Can B vitamins prevent cognitive decline or dementia?

Looking at the link between B vitamins and the brain

Despite B vitamins playing an important role in cognitive function, can they prevent cognitive decline or dementia?

The Women’s Health Initiative Memory Study found that when dietary folate intake was below the recommended daily amount, this increased the risk of mild cognitive impairment (MCI) and possibly dementia later in life.1

A meta-analysis identified 12 studies that found lowered serum folate levels, but not B12, was associated with cognitive impairment in seniors.2

The Framingham Heart Study demonstrated that plasma B12 levels in the lower end of normal range (187-256.8pmol/L) or below were associated with cognitive decline but particularly when plasma folate levels were high or in conjunction with use of folate supplements.3

A meta-analysis of five randomised controlled trials5 (RCTs) found benefit of vitamin B supplements on