Fish oil reached the brain and still did nothing for memory

Fish oil is one of the most popular supplements taken for brain health, yet its value for people facing a raised risk of dementia has never been settled. Carriers of the APOE ε4 gene variant have a higher risk of Alzheimer's disease and handle omega-3 fats differently, which makes them a natural group in which to test supplementation early, before any dementia appears. No large prevention trial had tried high-dose DHA specifically in non-demented ε4 carriers with low omega-3 intake. A prior question also needed answering first: does swallowed DHA actually reach the central nervous system in meaningful amounts?

Researchers at the University of Southern California ran a phase IIa randomised, double-blind, placebo-controlled trial between September 2018 and May 2024. From 739 people screened, 365 adults aged 55 to 80 without dementia were enrolled; all ate little DHA (under 200 mg a day) and carried at least one dementia risk factor. Participants were grouped by willingness to undergo a lumbar puncture, then assigned at random to 2 g of DHA daily or placebo, balanced for APOE ε4 status. The primary outcome was the six-month change in the ratio of DHA to arachidonic acid in cerebrospinal fluid, with brain imaging and cognitive testing tracked over 24 months.

The supplement unambiguously reached its target: the DHA to arachidonic acid ratio in cerebrospinal fluid rose on DHA while drifting slightly down on placebo, a between-group difference of 0.19, and the effect was the same whether or not participants carried APOE ε4. Despite that, brain volumes and cognitive performance showed no treatment differences at all across the 24 months. Dropout was substantial at 38%, mostly put down to COVID-19. Adverse events were comparable between the groups and no serious adverse events were attributed to the supplement.

This result complicates the simplest version of the fish oil story. Earlier null findings could always be excused on the grounds that the supplement might never have reached the brain; here the biochemical target was clearly engaged and cognition still did not separate from placebo. The authors' own conclusion is pointed: future research should prioritise understanding how the brain metabolises DHA rather than running further supplementation trials. Open questions remain about longer treatment windows, different doses and whether starting even earlier would matter.

If fish oil sits in your supplement drawer as insurance for your memory, this trial is a reason to hold that belief more loosely. In older adults with dementia risk factors and low omega-3 diets, arguably the group most likely to benefit, two years of a high dose changed brain chemistry without changing thinking or brain structure. That does not rule out effects over longer horizons or for other outcomes, but it removes one of the stronger arguments the supplement had. Anyone weighing the cost of a daily capsule now has a clearer picture of what it bought in this setting: target engagement, not measurable benefit.