Two-Year Fish Oil Trial Finds No Memory or Brain-Shrinkage Benefit

The work is a randomised, double-blind, placebo-controlled trial, published in 2026 in the journal EBioMedicine. It enrolled 365 adults aged 55 to 80 at the University of Southern California. None had dementia. All ate little DHA (under 200 mg a day), and each had at least one risk factor, such as obesity, high blood pressure, high cholesterol or low physical activity. DHA is an omega-3 fat found in fish oil.

Participants were randomly assigned 1:1 to take 2,000 mg of DHA daily or a matching placebo for 24 months. The placebo was a mix of corn and soybean oils. Everyone also took a B-complex vitamin. Of those enrolled, 58% were women, 39% were Hispanic, and 173 (47%) carried APOE ε4, a gene variant linked to higher Alzheimer's risk.

The first question was simple: does the supplement get into the brain? Some participants agreed to a lumbar puncture, which collects the fluid around the brain and spinal cord. The researchers measured the ratio of DHA to another fat, arachidonic acid, in that fluid. At six months it rose by 0.17 in the DHA group and fell slightly, by 0.02, in the placebo group. The study also reports that spinal fluid DHA rose 17%. Red blood cell omega-3 levels went from 4.9% to 11.0%. According to the authors, this held regardless of APOE ε4 status.

The second question was whether that mattered. The answer, per the authors, was no. Over 24 months, the DHA group showed no differences from placebo in hippocampal volume, the size of a brain region involved in memory that shrinks in Alzheimer's disease. Cognition, tested with a standard memory and thinking battery, also did not differ. Cognitive tests were exploratory outcomes in the trial design; brain structure was a secondary outcome. The primary outcome was the spinal fluid measure.

The authors conclude that DHA alone is not enough to improve cognition in nondemented older adults with low omega-3 intake. They note that earlier trials of DHA in people without dementia were mostly unconvincing: in a review they cite, 5 of 24 showed positive cognitive effects. Trials in people with established Alzheimer's were all null.

There are limits. This was a single-centre, phase 2a trial built mainly to test whether DHA reaches the brain. Dropout approached 30% during the study, partly because of COVID-19 disruptions. The results apply to people like those enrolled: older adults who rarely eat fish. They say little about people who already eat plenty of fish, or about other doses and omega-3 products.

Lead investigator Hussein Naji Yassine told the university press office, as reported by ScienceDaily: "Our results do not support fish oil supplements as a preventive measure against Alzheimer's." The coverage also suggests that omega-3s from a whole diet, such as a Mediterranean-style pattern, may work better. That idea comes from the researchers' earlier work and was not tested in this trial. The coverage also says Americans spend more than $1 billion a year on omega-3 supplements, a figure that does not come from the study.

For you

For readers weighing the cost of fish oil pills, the trial offers one clear point. In this group, a high dose raised brain DHA without a measurable gain in memory or hippocampal size over two years. It does not show that fish or dietary omega-3s are useless, and it did not test other reasons people take fish oil. The authors say future work should focus on how the brain handles DHA, especially in APOE ε4 carriers, rather than on more supplement trials.

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