I have discussed nutritional effects on memory and warned readers that many claims are just so much undocumented hype. Here is a study, apparently very well done by a group at Oxford University, that shows supplementing diet with B vitamins can help prevent mental decline in the elderly.
As people age, the brain tends to atrophy, even in healthy people, and this of course can contribute to mental decline and senility. A risk factor for brain atrophy in the elderly is homocysteine, an amino acid best known as being a risk factor for coronary heart disease, stroke and peripheral vascular disease. B vitamins (folic acid, B6 and B12) reduce the blood level of homocysteine. In the Oxford study, 271 people over 70 years old with mild cognitive impairment were given a mixture of the B vitamins daily for two years. Brain size changes were monitored with brain scans.
During the test period, brains shrunk 1.08% in the non-supplemented controls and 0.76% in those that got the B vitamins. There were correlated changes in blood homocysteine levels and performance on mental tests. Daily doses used were folic acid: 0.8 mg; B6: 20 mg; and B12: 0.5 mg.(I just checked my own brand-name one-a-day multi-vitamin pills, and they have a lot less of B6 and B12.)
If brains shrink at this rate (actually shrinkage probably accelerates over time), the difference by age 90, for example, would be substantial. Shrinkage would be even greater if there were other factors such as large alcohol consumption or Alzheimer's disease. One could also expect B vitamins to provide some benefit for the cardiovascular system, though this was not evaluated in this study.
Smith, A. D. et al. 2010. Homocystein-lowering by B vitamins slows the rate of accelerated brain atrophy in mild cognitive impairment: a randomized controlled trial. PLoS ONE. doi/10.1371/journal.pone.0012244