A randomized trial from Keck Medicine of USC found that two years of daily fish oil did not protect memory or thinking in older adults at elevated risk of Alzheimer’s disease. The 365 participants who took 2,000 milligrams of the omega-3 fatty acid DHA each day performed no better on cognitive tests than those given a placebo, and brain scans showed no sign that the supplement slowed shrinkage of the hippocampus.
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The result matters because the supplement did reach the brain. DHA levels in the fluid surrounding the brain rose by about 17% on average after six months of treatment, so the negative finding cannot be put down to the omega-3 failing to get where it was supposed to go. The trial’s title describes it as a test of whether high-dose DHA engages a central nervous system target, and on that narrow measure the answer was yes. The disappointment came further downstream, in the memory and imaging outcomes that people taking fish oil for brain health are generally hoping to change.
The trial behind the finding
The study, published in eBioMedicine as “CNS target engagement of high-dose DHA supplementation in older adults at risk for dementia,” was a randomized, double-blind, placebo-controlled trial. Hussein Naji Yassine, MD, director of the USC Center for Personalized Brain Health, led it. Participants were 365 adults aged 55 to 80 who rarely ate fish and had elevated Alzheimer’s risk, and about 47% carried the APOE4 gene variant, the strongest known genetic risk factor for late-onset Alzheimer’s. Neither the volunteers nor the researchers knew who received the active capsules.
Cognitive testing took place at the start and again after two years, while DHA in cerebrospinal fluid was measured at six months. According to AlzInfo, none of the volunteers had serious memory problems at the outset, but all had at least one dementia risk factor, and carrying APOE4 did not change the outcome. The paper’s reported results were consistent across the two main measures: thinking scores matched placebo, and the supplement did not prevent shrinkage of the hippocampus, a region commonly used as a marker of brain aging and Alzheimer’s risk.
What the researchers conclude and what they hedge
Yassine drew a plain conclusion in the release carried by ScienceDaily, saying the results do not support fish oil supplements as a way to prevent Alzheimer’s. In the original Keck Medicine of USC release on Newswise, he said the findings showed fish oil supplements “do not appear to protect brain health,” and added that staying healthy throughout life remains the most powerful tool for reducing Alzheimer’s risk.
The team is careful about what the trial cannot say. The findings apply to the population studied and do not mean omega-3s are unnecessary or that fish lacks health benefits. The trial tested DHA only, at a single dose, not omega-3s in general, and it did not directly test lifestyle factors such as exercise, sleep or diet. Fish supplies omega-3s beyond DHA, and the researchers suggest the nutrients may work better as part of an overall nutrient-rich pattern such as a Mediterranean-style diet, an idea that rests on earlier association studies rather than on this trial.
Why the supplement failed to help is, by the researchers’ own description, a hypothesis. They suspect that poor health, diet, genetics and age may affect how well the brain uses omega-3s, and they say they are still investigating that. Two years may also be too short for dietary benefits to build, a possibility AlzInfo raises, and the participants rarely ate fish, so the results may not carry over to people with different habits.
The research group around Yassine included colleagues from across USC, among them Helena Chui, Wendy Mack, Meredith Braskie and Lon Schneider, who are listed among the co-authors alongside more than a dozen others. The size of the team reflects the trial’s design, which paired two years of cognitive testing with spinal fluid sampling and brain imaging rather than relying on a single questionnaire or blood test.
Where the trial sits among other evidence
Nutrition Insight, reporting on the trial, set it against observational research, citing a study of more than 217,000 adults that linked higher blood omega-3 levels to a 35% to 40% lower risk of early-onset dementia. Observational studies of that kind cannot show that the omega-3s caused the lower risk, which is the gap a randomized trial is built to close. This one, in a group at elevated risk, did not find a cognitive benefit.Funding came from the National Institutes of Health, through grants including RF1AG076124 and R01AG055770, and from the Alzheimer’s Drug Discovery Foundation under grant GC-201711-2014197. The source texts read for this article did not name the cognitive tests, describe the placebo or report effect sizes and dropout rates, details that sit in the full paper.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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