Vitamin D, fish oil, and glucosamine are familiar supplements, especially among older adults. In 2026, however, all three have been pulled into an uncomfortable scientific question: could a supplement taken for health actually become less helpful, or even harmful, as the brain ages?
Several new studies have reported associations between supplement use and faster cognitive decline or Alzheimer’s disease progression. The headlines are provocative, but the evidence is not strong enough to conclude that these supplements broadly damage the brain. The studies differ substantially in design, population, and biological plausibility, and randomized trials sometimes point in a different direction.
Why three familiar supplements are suddenly under scrutiny
The concern does not come from one trial showing that vitamin D, fish oil, and glucosamine cause dementia. Instead, several separate 2026 studies produced signals that are now being discussed together.
That distinction matters. Observational research can reveal patterns in real people, but supplement users can differ from nonusers in ways that are difficult to measure. People may start supplements because of an illness, diet, joint disease, perceived memory problems, or advice from a clinician. Those same factors can influence cognitive outcomes independently of the supplement.
The right question is therefore not simply whether a statistical association exists. It is whether the association survives better-controlled studies, whether there is a plausible biological mechanism, and whether randomized evidence points in the same direction.
Vitamin D: an association, not proof of harm
The vitamin D concern comes largely from a prospective cohort study published in BMJ Open in January 2026. Researchers analyzed 5,065 participants from the US Health and Retirement Study. Their average age was 67.5 years, and 39.6% reported using vitamin D supplements.
Over six years, supplement users had a slightly faster decline in global cognitive scores than nonusers. The estimated difference in the rate of change was -0.052 points per year. Executive-function scores also declined slightly faster.
A sensitivity analysis produced the most attention-grabbing result: the association with faster cognitive decline appeared among supplement users whose baseline blood level of 25-hydroxyvitamin D was already normal, but not among those who were insufficient or deficient.
That finding does not establish that vitamin D caused the decline. Supplement use was self-reported, the researchers did not have detailed information about supplement dose, and residual confounding or reverse causation remains possible. Someone worried about declining health, for example, might be more likely to begin taking supplements.
Randomized vitamin D evidence tells a less alarming story
The VITAL trial provides an important counterweight. In two cognitive substudies involving thousands of adults aged 60 and older, participants were randomly assigned to vitamin D3 at 2,000 IU per day or placebo. Vitamin D supplementation did not significantly accelerate or slow cognitive decline over roughly two to three years.
Randomization is important because it reduces many of the differences that complicate observational comparisons. The VITAL result therefore argues against interpreting the 2026 cohort study as evidence that ordinary vitamin D supplementation directly harms cognition.
It also does not mean more vitamin D is always better. Vitamin D is fat-soluble, excessive intake can cause toxicity, and supplementation has a clear role when deficiency or another clinical indication is present. What the cognitive evidence does not support is taking vitamin D simply as a proven way to prevent dementia.
Fish oil: another concerning observational signal
A 2026 study in The Journal of Prevention of Alzheimer’s Disease examined participants in the Alzheimer’s Disease Neuroimaging Initiative. Omega-3 supplement use was associated with faster worsening on several cognitive measures, including the Mini-Mental State Examination, ADAS-Cog13, and Clinical Dementia Rating Sum of Boxes.
The researchers also examined brain imaging. The association was not explained by amyloid deposition, tau pathology, or gray-matter atrophy. Changes in glucose metabolism in Alzheimer’s-vulnerable brain regions statistically mediated part of the association.
Again, however, this was not a randomized supplement trial. People choosing to take omega-3 supplements may differ systematically from those who do not. The study can identify an association and generate hypotheses, but it cannot demonstrate that fish oil caused the cognitive deterioration.
Large randomized trials have not shown cognitive harm from fish oil
In VITAL-Cog, 3,424 healthy community-dwelling older adults underwent cognitive testing as part of the randomized VITAL trial. Marine omega-3 supplementation did not significantly alter the rate of cognitive change compared with placebo.
That is an important contradiction. If fish oil were producing a substantial, general toxic effect on cognition in healthy older adults, randomized trials would be expected to detect a consistent signal. So far, they have not.
The evidence also should not be interpreted as proof that fish-oil supplements prevent dementia. The more defensible conclusion is narrower: current randomized evidence does not establish either a broad cognitive benefit or the broad cognitive harm implied by some recent observational headlines.
Glucosamine is the most biologically intriguing signal
The glucosamine story is different because researchers did more than compare supplement users with nonusers.
In a 2026 Nature Metabolism study, scientists investigated hyperglycosylation, an increase in the cellular pathways that build sugar-containing structures called glycans and attach them to proteins. Protein glycosylation is normal biology, but the researchers found excessive glycan biosynthesis in Alzheimer’s disease models and postmortem human Alzheimer’s brain tissue.
They then manipulated the pathway experimentally. Reducing expression of enzymes involved in glycan biosynthesis improved cognitive outcomes in Alzheimer’s mouse models. Oral glucosamine supplementation moved the biology in the opposite direction and worsened memory performance in the mice.
This gives the glucosamine observation something the vitamin D and omega-3 associations lack: an experimentally testable mechanism.
What happened in people taking glucosamine?
The same research team retrospectively analyzed electronic health records from patients with Alzheimer’s disease and related dementias or mild cognitive impairment. About 8% were recorded as glucosamine users.
Among people with mild cognitive impairment, glucosamine use was associated with about a 25% higher likelihood of progression to dementia. Among patients who already had Alzheimer’s disease and related dementias, use was associated with about a 25% higher mortality risk.
Those are associations, not randomized comparisons. Osteoarthritis and joint pain are obvious potential sources of confounding because they can influence mobility, frailty, physical activity, and overall health. The human data therefore cannot prove that glucosamine caused either outcome.
Still, the combination of health-record data, human brain tissue, metabolic mapping, and experimental manipulation in mice makes the glucosamine finding worth following closely. A controlled human study would be needed to determine whether the mechanism translates into clinically meaningful harm.
Why the aging brain could respond differently
These studies raise a broader biological point. Nutrients and metabolites do not operate in isolation. Their effects depend on dose, baseline nutrient status, metabolism, disease state, medications, and the condition of the tissue receiving them.
A molecule that corrects a deficiency is not necessarily beneficial when the body already has enough. Likewise, a metabolic pathway that functions normally in a healthy brain can behave differently in a brain affected by neurodegeneration.
This context-dependent biology is also why population studies need cautious interpretation. Our earlier explainer on obesity and brain health discusses how metabolic health, vascular disease, inflammation, and other factors can converge on cognitive aging. Supplement use sits within that much larger biological picture.
Should older adults stop taking these supplements?
The current evidence does not support a blanket recommendation to stop vitamin D, fish oil, or glucosamine solely because of these studies.
Vitamin D has established roles in bone and mineral health, and supplementation can be appropriate for people who are deficient or have specific clinical indications. Omega-3 fatty acids also have biological and cardiovascular roles, although eating fish and taking an over-the-counter supplement are not equivalent interventions. Glucosamine is commonly taken for joint symptoms, but the new Alzheimer’s findings create a more specific reason for people with cognitive impairment or dementia to discuss its use with a clinician.
The practical lesson is less dramatic than the headlines: supplements are biologically active exposures, not automatically harmless versions of food. Whether they help, do nothing, or cause harm can depend on who takes them and why.
The bottom line
Three supplement stories have converged on the aging brain, but they should not be treated as three proofs of the same phenomenon. Vitamin D and omega-3 studies have reported concerning associations that are not confirmed by randomized cognitive trials. Glucosamine has a more developed mechanistic case, including experimental evidence in Alzheimer’s mouse models, but its human outcome data remain observational.
For now, the evidence argues against using these supplements indiscriminately for “brain health.” It does not justify concluding that vitamin D, fish oil, or glucosamine universally causes cognitive decline.
References
- Hua R, Lam CS, Mok VCT, Cheung YT. Association between vitamin D supplement use and cognitive functioning: a longitudinal cohort study. BMJ Open. 2026;16:e106050. DOI: 10.1136/bmjopen-2025-106050.
- Kang JH, Vyas CM, Okereke OI, et al. Effect of vitamin D on cognitive decline: results from two ancillary studies of the VITAL randomized trial. Scientific Reports. 2021;11:23253. DOI: 10.1038/s41598-021-02485-8.
- Liao ZB, Hu ZC, Zeng GH, et al. The association between omega-3 supplementation and cognitive decline in older adults. Journal of Prevention of Alzheimer’s Disease. 2026;13:100569. DOI: 10.1016/j.tjpad.2026.100569.
- Vyas CM, et al. Marine n-3 fatty acids and cognitive change among older adults in the VITAL randomized trial. Alzheimer’s & Dementia. 2022. DOI: 10.1002/trc2.12288.
- Hawkinson TR, Liu Z, Ribas RA, et al. Hyperglycosylation is a metabolic driver of Alzheimer’s disease. Nature Metabolism. 2026;8:1410-1425. DOI: 10.1038/s42255-026-01538-4.