Based on the available studies, there is no convincing evidence that fish oil supplements protect memory or prevent cognitive decline. In people with existing memory complaints, one RCT even showed a slight decline on some outcomes. Eating fish as part of a varied diet appears more promising than a standalone supplement.
Fish oil contains omega-3 fatty acids, particularly DHA and EPA, which receive a great deal of attention as potential memory-supporting agents because of their role in brain cell membranes. Observational studies show that people who eat a lot of fish sometimes perform better cognitively, and in animal models DHA improves cognition and reduces harmful protein accumulation in the brain (PMID 20413879). But that does not automatically mean that a pill does the same thing.
When fish oil is tested as a standalone supplement in humans, the results are disappointing. A broad systematic review of treatments for dementia (Alzheimer's, vascular dementia, Lewy body dementia) classified omega-3 as not effective for cognitive symptoms (PMID 23870856). A systematic review added that omega-3 supplements appear less protective than, for example, the Mediterranean diet as a whole (PMID 31684843). The dietary context therefore seems to matter.
A randomised controlled trial with 197 older adults who had mild memory complaints gave participants DHA-rich fish oil combined with cocoa flavanols for 12 months. The result: no improvement on the primary outcome measure, and for executive functions (planning, organising) and alertness a small decline was even recorded compared with the control group. The volume of the cerebral cortex also decreased slightly in the intervention group (PMID 37315924). This is a cautionary signal, even if the effects are small.
There is one study that strikes a positive note: a 24-month RCT in 60 healthy adults aged 65 and over found that a combination supplement of fish oil, carotenoids and vitamin E significantly improved working memory compared with placebo (PMID 34999335). The effect sizes were small, however (0.09 to 0.11) and clinically modest. In addition, several investigators had financial ties to supplement manufacturers, and the trial did not test fish oil alone. It is therefore not possible to say which ingredient made the difference.
At the cellular level there is an interesting mechanism: DHA stimulates the production of phosphatidylserine, a fat molecule that supports signal transmission in brain cells (PMID 24992464). Whether this mechanism leads to noticeable memory gains in humans in practice has not been demonstrated, however. In summary: the biological story sounds plausible, but the clinical evidence for fish oil as a memory pill is weak and inconsistent.
Seven sources used: two systematic reviews, three RCTs, one experimental laboratory study and one observational review. No large homogeneous meta-analyses available. Most RCTs are small (n=60-197) and test combination supplements, not fish oil alone. Funding risk in one RCT.