A large clinical trial found that omega-3 fish oil supplements failed to improve memory or slow brain changes associated with Alzheimer’s disease, with the results shifting attention toward overall diet and lifestyle instead.
The latest research from Keck Medicine of University of Southern California, published in The Lancet eBioMedicine, suggests that boosting omega-3 levels through supplements may not provide the brain benefits many people expect.
The study found that, disappointingly, although omega-3s from fish oil successfully reached the brain, they did not improve measures of brain health in older adults at increased risk of Alzheimer’s disease.
Over a two-year, placebo-controlled, double-blinded clinical trial, high-dose omega-3 supplements failed to improve memory, cognitive performance, or brain cell loss in regions associated with Alzheimer’s.
“We all wish for a silver bullet for preventing Alzheimer’s, but our findings showed that fish oil supplements do not appear to protect brain health,” said Hussein Naji Yassine, MD, director of the USC Centre for Personalised Brain Health and lead investigator of the study.
“While omega-3s play an important role in forming brain cell connections needed for cognition, our results do not support fish oil supplements as a preventive measure against Alzheimer’s.”
Fish oil reached the brain . . .
The study included 365 adults aged 55 to 80 who rarely consumed fish, a major dietary source of omega-3 fatty acids. Researchers considered all participants to be at elevated risk for Alzheimer’s disease.
Nearly half (47%) carried the APOE4 gene, the strongest known genetic risk factor for late-onset Alzheimer’s.
Participants were randomly assigned to receive either a daily fish oil supplement or a placebo. Each supplement contained 2 000 mg of docosahexaenoic acid (DHA), an omega-3 fatty acid that plays an important role in brain function.
One of the researchers’ first goals was to determine whether DHA from the supplements could actually make its way into the brain.
To answer that question, they measured DHA levels in cerebrospinal fluid, which surrounds the brain and spinal cord. After six months, DHA levels had increased by an average of 17%, confirming that the nutrient had successfully reached its intended destination.
But no improvement . . .
Although DHA reached the brain, that did not translate into measurable cognitive benefits.
Researchers assessed participants’ memory and thinking abilities at the start of the study and again two years later. Those who took DHA supplements performed no better on cognitive tests than participants who received a placebo.
Brain imaging results told a similar story. The scans showed that fish oil supplements did not slow shrinkage of the hippocampus, a brain region critical for memory that is commonly used as a marker of brain ageing and Alzheimer’s risk.
Why not?
The findings have prompted researchers to investigate why omega-3s can reach the brain without producing noticeable improvements in brain health.
Based on earlier research, Yassine and his colleagues suspect that omega-3s may be more effective when consumed as part of an overall Mediterranean-style eating pattern rather than as a standalone supplement. The Mediterranean diet is naturally rich in omega-3s and has been associated with a lower risk of Alzheimer’s disease.
“We’re focused on better understanding how the brain processes omega-3s and whether factors, such as poor health, dietary pattern, genetic risk and age, may change the brain’s ability to effectively absorb and use the omega-3s,” said Yassine. “We are working to develop medications that may help the brain better utilise these nutrients to preserve cognitive function.”
Healthy living still matters most
Although lifestyle factors were not directly examined in this study, the researchers emphasise that maintaining overall health remains one of the most effective ways to support brain function and reduce Alzheimer’s risk.
“Staying healthy throughout life remains the most powerful tool we have for reducing Alzheimer’s risk, including regular exercise, quality sleep and a balanced diet,” said Yassine. “Living a healthy lifestyle is the brain’s equivalent of getting regular car maintenance and high-quality oil changes. The brain is more likely to lose greater function if health issues in other parts of the body go unaddressed, in the same way that car engines stop working if regular maintenance is skipped.”
Study details
CNS target engagement of high-dose DHA supplementation in older adults at risk for dementia: a randomised, double-blind, placebo-controlled trial
Hussein Yassine, Sara Ghasem Pour, Marlene Juarez et al.
Published in The Lancet eBioMedicine on 18 June 2026
Summary
Background
APOE ε4 carriers have increased Alzheimer’s disease risk and altered omega-3 metabolism. No large-scale prevention trials have tested high-dose docosahexaenoic acid (DHA) supplementation specifically in non-demented APOE ε4 carriers with low baseline omega-3 intake for early intervention.
Methods
We conducted a phase IIa 24-month, randomised, double-blind, placebo-controlled trial (NCT03613844) at the University of Southern California between September 2018 and May 2024. Participants aged 55–80 without dementia, low dietary DHA intake (<200 mg/day), and ≥1 dementia risk factor were stratified by cerebrospinal fluid (CSF) collection willingness into lumbar puncture (LP) or no-LP arms. Within each arm, participants were randomised 1:1 to receive 2 g/day DHA or placebo, stratified by APOE ε4 status. Primary outcome was the 6-month CSF DHA-to-arachidonic acid (AA) ratio change. Secondary and exploratory outcomes included 24-month neuroimaging and cognitive measures.
Findings
Of 739 screened individuals, 365 participants were randomised (181 LP arm, 184 no-LP arm). Mean age was 66.4 years (SD 5.7), 210 (58%) were female, 142 (39%) were Hispanic, 173 (47%) were APOE ε4 carriers. DHA supplementation increased CSF DHA/AA ratio at 6 months vs placebo (0.17 [95% CI 0.15–0.18] vs −0.02 [95% CI −0.04 to −0.0004]; difference 0.19 [95% CI 0.16–0.21]; p < 0.0001), independent of APOE ε4 status (interaction p = 0.71). Dropout was 38%, mainly due to COVID-19. No treatment differences were observed in brain volumes or cognitive performance over 24 months. Adverse events were comparable between groups, with no serious adverse events attributed to treatment.
Interpretation
High-dose DHA achieved CNS target engagement in non-demented older adults with low baseline omega-3 intake, independent of APOE ε4. Despite biochemical target engagement, no differences in cognition or brain structure were observed over 24 months. Future research should prioritise brain DHA metabolism over further supplementation trials.
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