Three of the most popular supplements in the United States — vitamin D, omega-3 fish oil and glucosamine — are facing unexpected questions about their effects on the ageing brain, after a run of recent studies suggested they may be linked to faster cognitive decline in some older adults.
In January, a study published in BMJ Open followed 5,065 Americans with an average age of 67.5 from the long-running Health and Retirement Study. Around 40% took vitamin D supplements. Over six years, users showed slightly faster declines in overall cognition and executive function — but only those who already had normal vitamin D blood levels. People who were deficient didn't show the same pattern. One hypothesis is that extra vitamin D could affect molecular signalling tied to amyloid beta plaques, the proteins associated with Alzheimer's.
April brought a second signal. An analysis in the Journal of Prevention of Alzheimer's Disease compared 273 omega-3 supplement users aged 55 and older with 546 non-users over a median of five years, drawing on the NIH-funded Alzheimer's Disease Neuroimaging Initiative. Supplement users declined faster on standardised measures of memory, attention, language and spatial processing. Notably, users also showed lower glucose metabolism in brain regions vulnerable to Alzheimer's. Related research from the Medical University of South Carolina has even linked a fish oil component to slower recovery after brain injury.
Then in June came the most striking result. A team led by University of Florida biochemistry professor Ramon Sun found that glucosamine — a joint-pain remedy taken mainly by middle-aged and older adults — was associated with roughly a 25% higher likelihood of progressing from mild cognitive impairment to dementia over five years, and, among those already with Alzheimer's or dementia, a roughly 25% higher death rate. The study combined mouse models, postmortem human brain analysis and electronic health records, pointing to abnormal sugar buildups on proteins in the Alzheimer's brain.
Before anyone empties the medicine cabinet, the caveats matter. All three studies are observational — they show associations, not proof of cause and effect. Supplement users differ from non-users in income, health and habits, and people often start taking supplements precisely because something is already wrong. The findings also contradict other research in places.
Still, researchers say the pattern is worth taking seriously. The ageing brain has less biological reserve, and the blood-brain barrier — the gateway controlling what reaches brain tissue — changes with age. The emerging question isn't whether a supplement is simply good or bad, but good for whom, and at what age. Doctors routinely adjust medications for older bodies; supplements rarely come with that kind of age-specific guidance.
The practical takeaway: if you're over 55 and taking any of these supplements daily, it's worth raising with your doctor rather than making assumptions either way. Getting vitamin D from sunlight and food, rather than capsules, has its advocates too.