Vitamin B12, also known as cobalamin, is a water-soluble vitamin involved in the metabolism of every cell of the human body. It is required for proper red blood cell formation, neurological function, and DNA synthesis. It is particularly important in the normal functioning of the nervous system via its role in the synthesis of myelin, and in the maturation of developing red blood cells in the bone marrow. 

Vitamin B12 supports bone health and prevent osteoporosis. One study in more than 2,500 adults showed that people with a vitamin B12 deficiency had lower than normal bone mineral density. Both men and women with low vitamin B12 levels had on average lower bone mineral densities—putting them at greater risk for osteoporosis—than men and women with higher levels. (1) Vitamin B12 is also involved in red blood cell formation. Low vitamin B12 levels cause a reduction in red blood cell formation and prevent them from developing regularly. Due to this larger and irregular shape, the red blood cells are unable to move from the bone marrow into the bloodstream at an appropriate rate, causing megaloblastic anemia. 

Vitamin B12 is needed for the production of serotonin, a chemical responsible for regulating mood. Vitamin B12 supplements may help improve mood in people with a deficiency. One study in people with depression and low vitamin B12 levels found that those who received both antidepressants and vitamin B12 were more likely to show improved depression symptoms, compared to those treated with antidepressants alone. (2) Another study discovered that vitamin B12 deficiency was associated with twice the risk of severe depression. (3) Vitamin B12 may also help prevent brain atrophy and memory loss. According to one study, Low Vitamin B12 concentrations within the normal range are associated with poorer memory performance, which is an effect that is partially mediated by the reduced microstructural integrity of the hippocampus. (4) MCI patients with low-normal Vitamin B12 showed significantly poorer learning ability and recognition performance than did patients with high-normal Vitamin B12. 

In addition, Vitamin B12 is involved in energy production in the body. Taking a supplement or increasing intake of Vitamin B12 will likely improve energy levels for those significantly deficient in vitamin B12. A study has shown that vitamin B12 also helps to decrease homocysteine levels, which may reduce your risk of heart disease. (5) 

Vitamin B12 deficiency is characterized by megaloblastic anemia, fatigue, weakness, constipation, loss of appetite, and weight loss. Neurological changes, such as numbness and tingling in the hands and feet, can also occur. A deficiency in vitamin B12 causes an accumulation of homocysteine in the blood and might decrease levels of substances needed to metabolize neurotransmitters. (6, 7) Observational studies show positive associations between elevated homocysteine levels and the incidence of both Alzheimer’s disease and dementia. Low vitamin B12 status has also been positively associated with cognitive decline. During infancy, signs of a vitamin B12 deficiency include failure to thrive, movement disorders, or developmental delays. (8, 9) Additional symptoms of vitamin B12 deficiency include difficulty maintaining balance, depression, confusion, dementia, poor memory, and soreness of the mouth or tongue. (10) 

References: 

1. Tucker KL, Hannan MT, Qiao N, Jacques PF, Selhub J, Cupples LA, Kiel DP. Low plasma vitamin B12 is associated with lower BMD: the Framingham Osteoporosis Study. J Bone Miner Res. 2005 Jan; 20 (1): 152-8. Epub 2004 Oct 25. 

2. Ehsan Ullah Syed, Mohammad Wasay, Safia Awan. Vitamin B12 Supplementation in Treating Major Depressive Disorder: A Randomized Controlled Trial Open Neurol J. 2013; 7: 44–48. 

3. Penninx BW, Guralnik JM, Ferrucci L, Fried LP, Allen RH, Stabler SP. Vitamin B(12) deficiency and depression in physically disabled older women: epidemiologic evidence from the Women's Health and Aging Study. Am J Psychiatry. 2000 May; 157 (5): 715-21 

4. Köbe T, Witte AV, Schnelle A, Grittner U, Tesky VA, Pantel J, Schuchardt JP, Hahn A, Bohlken J, Rujescu D, Flöel A. Vitamin B-12 concentration, memory performance, and hippocampal structure in patients with mild cognitive impairment. Am J Clin Nutr. 2016 Apr; 103 (4): 1045-54. 

5. Kira Leishear, Luigi Ferrucci, Fulvio Lauretani, Robert M. Boudreau, Stephanie A. Studenski, Caterina Rosano, Rosanna Abbate, Anna M. Gori, Anna M. Corsi, Angelo Di Iorio, Jack M. Guralnik, Stefania Bandinelli, Anne B. Newman, Elsa S. Strotmeyer. Vitamin B12 and Homocysteine Levels and 6-Year Change in Peripheral Nerve Function and Neurological Signs J Gerontol A Biol Sci Med Sci. 2012 May; 67A (5): 537–543. 

6. Clarke R. B-vitamins and prevention of dementia. Proc Nutr Soc. 2008 Feb; 67 (1): 75-81. 

7. Hutto BR. Folate and cobalamin in psychiatric illness. Compr Psychiatry. 1997 Nov-Dec; 38 (6): 305-14. 

8. Clarke R, Birks J, Nexo E, Ueland PM, Schneede J, Scott J, Molloy A, Evans JG. Low vitamin B-12 status and risk of cognitive decline in older adults. Am J Clin Nutr. 2007 Nov; 86 (5): 1384-91. 

9. Bjørke Monsen AL, Ueland PM. Homocysteine and methylmalonic acid in diagnosis and risk assessment from infancy to adolescence. Am J Clin Nutr. 2003 Jul; 78 (1): 7-21. 

10. Bottiglieri T. Folate, vitamin B12, and neuropsychiatric disorders. Nutr Rev. 1996 Dec; 54 (12): 382-90.