Fish Oil Supplements Fail to Protect Memory or Slow Brain Shrinkage in Older Adults, Two-Year USC Study Finds

Fish oil supplements taken by millions of people in hopes of protecting their memory did not improve thinking skills or slow brain shrinkage in older adults at risk of Alzheimer's disease, according to a two-year clinical trial led by researchers at the University of Southern California.

The study, published in the journal eBioMedicine, found that high-dose omega-3 supplements did reach the brain, raising levels of a key fatty acid in the fluid surrounding the brain and spinal cord. But participants who took the supplements performed no better on memory and cognitive tests than those who took a placebo, and brain scans showed no protection against shrinkage in a region critical 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 Dr. Hussein Naji Yassine, director of the USC Center for Personalized Brain Health and lead investigator of the study.

A popular supplement put to the test

Omega-3 fatty acids are essential nutrients found in fatty fish such as salmon, sardines and mackerel. They help build and maintain the connections between brain cells that support memory and learning.

Fish oil has become one of the most widely used dietary supplements in the United States, with Americans spending more than $1 billion a year on omega-3 products. Many people take them in the belief that they can help keep the brain sharp with age.

Yassine said the nutrients themselves remain important, but the trial does not support using supplements to prevent the disease.

"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," he said.

How the trial worked

The researchers, from Keck Medicine of USC, enrolled 365 adults between the ages of 55 and 80 who rarely ate fish and were considered at increased risk of developing Alzheimer's disease.

Nearly half of the participants, 47%, carried a gene variant called APOE4, the strongest known genetic risk factor for late-onset Alzheimer's. Carrying the variant raises a person's risk of developing the disease but does not mean they will necessarily get it.

Participants were randomly assigned to take either a daily supplement containing 2,000 milligrams of docosahexaenoic acid, or DHA, an omega-3 fatty acid that is especially important for brain cell membranes, or a placebo.

The trial was double-blind, meaning neither the participants nor the researchers knew who was receiving the active supplement. That design is considered the gold standard for clinical research because it reduces the risk of bias.

Supplements reached the brain

A key question for the researchers was whether the supplements were reaching the brain at all, since earlier studies of fish oil and brain health had produced mixed results.

To find out, the team measured DHA levels in participants' cerebrospinal fluid, the clear liquid that surrounds the brain and spinal cord.

After six months, DHA levels in that fluid had risen by an average of 17% among those taking the supplements, confirming that the omega-3s were entering the central nervous system.

But no benefit for memory

Despite that increase, the researchers found no meaningful difference between the two groups after two years.

Participants taking fish oil did not perform better on memory and other cognitive tests than those on the placebo.

Brain imaging also showed that the supplements did not prevent shrinkage of the hippocampus, a region deep in the brain that plays a central role in forming memories. The hippocampus is especially vulnerable to aging and Alzheimer's disease, and its size is commonly used by researchers as a marker of brain health.

The results suggest that simply raising DHA levels in the brain is not enough to deliver measurable benefits, at least in this group of older adults.

Why food may differ from pills

The findings raise a new question: if omega-3s reach the brain, why don't they help?

Yassine and his colleagues are now studying how the brain absorbs, processes and uses these nutrients. Their earlier research suggests omega-3s may be more beneficial when consumed as part of a healthy overall diet than when taken alone as a supplement.

The Mediterranean-style diet, which emphasizes fish, vegetables, fruits, whole grains, legumes, nuts and olive oil, has been linked in many studies to a lower risk of Alzheimer's. The researchers suspect that the combination of nutrients in such a diet, along with a person's overall health, may influence how well the brain uses omega-3s.

"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 team is also working to develop medications that could help the brain make better use of these nutrients.

What the study does not show

The researchers stressed that the findings do not mean omega-3 fatty acids are unimportant or that eating fish has no health benefits.

Rather, the study shows that high-dose DHA supplements alone did not provide measurable protection against cognitive decline or brain shrinkage in the population studied, which was limited to older adults who rarely ate fish and were at elevated risk for Alzheimer's.

Omega-3 supplements have shown benefits in some other areas of health. For example, studies have linked fish oil to reduced heart risks in certain patient groups. The new findings apply specifically to brain health and Alzheimer's prevention.

Lifestyle remains key

Rather than relying on a single supplement, the researchers recommend healthy habits throughout life, including regular exercise, sufficient high-quality sleep and a balanced diet. Those factors were not tested in the trial, but the researchers said they offer a stronger foundation for long-term brain health.

"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.

He compared a healthy lifestyle to regular car maintenance, saying the brain is more likely to lose function if health problems elsewhere in the body go unaddressed.

Growing burden of Alzheimer's

Alzheimer's disease is the most common cause of dementia, a group of conditions that affect memory, thinking and the ability to perform everyday tasks. As populations age, the number of people living with the disease is expected to rise sharply, increasing pressure on families and health care systems.

While new drugs have been approved in recent years to slow the progression of Alzheimer's in some patients, there is still no cure, and researchers continue to search for ways to prevent or delay the disease.

The study was supported by the National Institutes of Health and the Alzheimer's Drug Discovery Foundation.