Among the B vitamins, three vitamins are directly related to specific nervous system disorders:
- Niacin (B3),
- Thiamin (B1) and
- Cobalamin (B12)
Thiamine deficiency is associated with Wernicke-Korsakoff syndrome, which leads to damage to the central and peripheral nervous system, vision and coordination problems and subsequent dementia (memory loss, confabulations and hallucinations).
Pellagra is caused by niacin (vitamin B3) deficiency and is characterized by dementia (disorientation, memory loss and confusion), diarrhea and dermatitis (also known as ” the disease of 3 Ds”)
Vitamin B12 deficiency is an important cause of treatable dementia. In addition to dementia, it can cause various clinical syndromes: Myelopathy (subacute combined degeneration), optical neuropathy and peripheral polyneuropathy. Myelopathy associated with vitamin B12 is damage to the posterior and lateral spine leading to incoordination (ataxia), gait abnormalities and numbness or paresthesia. Peripheral neuropathy can lead to neuropathic pain, numbness and ascending paresthesia. General weakness can also be observed.
Vitamin B12 has another very important function: together with vitamin B6 (pyridoxine) and folic acid (B9)it can lower homocysteine levels (an important inflammatory marker). Homocysteine is an amino acid produced by protein metabolism. High blood levels of this substance (hyper-homocysteinemia) destroy cells that line the inner walls of blood vessels (endothelium), promote the formation of thromboses and increase LDL cholesterol levels – all factors that promote the occurrence of atherosclerosis and cardiovascular disease. Homocysteine also has a direct neurotoxic effect.
Prospective observational studies have shown that total plasma concentrations of homocysteine are strongly associated with the risk of dementia and AD, and an increase in homocysteine concentration by 5 μmol/L increases the risk of AD by 40% (even after adjustment for variables such as age, sex, apolipoprotein E (ApoE) genotype and serum folate, vitamin B6 and vitamin B12 concentrations).
Given its role in lowering homocysteine plasma levels, folic acid, vitamin B12 and vitamin B6 may reduce the risk of dementia, enabling healthy cerebral aging.
Smith AD, Smith SM, de Jager CA, et al. Homocysteine-lowering by B vitamins slows the rate of accelerated brain atrophy in mild cognitive impairment: a randomized controlled trial. PLoS One. 2010;5(9):e12244. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2935890/pdf/pone.0012244.pdf
Seshadri S, Beiser A, Selhub J, Jacques PF et al, Plasmahomocysteine as a risk factor for dementia and Alzheimer’s disease, N Engl J Med 2002; 346:476-483 https://www.nejm.org/doi/10.1056/NEJMoa011613?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub%3dwww.ncbi.nlm.nih.gov