A study involving 266 people with mild cognitive impairment (aged 70+) has found that B vitamins are more effective in slowing cognitive decline when people have higher omega 3 levels.
Participants were randomly selected to receive either a B-vitamin supplement (folic acid, vitamins B6 and B12) or a placebo pill for two years. The vitamins had little to no effect for those with low levels of omega-3 fatty acids, but were very effective for those with high baseline omega-3 levels.
Levels of DHA appeared to be more important than levels of EPA, but more research is needed to confirm that.
The finding may help to explain why research looking at the effects of B vitamins, or the effects of omega-3 oils, have produced inconsistent findings.
The study followed research showing that B vitamins can slow or prevent brain atrophy and memory decline in people with MCI, and they were most effective in those who had above average blood levels of homocysteine.
(2016). Omega-3 Fatty Acid Status Enhances the Prevention of Cognitive Decline by B Vitamins in Mild Cognitive Impairment.
Journal of Alzheimer's Disease. 50(2), 547 - 557.