A large clinical trial from the USC School of Medicine has cast doubt on the efficacy of fish oil supplements in preventing cognitive decline. The results, published in a peer-reviewed journal, show that daily intake of 2000 mg of DHA for two years did not improve memory or cognitive function in older adults at risk for Alzheimer's disease.
The randomized, placebo-controlled trial involved 365 participants
Led by Dr. Hussein Naji Yassine, director of the Personalized Brain Health Center at USC, the study enrolled men and women aged 55 to 80 who rarely ate fish. Nearly half (47 percent) carried the APOE ε4 allele, the strongest genetic risk factor for late-onset Alzheimer's. Participants were randomly assigned to receive either a daily supplement of 2000 mg DHA or a placebo made from corn and soybean oil, identical in appearance, taste, and smell. Neither participants nor researchers knew the group assignments.
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DHA reached the brain but showed no measurable cognitive benefits
Researchers first confirmed that DHA entered the brain. Measurements in cerebrospinal fluid showed a 17 percent increase after six months in the DHA group, with no difference between APOE ε4 carriers and non-carriers. However, after 24 months, cognitive tests and MRI scans of the hippocampus revealed no significant differences between the DHA and placebo groups. Despite reaching the brain, DHA did not slow brain atrophy or enhance cognitive performance.
Possible explanations: enzymes, inflammation, and the pandemic
The authors suggest that an enzyme called calcium-dependent phospholipase A2 (cPLA2) may break down DHA before it can be incorporated into synaptic membranes. Additionally, many participants had cardiovascular risk factors such as obesity and hypertension, causing chronic inflammation that might have blunted the supplement's effects. The Covid-19 pandemic also affected the study, with 38 percent of participants dropping out. Furthermore, the relatively young average age (66) and minimal cognitive decline during the trial may have hidden any potential benefits.
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The researchers emphasize that rather than relying on supplements, the most effective ways to reduce Alzheimer's risk remain lifestyle measures: regular physical activity, quality sleep, and a balanced diet. While the UK generational tobacco ban represents a policy approach to prevention, this study shows there is no magic pill for brain health. For more information, see the Wikipedia page on fish oil.
Source: https://www.wired.com/story/what-are-fish-oil-supplements-good-for-heres-your-crash-course
