Health benefits of vitamin B12 are well-known. Popularly known as an energy vitamin (it works with other B vitamins to convert food into glucose), this water-soluble vitamin helps in red blood cells formation, healthy digestion and boosting immune functions.
However, these benefits are not just limited to physiological dimensions. Vitamin B12 also plays a crucial role in your brain and nervous system health. In fact, experts believe that B12 and folate deficiency may play a significant role in age related cognitive decline, dementia  and other neuro-degenerative disorders like Alzheimer’s.
Role of Vitamin B12 in brain and nerve functions
The exact biochemical play that leads to nerve and cognitive damage with low B12 status is not yet completely clear. However, science points out that the role of vitamin B12 in the ‘One Carbon’ pathway may hold some answers. This pathway is a series of complex biochemical reactions that leads to the production of many important proteins, hormones and other substances.
Vitamin B12 works as a co-factor for the synthesis of methionine, an important milestone in One Carbon pathway. Methionine is converted to S-adenosyl methionine (SAMe). It is an amino acid that donates methyl groups during methylation reactions in the body.
Methylation is an extremely important process where methyl groups are transferred to amino acids, enzymes, neurotransmitters, proteins and DNA. These reactions help to regulate healing, energy, DNA functions, immunity, detoxification and neurological functions. Myelin and neurotransmitters, required for healthy brain and nervous system functions, are also synthesised during these reactions.
In fact, SAMe is a master methyl donor in the body and is involved in a wide range of detoxification processes. Inadequate B12 disrupts the production of SAMe, leading to the build-up all kinds of toxins in the brain and other tissues.
Myelin synthesis and stabilization
Vitamin B12 helps in the synthesis and regeneration of myelin, a fatty sheath that covers the nerves of central and the peripheral nervous system. Nerves carry messages between different parts of the body with the help of electrical impulses or signals. What myelin does is that it protects nerve fibres from damage and helps in effective and faster transmission of electrical impulses across nerves.
B12 is also required for the conversion of methylmelonyl-CoA into succinyl CoA. If this conversion doesn’t take place, methylmelonyl-CoA is converted into MMA, a substance that leads to the formation of a weak myelin sheath.
A damaged, weak or destroyed myelin sheath, as with B12 deficiency, impairs the transmission of signals across the nerves, causing it to slow down. This leads to poor nerve function and neurological problems. Point in case: if you are not getting enough B12, you are at a high risk of developing nerve damage, sometimes irreversible if deficiency remains unresolved for a long time.
Vitamin B12 and folic acid are involved in the breakdown of homocysteine, an amino acid produced naturally in the body when you digest proteins. This is how it works. When SAMe donates its methyl group, it becomes SAH (S−Adenosyl Homocysteine), and is eventually converted to homocysteine.
Homocysteine is converted back to methionine with the help of B12 and folic acid. (Homocysteine may also enter another pathway where it becomes cysteine, and this process requires B6)
When you are deficient in B12 and folate, levels of homocysteine may build-up. High levels of homocysteine are believed to damage nerves and blood vessels and are associated with increased risk of heart attack, stroke, dementia, cognitive decline and Alzheimer’s disease. An increase in homocysteine levels cause also the brain to shrink.
Emerging research suggests that B12 supplementation can improve cognitive performance and reduce the risk of brain atrophy (shrinkage).  An Oxford University study found that vitamin B12, when taken with vitamin B6 and folic acid, improved cognitive functions and lowered brain atrophy in people with Alzheimer’s disease. 
Production of neurotransmitters
Vitamin B12 and folate are involved in the synthesis of neurotransmitters such as dopamine, serotonin, melatonin and norepinephrine. These brain chemicals are known to regulate many brain functions including mood, sleep, learning and memory.
Neurological symptoms of Vitamin B12 deficiency
- Pain, numbness, and tingling in hands or feet
- Depression and anxiety
- Cognitive decline
- Memory loss and dementia
- Sensory loss
- Lack of focus and co-ordination
- Abnormal reflexes or loss of reflexes
- Brain fog and confusion
- Behavioral disturbances
- Abnormal gait
- Brain shrinkage
- Bladder dysfunction
Several factors increase the risk for vitamin B12 deficiency – including pernicious anemia, strict vegetarian diet, reduced amount of stomach acid (due to age, prolonged use of antacids and drugs like metformin, H2 blockers and proton pump inhibitors), heavy alcohol consumption, bacterial overgrowth in intestines, weight loss surgery, intestinal surgery, Crohn’s disease and celiac disease. If you are over 40 or think any of these risk factors affect you, taking vitamin B12 supplements may help.
- Clarke R, Birks J, Nexo E, Ueland PM, Schneede J, Scott J, et al. Low vitamin B-12 status and risk of cognitive decline in older adults. Am J Clin Nutr 2007;86:1384-91.
- Grober at al. Neuroenhancement with Vitamin B12—Underestimated Neurological Significance. Nutrients. 2013.
- Gwenaëlle Douauda et al. Preventing Alzheimer’s disease-related gray matter atrophy by B-vitamin treatment. PNAS. 2013