Millions of older Americans reach for vitamin D, fish oil, and glucosamine to protect their brain and joints.
Three separate observational studies published this year have now linked those same supplements to faster cognitive decline in adults 55 and older.
None of the studies proves cause and effect.
Each one shows an association pulled from health records or cohort data, and the researchers behind them stress the limits of that design.
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Vitamin D
A study published in BMJ Open in January 2026 followed 5,065 older adults over about six years.
It found that vitamin D users had a faster decline in global thinking skills than non-users.
The effect showed up only in participants whose vitamin D levels were already normal at baseline.
Users lost about 0.05 points a year more on the study's overall cognitive scale.
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They also lost 0.02 points a year more on tests of executive function, which cover planning and problem solving.
Memory scores did not differ significantly.
Fish oil
A study in The Journal of Prevention of Alzheimer's Disease in April 2026 compared 273 omega-3 supplement users with 546 matched non-users over a median of five years.
Users declined faster on three standard cognitive tests.
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The researchers linked the pattern to reduced glucose use in brain scans.
Amyloid and tau, the proteins usually blamed for Alzheimer's disease, showed no clear role.
Glucosamine
In June 2026, University of Florida neuroscientists reported in Nature Metabolism that glucosamine, a joint supplement, may accelerate cognitive decline.
People with mild cognitive impairment who used it had about a 25 percent higher likelihood of progressing to dementia during follow-up.
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The team combined records from more than 2,750 patients with laboratory work on brain tissue and mouse models.
Senior author Ramon Sun said the results are preliminary and need to be confirmed in a clinical trial.
A large randomized trial found no effect either way
The observational data sit against a very different result from the VITAL trial, a randomized study of about 3,400 older adults launched in 2009.
Christopher Weber, senior director at the Alzheimer's Association, told Northeastern Global News that "neither vitamin D nor fish oil helped or harmed cognition" in that data.
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Randomized trials assign people to a supplement or a placebo, so the two groups are similar on average.
Observational studies compare people who already chose to take a supplement with those who did not, and those groups often differ in diet, income, and health status.
David Madigan, a Khoury College professor at Northeastern University, said "not much" can be pulled out of the newer observational papers on their own.
What should you do?
The upper safe limit for vitamin D in adults is 4,000 international units a day, and typical store bottles run from 1,000 to 5,000 units.
Carmen Sceppa, dean of Bouvé College of Health Sciences at Northeastern, advised talking to a doctor before starting or stopping any supplement.
Nutrition experts continue to point to the Mediterranean or MIND diet as the strongest evidence-based approach for brain health during aging.
This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.