Thirty-three studies were included in the review: 14 RCTs (2,438 participants, range seven to 211 participants) and 19 cohort studies (19,485 participants, range 79 to 5,395 participants). The included studies were of variable quality: 10 of the RCTs reported adequate methods of randomisation and nine reported adequate similarities at baseline and blinding. Losses to follow-up were not reported.
Folate and other B-vitamins (10 RCTs): Compared to placebo, folic acid supplementation resulted in a significant improvement in memory and cognitive function in three RCTs but a decline in one cognitive domain was reported in one RCT and one RCT reported no significant differences between groups for folic acid supplementation. No trials reported increased cognitive performance following supplementation with folic acid combined with other B-vitamins (six RCTs) but a trend for increased performance or slower decline was noted in placebo groups compared to intervention groups (three RCTs).
Fish and fatty acids (four RCTs): One small RCT reported improvements in cognitive measures between one group that received an intervention and another that received no intervention or placebo (statistical data not reported). One RCT reported improvements in quality of life for the intervention group compared to placebo (statistical data not reported). Two RCTs reported no differences in cognitive function tests between intervention and placebo groups.
The results of the cohort studies are reported in the paper.