Three Studies, Three Different Answers on Supplements and the Brain

Episode SummaryA friend sent Mikki a link in a WhatsApp group — an NZ Herald article, originally published in the Washington Post, under the headline that popular supplements could harm the ageing brain. It named three: vitamin D, omega-3 and glucosamine. In this solo episode Mikki goes back to the three separate studies behind that single article and works through what each one actually found.Along the way she lays out the difference between observational research and randomised controlled trials, and why healthy user bias makes supplement users hard to compare with non-users in the first place. The three studies do not point the same way: the vitamin D finding is statistically significant but amounts to about a third of a point on a 27-point scale over six years, the omega-3 signal is more substantial but runs directly against the randomised evidence from VITAL, and the glucosamine work is the most interesting of the three because it pairs experimental biology with human health records — though it concerns people who already have cognitive impairment. Mikki finishes with the questions worth asking of any study like this, and where she has landed on each supplement.Key TopicsWhy the NZ Herald headline claims more than the evidence behind it can support, even though the article itself acknowledges the findings are preliminary, mostly observational and sometimes contradictoryObservational studies versus randomised controlled trials, and why observational work is hypothesis-producing rather than conclusiveHealthy user bias — researchers can statistically adjust for some differences between supplement users and non-users, but not for a healthy user lifestyle, and not for anything that was never measuredThe vitamin D observational study: over 5,000 adults, average age 67.5, followed six years, showing faster decline in global cognition and executive function — but only about 0.31 points on a scale running to 27Why very large databases can make clinically trivial differences statistically significant, and why the vitamin D result disappeared when users were compared against people taking neither vitamin D nor a multivitaminThe omega-3 analysis using ADNI data, which found faster decline on three cognitive measures plus greater FDG hypometabolism in Alzheimer's-vulnerable brain regions — a real signal, but an observational oneWhy that omega-3 analysis has been challenged in published letters, covering exposure classification, residual confounding, omega-3 formulation, and its inconsistency with previous evidenceWhat the VITAL randomised trial found for both vitamin D and omega-3: no significant difference in cognitive decline, which means no demonstrated benefit but also no demonstrated harmThe glucosamine paper, which combined post-mortem human brains, Alzheimer's mouse models and electronic health records, and found roughly 25% greater progression from mild cognitive impairment to dementia among usersThe questions to ask of any supplement study — who was studied, were they deficient, what dose, was exposure randomised, how big was the effect, does it agree with the randomised trials, and has it been replicatedChapters00:00 A headline about supplements and the brain02:11 Observational studies versus randomised trials05:18 Vitamin D and cognitive decline07:53 Small effects, big databases, real limitations11:11 Omega-3, fish oil and the ADNI study15:00 Why the omega-3 findings were challenged18:55 Glucosamine, mice and health records21:59 Bringing the three studies together23:55 Practical bottom lines for each supplementGuest / ResourcesThe article that prompted the episodeNZ Herald: "Why scientists are starting to worry popular supplements could harm the ageing brain" — https://www.nzherald.co.nz/lifestyle/why-scientists-are-starting-to-worry-popular-supplements-could-harm-the-ageing-brain/premium/VHMO7A5XQBAORA7PZPHXVCNHLE/Originally published in the Washington Post, 10 September 2026: "Why scientists are starting to worry popular supplements could harm the aging brain" — https://www.washingtonpost.com/health/2026/09/10/scientists-are-investigating-whether-popular-supplements-could-harm-aging-brain/The three studies discussedVitamin D — "Association between vitamin D supplement use and cognitive functioning: a longitudinal cohort study", BMJ Open, January 2026. Used the US Heal