What does this score mean?
The Trust Index is an editorial methodology score indicating the extent to which the scientific finding evaluated on this page can be trusted, rather than the success rate of the treatment. Study design, risk of bias, sample size, precision, direct applicability, reproducibility, transparency, and clinical relevance are evaluated together.
What did the study investigate?
It addressed two observational human studies evaluating how vitamin D and omega-3 supplement use is associated with cognitive changes in older age.
How was it conducted?
The vitamin D analysis was based on a longitudinal cohort of 5,065 people; the omega-3 analysis was based on 273 users and 546 non-users following propensity-score matching in ADNI data.
What was found?
Associations with faster decline were reported in supplement-using groups on some cognitive measures; however, these findings do not demonstrate causality and may be influenced particularly by factors such as baseline levels, reason for use, and lifestyle.
Limitations of the study
Neither study is a randomized controlled trial. Dose, product content, indication for use, and unmeasured confounding factors may influence the results.
What does it mean clinically?
The data serve as a reminder that correcting a deficiency and taking additional or high-dose supplements when levels are sufficient are not the same thing; the results should not be interpreted as meaning that vitamin D or omega-3 is generally harmful.
What do we not know yet?
Randomized studies are needed to demonstrate at which baseline levels, in which doses, and in which patient populations the long-term cognitive effect differs.
Source
Full citations and links to the sources have been preserved in the detailed evaluation section of this page.
Written and medically reviewed by: Dr. Kerem Çağlayan · Healthy aging and longevity · All Scientific Research
Detailed evaluation
Research evaluation · September 18, 2026
The assumption of “higher dose = more protection” with supplements is frequently seen, especially in the field of longevity. Two observational studies published in 2026 reported signals questioning this assumption for vitamin D and omega-3.
The vitamin D study
In a longitudinal cohort analysis published in BMJ Open, 5,065 individuals from the US Health and Retirement Study were evaluated. The mean age was 67.5 years, and 2,004 individuals were using vitamin D supplements. Over a six-year follow-up, an association was reported with a faster decline in global cognition and executive function scores among supplement users.
In the sensitivity analysis, accelerated decline was observed in supplement users with normal baseline 25(OH)D levels, whereas the same signal was not apparent in the deficient/insufficient group. This study does not prove that vitamin D impairs cognition.
The omega-3 study
In an ADNI analysis published in the Journal of Prevention of Alzheimer’s Disease, 273 omega-3 users were compared with 546 non-users after propensity-score matching. Omega-3 use was associated with faster worsening on some cognitive scales. The researchers reported that FDG hypometabolism might mediate part of the association.
This, too, is not a randomized controlled trial; the reason for use, dose, product content, and lifestyle factors cannot be fully controlled.
Clinical message
These data do not support the conclusion that “vitamin D or omega-3 is harmful.” The more accurate message is that correcting a deficiency and pursuing additional or high-dose optimization when laboratory levels are already sufficient are not the same thing.
Quality of evidence
Observational human data; not proof of causality.
References
- Hua R, Lam CS, Mok VCT, Cheung YT. Association between vitamin D supplement use and cognitive functioning: a longitudinal cohort study. BMJ Open. 2026;16:e106050.
- Liao ZB, Hu ZC, Zeng GH, et al. The association between omega-3 supplementation and cognitive decline in older adults. J Prev Alzheimers Dis. 2026;13(6):100569.
- The Washington Post – September 10, 2026.
This content is an evaluation of the scientific literature; it is not an individual supplement dosage recommendation.
