Low intakes of vitamin B6, B12 and folate are know to cause elevations in plasma levels of homocysteine. High levels of homocysteine in turn are associated with dementia, possibly because they are a risk factor for cardiovascular disease. It is known that low intakes of B vitamins are associated with reversible dementia (vitamin B12 and folate), depression (folate) and convulsions (vitamin B6), and it is well established that the B vitamins are needed for nerve function and that deficiency of the B vitamins leads to dysfunction of the nervous system. If deficiencies of B vitamins are involved in cognitive decline, it would be expected that the effects of poor B vitamin status would be seen most easily in the elderly as certain cognitive functions show declines with age. In fact, supplementation of B vitamins shows beneficial effects on some aspects of cognitive ability in elderly subjects.
For example, researchers1 have investigated the associations between plasma concentrations of homocysteine, vitamin B6, vitamin B12 and folate with scores on cognitive tests in elderly male subjects aged 54 to 71 years. The tests involved language tests, speed and attention tests, memory tests, spatial copying tests and spatial reasoning tests. Plasma was taken by blood sample in order to test homocysteine and B vitamin status. The results showed that lower concentrations of vitamin B12 and folate, and higher concentrations of homocysteine were associated with poorer spatial copying skills. This association was not explained by the presence of clinically diagnosed cardiovascular disease in the subjects. High homocysteine levels were a stronger predictor of copying performance than either folate or vitamin B12. Higher concentrations of vitamin B6 were associated with better performance in two measurements of memory.
These results support other studies that have shown a benefit to cognitive performance with improved B vitamin status in elderly individuals. However, this study shows that different B vitamins may affect cognitive performance in different ways. Interestingly, nearly half of the subjects in this study had plasma concentrations of vitamin B6 of less that 30µmol/L which provides more evidence of widespread micronutrient deficiencies in Western populations. Folate, vitamin B12 and vitamin B6 are all required for the production of serotonin, taurine, γ-amino butyric acid and catecholamine neurotransmitters (dopamine and adrenaline) which are essential for cognitive function, as well as S-adenosylmethionine which has antidepressant effects. Homocysteine appears to cause endothelial damage and atherosclerosis and may be associated with carotid stenosis. Homocysteine and B vitamins could therefore affect cognitive function in many ways.