – and is commonly added to fortified foods. Vitamin B12 supplements often include vitamin B6 and folic acid (vitamin B9) because the three are related in their biological function and proposed benefits.
Vitamin B12 is involved in several important biochemical reactions in the body, primarily the production of red blood cells, energy from fatty acids and neurotransmitters. The latter are chemicals that relay messages in the nervous system and brain. Neurotransmitters like serotonin and dopamine are involved in cognitive function and mood.
A deficiency in vitamin B12 can lead to anaemia and neurological damage. Prior to these more serious symptoms, changes such as memory loss, reduced attention and difficulty solving problems can develop. This has led to much interest in the role of vitamin B12 in cognitive decline and dementia among the elderly.
As people get older, vitamin B12 is not absorbed as well from dietary sources, leading to proposals that older people should use supplements of vitamin B12, B6 and folic acid to promote cognitive function.
EVIDENCE FROM STUDIES
A deficiency in vitamin B12, vitamin B6 or folic acid leads to higher blood levels of the compound homocysteine. Some studies have shown that people with dementia or Alzheimer’s disease tend to have lower levels of vitamin B12 and elevated homocysteine levels. One hypothesis is that elevated homocysteine levels may contribute to cognitive decline.
A few small studies gave vitamin B12 supplements to people with Alzheimer’s disease. Most of these were not controlled studies. While they found that serum levels of the vitamin increased, and homocysteine levels decreased, scores on cognitive tests did not change. These results suggest that vitamin B12 may not be able to reverse cognitive decline.
A Cochrane systematic review identified three randomised controlled trials in which older adults with low serum levels of vitamin B12 were given either vitamin B12 or placebo. The participants scored from normal to mild dementia in cognitive tests. No differences in scores were found between the groups in the studies. However, the longest study lasted only three months.
Another Cochrane review identified randomised controlled trials, in which supplements contained folic acid with or without vitamin B12. The review concluded that supplements of folic acid or vitamin B12 did not provide cognitive benefits, either for healthy adults or those with dementia. However, one of the studies with folic acid only did show some benefits.
Since these reviews were published, more studies have reported that these supplements have not produced improvements in cognitive function.
PROBLEMATIC ASPECTS
Vitamin B12 is usually safe, even in high doses. Most vitamins have a recommended upper tolerable limit, but vitamin B12 does not because adverse effects are so rarely reported.
RECOMMENDATIONS
The available information on vitamin B12 reflects the complexity of our bodies, our minds and nutritional requirements. Deficiencies in vitamin B12 in the diet and serum levels are associated with cognitive health. However, the evidence that vitamin B supplements prevent or reverse cognitive decline is not convincing.
Those at risk of vitamin B12 deficiency should consider a supplement for overall health. This would include strict vegetarians and those taking medications to reduce stomach acid production. Stomach acid is required to release vitamin B12 from food sources. Most people in developed countries get sufficient vitamin B12 from diet and fortified foods. The best source of vitamin B12 is from a balanced diet that includes the foods mentioned above.
Dónal OMathúna has a PhD in pharmacy, researching herbal remedies, and an MA in bioethics, and is a senior lecturer in the School of Nursing, Dublin City University
