Three separate studies published in 2026 have reported that older adults taking vitamin D, omega-3 supplements fish oil, or glucosamine supplements showed faster cognitive decline than non-users, according to a roundup published by Medical Daily on 14 September 2026. A BMJ Open study of 5,065 Americans with a mean age of 67.5, followed for six years, found vitamin D users had an accelerated decline in global cognitive function and executive function, though this appeared only in users whose blood vitamin D levels were already normal, not those who were deficient. A separate study of more than 800 people in the Alzheimer's Disease Neuroimaging Initiative, published in the Journal of Prevention of Alzheimer's Disease, found omega-3 users declined faster on cognitive tests than non-users. A third analysis of electronic health records, published in Nature Metabolism, linked glucosamine use to a higher rate of progression to dementia.
Observed link, not proven cause
All three studies were observational, comparing supplement users with non-users rather than randomly assigning people to take the supplement or not. The Journal of Prevention of Alzheimer's Disease authors were explicit about this limit, writing that their findings "don't prove that omega-3 supplements cause accelerated brain decline" and calling for "a more nuanced assessment and further research," according to ScienceAlert's coverage of the same study. Researchers across the three papers noted that people who choose to take a supplement may already differ from those who do not, in ways that could explain the pattern without the supplement itself playing any causal role.
What this means for your supplement routine
None of the three studies changes the authorised UK position on vitamin D, which contributes to the normal function of the immune system, or on EPA and DHA, which contribute to the normal function of the heart, under the conditions of use set out on the GB Nutrition and Health Claims Register. What they add is a reminder that observational findings on long-term supplement use, in either direction, need controlled trials before they can be treated as settled. Anyone checking what they currently pay for vitamin D or fish oil can compare prices in the vitamins and minerals collection.
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This article reports published news and research and is for information only. Food supplements are not a substitute for a varied diet or medical care, and no product mentioned is intended to diagnose, treat, cure or prevent any disease. Speak to your GP or pharmacist before changing what you take, especially if you are pregnant, breastfeeding, or taking prescribed medicines.