Three Studies, Three Different Answers on Supplements and the Brain

Episode Summary
A 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 Topics
  • Why 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 contradictory
  • Observational studies versus randomised controlled trials, and why observational work is hypothesis-producing rather than conclusive
  • Healthy 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 measured
  • The 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 27
  • Why 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 multivitamin
  • The 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 one
  • Why that omega-3 analysis has been challenged in published letters, covering exposure classification, residual confounding, omega-3 formulation, and its inconsistency with previous evidence
  • What 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 harm
  • The 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 users
  • The 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 replicated
Chapters
00:00 A headline about supplements and the brain
02:11 Observational studies versus randomised trials
05:18 Vitamin D and cognitive decline
07:53 Small effects, big databases, real limitations
11:11 Omega-3, fish oil and the ADNI study
15:00 Why the omega-3 findings were challenged
18:55 Glucosamine, mice and health records
21:59 Bringing the three studies together
23:55 Practical bottom lines for each supplement
Guest / Resources
The article that prompted the episode
The three studies discussed
  • Vitamin D — "Association between vitamin D supplement use and cognitive functioning: a longitudinal cohort study", BMJ Open, January 2026. Used the US Health and Retirement Study; 5,065 participants, mean age 67.5, 2,004 (39.6%) vitamin D users, six years of follow-up. Global cognition declined by an additional 0.052 points per year and executive function by 0.021 points per year among users. https://pubmed.ncbi.nlm.nih.gov/41571412/
  • Omega-3 — "The association between omega-3 supplementation and cognitive decline in older adults", The Journal of Prevention of Alzheimer's Disease, 2026. Used Alzheimer's Disease Neuroimaging Initiative (ADNI) data; 273 omega-3 users compared with 546 matched non-users over about five years, with FDG hypometabolism in Alzheimer's-vulnerable regions acting as a mediating pathway. https://pmc.ncbi.nlm.nih.gov/articles/PMC13099475/
  • Glucosamine — "Hyperglycosylation is a metabolic driver of Alzheimer's disease", Nature Metabolism, 2026. Combined post-mortem human brain tissue, Alzheimer's mouse models, experimental manipulation of glycan biosynthesis, and electronic health records. https://www.nature.com/articles/s42255-026-01538-4
Published responses to the omega-3 analysis
The randomised trial referenced throughout
  • VITAL (Vitamin D and Omega-3 Trial) — randomised older adults to 2,000 IU vitamin D3 daily and to 1 g marine omega-3 daily providing 460 mg EPA and 380 mg DHA, against placebo. https://clinicaltrials.gov/study/NCT01169259
Cognitive measures mentioned
  • MMSE (Mini Mental State Examination), ADAS-Cog-13 (Alzheimer's Disease Assessment Scale), CDR-SB (Clinical Dementia Rating Sum of Boxes), FDG PET imaging, APOE4 status
Also mentioned
  • Pillar Performance — the omega-3 supplement Mikki takes herself. She states in the episode that this is not an ad.
  • Mikki's recipe portal at mikkiwilliden.com — over a thousand recipes, $12 a month
  • Instagram, Threads and X: Mikki Williden · Facebook: Mikki Williden Nutrition · Website: mikkiwilliden.com

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