Neural tube defects (NTD) may result in anencephaly or spina bifida, which are mostly fatal congenital malformations of the central nervous system. The defects arise from failure of embryonic neural tube to close, which occurs between the 21 st and 28 th days after conception, a time when many women are unaware of their pregnancy (38). Randomized controlled trials have demonstrated 60% to 100% reductions in NTD cases when women consumed folic acid supplements in addition to a varied diet during the month before and the month after conception. Increasing evidence indicates that the homocysteine-lowering effect of folic acid plays a critical role in reducing the risk of NTD (39). twelve for effective functioning of the methionine synthase enzyme. in B12 levels and elevated homocysteine concentrations have been found in the blood and amniotic fluid of pregnant women at high risk of NTD (40). The recent meta-analysis of 12 case-control studies, including 567 mothers with current or prior NTD-affected pregnancy and 1,566 unaffected mothers, showed that low maternal vitamin B12 status was associated with an increased risk of NTD (41). Yet, whether vitamin B12 supplementation may be beneficial in the prevention of NTD has not been evaluated (42).
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The occurrence of vitamin B12 deficiency prevails in the elderly population and has been frequently associated with Alzheimer’s disease (reviewed in 43). One study found lower vitamin B12 levels in the cerebrospinal fluid of patients with Alzheimer’s disease than in patients with other types of dementia, though blood levels of vitamin B12 did not differ (44). The reason for the association of low vitamin B12 status with Alzheimer’s disease is not clear. Vitamin B12 deficiency, like folate deficiency, may lead to decreased synthesis of methionine and S-adenosylmethionine (SAM), thereby adversely affecting methylation reactions. Methylation reactions are essential for the metabolism of components of the myelin sheath of nerve cells as well as for synthesis of neurotransmitters (18). Other metabolic implications of vitamin B12 deficiency include the accumulation of homocysteine and methylmalonic acid, which might contribute to the neuropathologic features of dementia (43).
A large majority of cross-sectional and prospective cohort studies have associated elevated homocysteine concentrations with measures of poor cognitive scores and increased risk of dementia, including Alzheimer’s disease (reviewed in 45). A case-control study of 164 patients with dementia of Alzheimer’s type included 76 cases in which the diagnosis of Alzheimer’s disease was confirmed by examination of brain cells after deathpared to 108 control subjects without evidence of dementia, subjects with dementia of Alzheimer’s type and confirmed Alzheimer’s disease had higher blood homocysteine levels and lower blood levels of folate and vitamin B12. Measures of general nutritional status indicated that the association of increased homocysteine levels and diminished vitamin B12 status with Alzheimer’s disease was not due to dementia-related malnutrition (46). In a sample of 1,092 men and women without dementia followed for an average of 10 years, those with higher plasma homocysteine levels at baseline had a significantly higher risk of developing Alzheimer’s disease and other types of dementia. Specifically, those with plasma homocysteine levels greater than 14 ?mol/L had nearly double the risk of developing Alzheimer’s disease (47). A study in 650 elderly men and women reported that the risk of elevated plasma homocysteine levels was significantly higher in those with lower cognitive function scores (48). A prospective study in 816 elderly men and women reported that those with hyperhomocysteinemia (homocysteine levels >15 ?mol/L) had a significantly higher risk of developing Alzheimer’s disease or dementia. Although raised homocysteine levels might be partly due to a poor vitamin B12 status, the latter was not related to risk of Alzheimer’s disease or dementia in this study (49).