The brain needs to metabolize neurotransmitters, and to do this it produces S- adenosylmethionine, for which B12 is a very important part of production. It can then be seen how vitamin B12 deficiencies could contribute to a wide array of mental health problems like depression, demyelinating myelopathy and dementia or Alzheimer's.
Norepinephrine is an essential neurotransmitter for maintaining a positive mood, and those who are deficient in B12 have shown to be more prone to depression and other neuropsychiatric symptoms. Diseases such as pernicious anemia and dementia, which are most common in the elderly, have also been linked to B12 deficiencies. A book by Syd Baumel called "A Shot in Time Saves Mind" discusses the possibility that patients suffering depression often have difficulty absorbing B12 without showing signs of pernicious anemia. For more info see http://www.omegaflaxseedoil.com/benefits/vitaminb12/
S-adenosylmethionine is what most people know to be called Serotonin, and is an important neurotransmitter in the brain. B12 is detrimental to the manufacture of serotonin, so therefore has a very large role in the maintaining of both emotional and mental health.
It is a fairly common fact that those who suffer from depression have a problem with serotonin. Without adequate serotonin levels a person will have trouble with controlling their emotions, moodiness, sexual arousal, body temperature, skin problems, insomnia and headaches. No matter how hard someone with low serotonin levels tries to lift their mood, the poor communication between the cells will prevent it from being possible. As soon as these symptoms are noticed, a supplement should be introduced in the diet.
Vitamin B12 and folic acid deficiencies can both contribute to the onset of serious depression. Many studies have been conducted that have proven supplementing B12 and folic acid simultaneously in depression patients along with a good dietary intake will elevate moods dramatically. It is for this reason vitamin B12 and folic acid are considered to be anti-depressant vitamins.
Mental health is detrimental to leading a happy life, and neurotransmitters being produced at the right levels are a huge part of that. In order to make the neurotransmitters, the brain must have adequate B12, which has to come from your dietary intake. Eat healthy foods high in vitamin B and definitely consider taking a good B supplement that has all the B vitamins and folate.