FISH OIL: LITTLE BRAIN IMPACT

Millions of people take fish oil supplements hoping to keep their minds sharp as they age. A two-year clinical trial put that hope to the test—and found a striking gap between what the supplements delivered and what they achieved.

The omega-3 fatty acid DHA reached participants’ central nervous systems. But people taking the supplements did no better on memory and thinking tests than people taking a placebo. The supplements also did not prevent shrinkage of the hippocampus, a brain region involved in memory that is vulnerable to Alzheimer’s-related changes.

“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.

A direct test of a popular supplement

The study, published in eBioMedicine, enrolled 365 adults ages 55 to 80 who rarely ate fish and were considered at elevated risk for Alzheimer’s disease. About 47% carried APOE4, a gene variant linked to a higher risk of late-onset Alzheimer’s. Carrying the variant does not mean a person will develop the disease.

Researchers randomly assigned participants to take either a daily supplement containing 2,000 milligrams of DHA or a placebo. Neither the participants nor the researchers knew who received which treatment during the trial.

DHA is an omega-3 fatty acid that helps form brain cell membranes and supports communication between neurons. The researchers wanted to know whether giving people more of it could protect memory and brain structure over time.

First, they checked whether the supplement was reaching the central nervous system at all. After six months, DHA levels in cerebrospinal fluid—the liquid surrounding the brain and spinal cord—had risen by an average of 17% among those taking the supplement.

That was encouraging on its own. But after two years, cognitive testing showed no significant advantage for the supplement group. Brain scans told a similar story: Fish oil did not prevent hippocampal shrinkage compared with the placebo.

The distinction matters. Raising a nutrient level in the central nervous system is not the same as showing that a supplement protects against cognitive decline.

Food and supplements may not have the same effects

The findings leave researchers with a question: If DHA reached the central nervous system, why didn’t it produce a measurable benefit?

Yassine and his colleagues are studying how the brain absorbs and uses omega-3s—and whether age, genetic risk, diet, or other health factors affect that process. Their earlier work suggests omega-3s may be more useful as part of a broader eating pattern than as an isolated supplement.

A Mediterranean-style diet, for instance, includes fish alongside vegetables, fruit, whole grains, legumes, nuts and olive oil. Research has associated this eating pattern with a lower risk of Alzheimer’s disease, though this trial did not test the diet.

“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,” Yassine said.

The trial does not show that omega-3s are unimportant or that eating fish has no benefits. It shows that, in this group of older adults at elevated Alzheimer’s risk, a high-dose DHA supplement did not measurably improve thinking or prevent hippocampal shrinkage over two years.

For now, the researchers advise against counting on a single supplement to protect the brain. Regular exercise, good sleep, and a balanced diet remain practical priorities for overall health, even though this trial did not directly test their effects on Alzheimer’s risk.

The National Institutes of Health and the Alzheimer’s Drug Discovery Foundation supported the study.