Fish oil is one of the best-selling supplements in the country — Americans spend more than a billion dollars on it annually, much of it driven by hopes that omega-3 fatty acids can protect memory and stave off Alzheimer’s disease. A rigorous new clinical trial from Keck Medicine of USC suggests that hope may not hold up.
The two-year, double-blind, placebo-controlled study, published in eBioMedicine, followed 365 adults aged 55 to 80 who had no dementia diagnosis but carried at least one risk factor for cognitive decline, such as obesity, high blood pressure, or physical inactivity. Nearly half also carried the APOE ε4 gene variant, the strongest known genetic risk factor for Alzheimer’s. Participants took either 2,000 milligrams of daily DHA — an omega-3 fatty acid derived from fish oil — or a placebo.
The trial was designed to close a long-standing loophole in omega-3 research: critics of earlier null results argued the supplements simply weren’t reaching the brain in sufficient quantities. This time, researchers directly measured DHA levels in participants’ cerebrospinal fluid and confirmed a 17% increase after six months, proving the nutrient was arriving where it needed to go.
It didn’t matter. After two years, participants taking DHA performed no better on cognitive tests than those on placebo, and brain scans showed no meaningful difference in hippocampal volume, a region highly susceptible to shrinkage in Alzheimer’s disease.
“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 lead investigator Dr. Hussein Yassine, director of the USC Center for Personalized Brain Health.
The study doesn’t necessarily mean omega-3s are irrelevant to brain function — DHA plays a documented role in maintaining connections between brain cells — but it does suggest that simply raising DHA levels through supplementation isn’t enough to move the needle on cognition, at least over a two-year window in this population. Researchers say future work should investigate why some brains may struggle to effectively use the omega-3s they receive, and whether specific subgroups, or omega-3s from whole foods rather than pills, might behave differently. In the meantime, the strongest evidence for protecting brain health still points to familiar, unglamorous basics: aerobic exercise, quality sleep, and cardiovascular health.
