Fish oil against Alzheimer's? Two-year study finds no benefit for memory

Omega-3 fish oil capsules are among the most popular dietary supplements – according to Keck Medicine of USC, Americans alone spend more than $1 billion a year on them. A randomized, double-blind, placebo-controlled trial by the University of Southern California now calls the hoped-for protection against Alzheimer's into question. The results appeared in the journal eBioMedicine and were presented by the university on Oct. 22, 2026. The trial enrolled 365 adults aged 55 to 80 who rarely ate fish and were at elevated risk of Alzheimer's; 47% carried the APOE4 gene variant.

Half of the participants took 2,000 milligrams a day of the omega-3 fatty acid DHA for two years, the rest a placebo. After six months, DHA levels in the cerebrospinal fluid of the DHA group were on average 17% higher – the substance did reach the central nervous system. After two years, however, participants performed no better than the placebo group on memory and thinking tests. Imaging also showed no slowing of shrinkage of the hippocampus, a brain region important for memory.

"Our results do not support fish oil supplements as a preventive measure against Alzheimer's," said study leader Hussein Naji Yassine. The researchers also stress the study's limits: only high-dose DHA supplements were tested, not fish or a dietary pattern such as the Mediterranean diet. The results also apply to older people at elevated risk who eat little fish. The study does not show that omega-3 fats or fish are useless in general. It was funded in part by the U.S. National Institutes of Health (NIH).