Vitamin B12: Alzheimer's Disease and Dementia

Individuals with Alzheimer's disease often have low blood levels of vitamin B₁₂.

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. 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, adversely affecting methylation reactions essential for the metabolism of components of the myelin sheath of nerve cells, as well as neurotransmitters. Moderately increased homocysteine levels, as well as decreased folate and vitamin B₁₂ levels have also been associated with Alzheimer's disease and vascular dementia.

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 the brain cells after death. Compared 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 folate and vitamin B₁₂ levels.

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. Low serum vitamin B₁₂ (< 150 pmol/L) or folate (< 10 nmol/L) levels were associated with a doubling of the risk of developing Alzheimer's disease in 370 elderly men and women followed over 3 years.

In a sample of 1092 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. Those with plasma homocysteine levels greater than 14 mmol/L had nearly double the risk of developing Alzheimer's disease.

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