In a randomized study of 299 older men, daily supplementation with 2 mg folate, 25 mg vitamin B6, and 400 microg vitamin B12 for a period of 2 years was found to be associated with a significantly lower increase in the amyloid beta protein 1-40 found in tangles/plaques in the brains of patients with Alzheimer's disease—as compared to subjects who received a placebo. While A beta 40 increased by an average of 18.5% (26.8pg/ml) among subjects who received a placebo, it increased by an average of only 4.9% (7.0 pg/ml) among those who received supplementation with B vitamins. The authors state, "We conclude that B vitamins may decrease the plasma level of A beta 40 and have a role in the prevention of AD."
A good multivitamin powder should have more B12 and B6 and a bit less folate, and would likely have the same effect.
"B-vitamins reduce plasma levels of beta amyloid," Flicker L, Martins RN, et al, Neurobiol Aging, 2008; 29(2): 303-5.