Interestingly, Yassine, the PI behind the failed PreventE4 trial wrote this paper last year (before the PreventE4 results were known): Designing Newer Omega-3 Supplementation Trials for Cognitive Outcomes: A Systematic Review Guided Analysis 2024.
In his words (those of a high believer in omega 3 supplementation at least when he started the PreventE4 trial…):
Conclusions: We recommend that newer n-3 PUFA supplement trials targeting AD prevention be personalized. For the general population, the null hypothesis appears to be correct, and future interventions are needed to identify and test dietary patterns that include PUFA-rich food rather than supplements.
Given that the majority of clinical trials involving n-3 PUFA supplements and cognition reported null findings, we must consider the possibility that the null hypothesis is correct. That is, there is no true difference between n-3 PUFA supplements and placebo in AD prevention or treatment in the older general population. […] However, it is plausible that these clinical trial interventions were not long enough to find a true effect, given that the prodromal phase of AD can last 10–20 years. […] We propose that carefully selected populations may benefit from n-3 PUFA supplementation and present some of the factors that can help define these individuals for personalized intervention with either an n-3 PUFA supplement or an n-3 PUFA dietary pattern.
This association between DHA intake and dementia outcomes is more evident when comparing individuals who do not consume fatty fish to those who consume one or two servings. However, the association between baseline n-3 PUFA-enriched dietary intake and dementia differs according to the population characteristics. Some vegetarian and vegan populations that do not consume red meat, chicken, or fish do not have an increased risk of dementia despite lower n-3 PUFA levels, suggesting compensation for lower n-3 PUFA intake. It is plausible that such populations have lower vascular risk factors and less vascular dementia; however, further research is needed. We recommend that future trials select participants based on low baseline n-3 PUFA levels in the context of a Western dietary lifestyle.
Overall, n-3 PUFA doses of less than 1 g/day are not likely to significantly increase brain levels within a short period of time of supplementation. In the DHA pilot trial, we observed a 28% increase in CSF DHA levels after 2 g of algal DHA per day over 6 months compared with placebo, despite a > 200% increase in plasma DHA levels. These findings suggest that lower doses (<1 g of DHA per day) are likely to be associated with lower brain DHA delivery. Dementia prevention trials using omega-3 supplementation doses equal to or lower than 1 g/day may have reduced brain effects, particularly in APOE4 carriers owing to lower brain uptake or greater brain consumption […] The clinical evidence regarding whether DHA or EPA in triglyceride or phospholipid form is superior remains inconclusive.
N-3 PUFAs, particularly DHA, have a long half-life in the brain of around 2.5 years, and their effects on brain structure may take time to manifest. Therefore, long-term interventions with lower doses of omega-3 fatty acids may not be sufficient to produce significant changes in brain levels or structural changes such as brain volume. […] Overall, a supplementation duration longer than 6 months is recommended.
Among n-3 PUFAs, EPA intake appears to be more advantageous than DHA in reducing “brain effort” relative to cognitive performance.
All participants in PreventE4 receive high dose vitamin B supplementation to eliminate any confounding effects of high homocysteine levels on the outcomes. The trial is expected to be reported in 2025.
So, according to Yassine (and that was before he got the results of his PreventE4 trial!), most likely, “there is no true difference between n-3 PUFA supplements and placebo in AD prevention or treatment in the older general population”. But there might be some benefits of high-dose (>1g/day) supplementation in people with a Western diet that don’t eat fish (but not vegans who eat healthy) and new trials are necessary to prove it. He adds: “Combination interventions, such as multi-domain, multi-nutrient, or dietary patterns, are likely more effective than n-3 PUFA supplementation alone.”
It seems that Yassine now shifted his work on cPLA2 inhibitors that can increase brain omega-3 levels: Novel mechanisms to enhance omega-3 brain metabolism with APOE4 2023
Is there any strong paper in favor of DHA supplementation for dementia prevention given the massive amount of evidence showing that it is useless?