Vitamin B12 is essential for the manufacture of S-adenosylmethionine which is involved in neurotransmitter metabolism. A deficiency of vitamin B12 may therefore lead to a number of mental disorders such as depression, dementia and demyelinating myelopathy. In order to ensure mental wellbeing it is therefore necessary to ensure that you get the correct amount of vitamin B12.
In "A Shot in Time Saves Mind" by Syd Baumel, he says, "Because pernicious anemia is a highly age-related disease, most doctors today are alert for it in elderly patients with neuropsychiatric symptoms. But studies suggest any depressed person has a 10 to 30 percent chance of being B12-shy, usually without pernicious anemia as the cause. That deficiency, according to Cees van Tiggelen et al, 'has profound effects on several neurotransmitter systems and results in significantly reduced norepinephrine levels in the brain.' Norepinephrine is one of the brain's most important good-mood neurotransmitters." (http://www.mts.net/~baumel/B12.html)
Vitamin B12 is also involved in the manufacture of S-adenosylmethionine. S-adenosylmethionine is the precursor to what we commonly know as serotonin. Serotonin is a neurotransmitter in the brain which plays a large part in our emotional health.
In the University of Berkeley's term paper on "Nutritional Factors: Natural Treatment for Depression" the role of serotonin as well as various other neurotransmitters are explained: "In depression there are reduced levels of serotonin. They are 'well-being' neurotransmitters that are involved with control of emotions, mood, arousal, temperature regulation, etc. If a person has too little serotonin, there is 'less' communication between cells which will eventually lead to lowering of mood. In addition, low brain levels of dopamine and norepinephrine can cause depression. (Robertson, 1997). These are 'energizer' neurotransmitters creating alertness and giving rise to action and excitement. If low brain contents of these mood neurotransmitters produce the symptoms of depression, correcting it by increasing their number to the optimal level should help reducing the symptom." (http://sulcus.berkeley.edu/mcb/165_001/papers/manuscripts/_9.html)
This same paper goes on later to say "Although less common than that of folic acid, Vitamin B12 deficiency can also cause depression. Lots of evidence show that correcting the folic acid and/or vitamin B12 deficiency with supplements or proper food intake result in a dramatic improvement in mood. The serotonin-elevating effects are undoubtedly responsible for much of the antidepressive effects of folic acid and vitamin B12."
Neurotransmission and the correct production of neurotransmitters are therefore essential for mental wellbeing. Vitamin B12 is known to play a large role in the manufacture of these neurotransmitters and therefore in mental health and a "good mood". In order to ensure mental health therefore you should ensure that you are receiving the correct amount of vitamin B12 rich foods or supplements in your diet.
Glossary:
S-adenosylmethionine: neurotransmitter that is the precursor to serotonin
Demyelinating: removal of the myelin sheath around a nerve
Myelopathy: "a disorder in which the tissue of the spinal cord is diseased or damaged (Thomson Gale)