Multivitamins, Vitamin D, and Fish Oil for an Aging Brain: What the Evidence Says

Longevity Basics leaves out supplements on purpose. The six factors in the book have stronger evidence, and supplement research changes fast. This page is where we keep track of it. Here’s what the research on three popular supplements shows so far.

The short answer

• Multivitamins: in one large trial program, a daily multivitamin produced a modest benefit on memory and thinking tests in older adults. That's encouraging, but it isn't the same as preventing dementia.
• Vitamin D: in large trials of older adults who mostly weren't deficient, vitamin D didn't slow cognitive decline or lower dementia rates.
• Fish oil (omega-3): heavily studied, but in a large trial it didn't help memory or thinking, and guidelines favor eating fish over supplements for most people.
• None of them replaces the basics in the book: strength training, cardio, sleep, stress, and connection.
• Talk to your doctor before starting any of them, especially if you take medications.

Multivitamins: a modest, promising signal

COSMOS is a large randomized trial of older adults, run by Brigham and Women's Hospital with Wake Forest, Columbia, and Mass General.

• Memory and thinking: three cognition studies inside COSMOS, with more than 5,000 participants combined, found that a daily multivitamin modestly improved scores on memory and overall thinking tests compared with a placebo. The researchers estimate it slowed cognitive aging by the equivalent of about two years. The benefit was significant for memory but not for attention and executive function. [1]
• Biological aging: in 2026, a COSMOS study of 958 adults averaging 70 years old found that a multivitamin slowed "epigenetic clocks," a DNA-based estimate of biological age, by about four months over two years. The researchers note that more work is needed to know what that means for long-term health. [2]
• Living longer: that's a different question. In a study that followed about 390,000 generally healthy U.S. adults for more than 20 years, people who took a daily multivitamin didn't have a lower risk of death. [3]
• Heart disease and cancer: the U.S. Preventive Services Task Force says the evidence is insufficient to recommend multivitamins for preventing them. It recommends against beta-carotene and vitamin E supplements for this purpose. [4]
• Who paid: COSMOS is funded by the National Institutes of Health and by Mars Edge, a nutrition business of Mars. Pfizer Consumer Healthcare, now Haleon, supplied the multivitamin and placebo pills. [2, 5]

What it means: a multivitamin is a reasonable thing to ask your doctor about, especially if your diet is limited. The best evidence so far shows a small benefit on test scores, not protection from dementia.

Vitamin D: the trials haven't shown a brain benefit for most people

• VITAL: in a randomized trial, older adults took vitamin D or a placebo for about two to three years. Vitamin D didn't slow cognitive decline overall. There was a modest benefit among Black participants, a finding that needs confirming. [6]
• Finnish Vitamin D Trial: 2,492 adults with an average age of 68 took one of two daily doses of vitamin D or a placebo for about four years. Vitamin D didn't change how many people developed dementia. Most participants already had enough vitamin D. [7]
• Endocrine Society guideline (2024): the society suggests that healthy adults under 75 not take more than the standard recommended intake. It says adults 75 and older may benefit from supplements because of a possible lower risk of death, and it suggests against routine vitamin D blood tests for the groups it studied. [8]

Why headlines disagree: a study that follows people over time can show that low vitamin D and memory problems tend to go together. It can't show that taking vitamin D prevents them. Randomized trials test that directly, and so far they haven't shown a brain benefit in people with adequate levels.

What it means: if you're 75 or older, or you think you may be low in vitamin D, ask your doctor. Taking extra vitamin D for your brain isn't supported by the trials so far.

Fish oil: well studied, but no brain benefit so far

• Memory and thinking: in the VITAL trial, older adults took fish oil or a placebo for two to three years. There was no difference in how their memory and thinking changed. [9] The NIH Office of Dietary Supplements concludes that omega-3 supplements don't affect thinking in older adults without cognitive impairment. [10]
• Biological aging: in DO-HEALTH, a three-year trial of 777 Swiss adults over 70, daily omega-3 slowed several epigenetic clocks by up to about four months. As with the multivitamin study above, it isn't yet clear what that means for long-term health. [11]
• Heart: in VITAL, fish oil didn't lower major heart events overall, although heart attacks were lower. A high-dose prescription form of omega-3 reduced cardiovascular events by about 25% in people at high risk, but that's a medication taken under a doctor's care, not an over-the-counter supplement. [10]
• A risk to know about: high-dose trials found a slightly higher risk of atrial fibrillation, an irregular heartbeat. In a UK study of about 415,000 adults, regular fish oil users who didn't have heart disease had a 13% higher risk of atrial fibrillation and a 5% higher risk of stroke, while the pattern was more favorable in people who already had heart disease. That study was observational. [10, 12]
• Guidelines: the American Heart Association doesn't recommend omega-3 supplements for people who aren't at high heart risk, and favors eating fish. [10]

What it means: eating fish is the better-supported choice. If you're thinking about fish oil, especially for your heart or with a heart rhythm problem, talk to your doctor first.

What does protect your brain?

The six factors in Longevity Basics are where the stronger evidence is. That means muscle, cardio fitness, metabolic health, a fixed wake time, stress practice, and real connection. Start with one Minimum Viable Practice. Never zero.

Sources

1. Mass General Brigham, "Third major study finds evidence that daily multivitamin supplements improve memory and slow cognitive aging in older adults," January 2024 (summarizing Vyas et al., American Journal of Clinical Nutrition, 2024). https://www.sciencedaily.com/releases/2024/01/240118122110.htm · Paper: https://ajcn.nutrition.org/article/S0002-9165(23)66342-7/fulltext
2. "Scientists say a daily multivitamin may help slow aging," May 2026 (summarizing Nature Medicine, May 2026). https://www.sciencedaily.com/releases/2026/05/260512202345.htm · Paper: https://www.nature.com/articles/s41591-026-04239-3
3. National Institutes of Health, "For healthy adults, taking multivitamins daily is not associated with a lower risk of death," June 26, 2024 (Loftfield et al., JAMA Network Open, 2024). https://www.nih.gov/news-events/news-releases/healthy-adults-taking-multivitamins-daily-not-associated-lower-risk-death
4. U.S. Preventive Services Task Force, Vitamin, Mineral, and Multivitamin Supplementation to Prevent Cardiovascular Disease and Cancer, June 21, 2022. https://www.uspreventiveservicestaskforce.org/uspstf/recommendation/vitamin-supplementation-to-prevent-cvd-and-cancer-preventive-medication
5. COSMOS Trial, Q & A (funding and study pills). https://cosmostrial.org/q-a/
6. Effect of vitamin D on cognitive decline: results from two ancillary studies of the VITAL randomized trial. Scientific Reports, 2021. https://www.nature.com/articles/s41598-021-02485-8
7. The Effect of Vitamin D3 Supplementation on the Incidence of Diagnosed Dementia Among Healthy Older Adults — The Finnish Vitamin D Trial. Journals of Gerontology: Series A, 2025. https://academic.oup.com/biomedgerontology/article/80/7/glaf077/8115445
8. Endocrine Society, Vitamin D for the Prevention of Disease guideline, June 2024 (news release). https://www.endocrine.org/news-and-advocacy/news-room/2024/endocrine-society-recommends-healthy-adults-take-the-recommended-daily-allowance-of-vitamin-d
9. Kang JH, Vyas CM, Okereke OI, et al. Marine n-3 fatty acids and cognitive change among older adults in the VITAL randomized trial. Alzheimer's & Dementia: Translational Research & Clinical Interventions, 2022. https://doaj.org/article/567a00a9daa54fc58c3f03ad5cbf7049
10. NIH Office of Dietary Supplements, Omega-3 Fatty Acids: Fact Sheet for Health Professionals, updated September 20, 2026. https://ods.od.nih.gov/factsheets/Omega3FattyAcids-HealthProfessional/
11. University of Zurich, "Omega-3s can slow down aging process," February 2025 (summarizing Bischoff-Ferrari et al., Nature Aging, 2025). https://news.uzh.ch/en/articles/media/2025/omega-3s-aging-process.html · Paper: https://pubmed.ncbi.nlm.nih.gov/39900648/
12. Regular use of fish oil supplements and course of cardiovascular diseases: prospective cohort study. BMJ Medicine, 2024. https://doaj.org/article/6195dfa0d32e4576900d3292dc06c092

This page is for general education, not medical advice. Talk with your doctor about your own situation.