A two-year clinical trial led by Keck Medicine of the University of Southern California has found that high-dose omega-3 DHA supplements do not protect brain health or improve memory in older adults at heightened risk of Alzheimer’s disease, even though the fatty acids did reach the brain. The study enrolled 365 participants, aged 55 to 80, who rarely ate fish and were considered at elevated risk of developing the condition.
Participants were randomly assigned to receive daily fish oil or a placebo in a double-blind design, with the supplement delivering 2,000 mg of DHA, a key omega-3 fatty acid important for brain cell membranes and neuron communication.
Researchers assessed DHA levels in the cerebrospinal fluid, the clear liquid surrounding the brain and spinal cord, to confirm brain access. After six months, DHA in the cerebrospinal fluid increased by an average of 17% among those taking the supplements, demonstrating the nutrient did reach the central nervous system.
Despite this biochemical uptake, the study found no corresponding cognitive benefit. Memory and other cognitive assessments conducted at the outset and again after two years showed no advantage for the fish oil group over the placebo group. Brain imaging yielded similarly disappointing results, with no evidence that the supplements prevented shrinkage of the hippocampus, a brain region crucial for memory.
“We all wish there was 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 Center for Personalized 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.”
The researchers note that a key question emerging from the results is why omega-3s reach the brain but do not automatically translate into better function. They are exploring how the brain absorbs, processes and uses these nutrients, with earlier work suggesting omega-3s may be more advantageous when consumed as part of a broader dietary pattern rather than in isolation as a supplement. The Mediterranean-style diet, emphasising fish, vegetables, fruits, whole grains, legumes, nuts and olive oil, is being considered as a context in which omega-3s might work more effectively.
“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 omega-3s,” said Yassine. “We are working to develop medications that may help the brain better utilise these nutrients to preserve cognitive function.”
The findings do not suggest that omega-3 fatty acids are unnecessary or that eating fish offers no health benefits. Rather, they indicate that taking high-dose DHA supplements alone did not provide measurable protection against cognitive decline or hippocampal shrinkage in the studied population.
Healthy habits across life remain a central message. The researchers emphasise that protection against neurodegenerative decline relies on a broader approach, including regular physical activity, sufficient high-quality sleep and a nutritious, balanced diet. “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,” Yassine said. “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.”
The study was funded by the National Institutes of Health, with several grant numbers listed, and by the Alzheimer’s Drug Discovery Foundation.
