Supplement users show faster cognitive decline

The second study, published in 2026 in The Journal of Prevention of Alzheimer’s Disease, asked whether omega-3 supplementation was associated with the rate of cognitive decline over time.²

After matching participants for age, sex, APOE ε4 status, and clinical diagnosis, the final sample included 819 older adults: 273 omega-3 supplement users and 546 non-users, who were followed for about 5 years.

The participants were not all cognitively healthy. The cohort included cognitively normal adults, people with significant memory concerns, people in various stages of MCI, and even people already diagnosed with dementia due to Alzheimer’s disease. That matters because—unlike the earlier study—this was capturing what happens in people already showing signs of cognitive vulnerability.

Omega-3 use was self-reported, and the exposure was defined as the first follow-up visit after a participant reported starting supplementation. Fish oil was the primary formulation, although some participants reported other omega-3 sources, including flaxseed oil and krill oil.

• Omega-3 supplement users declined faster across all three cognitive measures used (the MMSE, ADAS-Cog13, and CDR-SB).

• The omega-3-associated decline corresponded to about 8% of the typical annual Alzheimer’s progression rate for MMSE, 15% for ADAS-Cog, and 11% for CDR-SB.

The next question was whether this decline tracked with the classic biological features of Alzheimer’s disease. And this is where the study gets even more interesting.

The faster cognitive decline was not explained by greater amyloid deposition, tau aggregation, or gray matter atrophy. Omega-3 supplement use was not associated with faster accumulation of the proteinopathies we usually think of as central to Alzheimer’s disease, nor was it associated with faster loss of gray matter volume.

Instead, omega-3 users showed a greater decline in regional cerebral glucose metabolism, which explained about 19–41% of the relationship between omega-3 supplementation and cognitive decline (depending on the cognitive test being looked at).

The problem of reverse causality

The obvious concern with this study is reverse causality: what if people started taking omega-3s because they, their family, or their clinicians noticed early memory problems or general health decline? The authors tried to address this by examining the pre-supplementation period. Future omega-3 users did not show faster cognitive decline or worse glucose metabolism trajectories before starting supplements compared with matched non-users, suggesting that the divergence emerged after supplementation began.

But that does not fully eliminate the concern. Subtle memory changes, family history, chronic pain, cardiovascular concerns, supplement-seeking behavior, or other unmeasured health factors could still influence who starts omega-3s and how they decline over time. The authors acknowledge this directly, noting that unmeasured factors could still have contributed to the association, and also that they lacked detailed information on dose, adherence, discontinuation, and specific supplement type.

Omega-3s have brain-protective mechanisms

So this study should not be read as definitive evidence that omega-3s cause cognitive decline. 

It does not overturn the broader evidence that omega-3s are biologically important for the brain. And that evidence doesn't just come from observational studies. There are mechanisms to support it.

• Omega-3s (in particular DHA) are required for glucose transporters to function at neuron cell membranes, and deficiency impairs glucose uptake (findings in direct contrast with the results observed in this latest study).
• Furthermore, neuroinflammation is a major driver of Alzheimer's disease, and omega-3s are among the most powerful anti-inflammatory compounds.
• Last (but not least), DHA is highly enriched in neuronal cell membranes, where it helps maintain membrane fluidity. Multiple receptors, including those for serotonin and dopamine, depend on membrane fluidity to function properly and bind neurotransmitters.

(You can read more about the benefits of omega-3s on our Omega-3 Topic Page).

Final thoughts

I’ve been trying to wrap my head around these two studies for a few weeks, because on the surface, they seem almost impossible to reconcile.

One study suggests omega-3s are associated with a lower risk of Alzheimer’s disease. The other suggests omega-3 supplementation is associated with faster cognitive decline… in participants from the same cohort.

But after spending more time with the details, I’m not sure these studies carry equal weight. Here's why:

The first major distinction is that cognitive decline is not the same thing as Alzheimer’s disease.

The 2026 study asked whether omega-3 supplementation was associated with changes in cognitive test scores over time. And the answer, in that analysis, was yes. But the 2023 Alzheimer's Neuroimaging cohort analysis asked whether people developed Alzheimer’s disease during follow-up.

A person can decline faster on cognitive testing without crossing the diagnostic threshold for Alzheimer’s disease. Conversely, someone may be classified as not having Alzheimer’s while still showing subtle or measurable cognitive deterioration. These outcomes overlap, but they are not interchangeable.

This becomes even more important when you look at the biomarker results from the 2026 study. The association between omega-3 supplementation and faster cognitive decline was not mediated by amyloid deposition, tau pathology, or gray matter atrophy—the classic biological hallmarks we tend to associate with Alzheimer’s disease.

In other words, the harmful association in that study did not map neatly onto “more Alzheimer’s pathology.” 

The second important point is that although both papers used Alzheimer's Neuroimaging cohort data, the most favorable cognitive-decline evidence in the 2023 paper did not come only from this cohort. 

That paper also included a broader meta-analysis drawing from dietary intake studies, plasma biomarker studies, and erythrocyte membrane biomarker studies. And the protective signal here linked higher dietary DHA/EPA intake and higher blood omega-3 status with a lower risk of cognitive decline or dementia.

That brings up a crucial distinction: supplement use, dietary omega-3 intake, and blood omega-3 status are not the same exposure!

A person who has high omega-3 levels in red blood cell membranes is likely someone with a long-term pattern of higher omega-3 intake and successful incorporation of those fatty acids into tissues. Erythrocyte omega-3s are often treated as one of the more stable biological markers of omega-3 status because they reflect longer-term intake more than plasma does.

This is where the 2026 study becomes tricky. The exposure was defined around the first follow-up visit after a participant reported initiating supplementation. That is useful because it tries to anchor omega-3 exposure to what happens afterward. But it also introduces a different kind of bias, one that we discussed earlier: people may start taking omega-3s because they, their family, or their clinician notice something changing. Maybe it's memory concerns, maybe it's pain, or maybe it's general health decline or some vague sense that they need to “do something” for their brain.

The third major difference is the type of people being studied.

The 2023 Alzheimer's Neuroimaging cohort analysis excluded people with dementia at baseline. The 2026 study included a much broader spectrum: cognitively normal adults, people with significant memory concerns, various stages of mild cognitive impairment, and even people diagnosed with Alzheimer’s dementia. That means the 2026 study may be telling us something very different—not necessarily that omega-3s are broadly harmful to the aging brain, but that omega-3 supplementation may not halt (and could potentially track with faster decline) in people already showing signs of cognitive impairment or neurodegenerative vulnerability.

That interpretation fits with the broader randomized-trial literature, where omega-3 supplementation has generally been much less impressive once cognitive impairment or Alzheimer’s disease is already present. Prevention and treatment are not the same thing. Maintaining healthy omega-3 status across decades may not have the same effect as introducing a fish-oil supplement after cognitive symptoms or brain metabolic dysfunction have already begun.

And finally, we can't even be sure that the participants in this recent study kept taking omega-3s beyond their "date of initiation." 

All we can conclude is that at some point, they reported that they started supplementing with omega-3s, not that they maintained diligent use over the next 5 years.

This may be the cleanest way to think about the discrepancy:

• The 2023 paper captures the reality of long-term omega-3 intake and stable biological omega-3 status, especially from diet and erythrocyte markers, in adults without dementia.
• The 2026 paper may capture another reality—the initiation of mostly commercial fish-oil supplementation in an older, more clinically vulnerable population, with unknown dose, unknown adherence, and a different underlying disease stage.

So where does this leave us?

For me, the takeaway is not to abandon omega-3s as a brain-health strategy.

Studies like the most recent 2026 Alzheimer's Neuroimaging cohort one are being presented in the media as a simple "fish oil may accelerate cognitive decline." That's an unfair conclusion that isn't supported by other rigorous research. It's why I chose to cover the topic this week.

Not because I wanted to dismiss the study's findings as outright false, but because there's much more complexity to this story that needs to be considered. The quality of this new analysis just isn't rigorous enough to warrant a sweeping change in recommendations.

That is especially true when it has the potential to change people's behaviors around a topic as sensitive and important as brain aging.

Go Deeper on Omega-3s

Today's newsletter focused on why two analyses of omega-3s can seem to point in opposite directions. That is exactly where the practical questions matter: DHA vs. EPA, dose, fish-oil quality, oxidation, AFib risk, and whether taking a supplement later in life is the same thing as maintaining healthy omega-3 status over decades.

I've answered these questions (and more) in several of my previous Q&A sessions.

• Q&A #76: EPA vs. DHA - which is more important for brain health

• Q&A #56: Are most fish oil supplements already oxidized?

• Q&A #56: Identifying quality fish oil by its TOTOX level

• Q&A #39: Optimal daily dose for omega-3 supplementation

• Q&A #60: Does fish oil supplementation increase AFib risk?

• Q&A #64: What omega-3 supplement does Rhonda take?