Could Some Popular Supplements Harm the 50+ Brain?

New research raises questions about vitamin D, fish oil and glucosamine—but scientists caution that the findings are preliminary and should not prompt people to abandon medically necessary supplements.

For years, vitamin D, fish oil and glucosamine have occupied the "good guys" side of the medicine cabinet. They are widely available, often described as “natural,” and commonly taken by older adults hoping to protect their bones, brains, hearts or joints.

But several studies now challenge the assumption that these supplements are always harmless—particularly as the brain ages. As The Washington Post recently reported, researchers have found troubling associations between the three popular supplements and cognitive decline in some older adults.

The evidence is far from conclusive. Most of the findings come from observational research, which can identify a relationship but cannot prove that a supplement caused a health problem. Still, the studies raise an important question for people over 50: Could a substance that is beneficial at one age—or for someone with a diagnosed deficiency—affect an older brain differently?

Three supplements under scrutiny

A study published in January in BMJ Open examined vitamin D supplement use among 5,065 Americans whose average age was about 68. Researchers used data from the University of Michigan’s nationally representative Health and Retirement Study.

Over six years, participants taking vitamin D experienced slightly faster declines in overall cognition and executive function—the mental skills involved in planning, concentrating and making decisions. Importantly, that association appeared only among people whose vitamin D blood levels were already normal. It was not found among participants who were deficient or had insufficient levels.

That distinction matters. Vitamin D is essential for bone and muscle health, and older adults are particularly susceptible to deficiency because their bodies become less efficient at producing and processing it. The new research does not suggest that people with a documented deficiency should refuse treatment.

A separate 2026 analysis examined adults ages 55 to 90 participating in the NIH-supported Alzheimer’s Disease Neuroimaging Initiative. Researchers compared 273 people who used omega-3 supplements with 546 nonusers over a median of five years.

Those taking omega-3 supplements declined more rapidly on three measures of memory and cognitive function. Brain imaging also indicated lower glucose metabolism in regions vulnerable to Alzheimer’s disease among supplement users.

Again, the study did not establish cause and effect. People may have begun taking fish oil because they were already worried about memory problems, for example. Dosages, formulations and reasons for using the supplements could also have differed.

The findings do, however, reinforce a distinction researchers have observed before: Eating fish as part of a healthy diet is not necessarily equivalent to swallowing concentrated fish-oil capsules. Foods deliver combinations of protein, fats, vitamins and other nutrients that supplements do not reproduce.

The third warning signal involves glucosamine, commonly taken for osteoarthritis.

Researchers studying how sugars attach to proteins in the brain found that giving glucosamine to mice genetically modified to develop Alzheimer’s-like disease worsened their cognitive problems. The scientists also found unusually high levels of these sugar structures, called glycans, in human Alzheimer’s brain tissue.

When they analyzed electronic health records, glucosamine use was associated with about a 25% greater likelihood that someone with mild cognitive impairment would progress to Alzheimer’s disease or another dementia. Among people who already had dementia, its use was associated with a roughly 25% higher death rate.

Those figures sound alarming, but the records could not capture every supplement people used, and unmeasured differences between users and nonusers might explain some of the results. A controlled human trial would be needed to determine whether glucosamine itself increases the risk.

The National Center for Complementary and Integrative Health already describes the evidence for glucosamine’s joint benefits as inconsistent. It also warns that glucosamine may raise blood sugar in some people and that glucosamine and chondroitin may increase bleeding risk when combined with warfarin.

Why aging could change the equation

The brain is not biologically identical at 70 and 30. With age, it may regulate inflammation less efficiently, recover more slowly from cellular stress and process energy differently. The blood-brain barrier—the protective system controlling which substances can move from the bloodstream into brain tissue—also changes. Scientists are investigating whether those changes alter how nutrients, medications and supplements reach the aging brain.

Doctors routinely adjust prescription medications as kidney function, metabolism and drug sensitivity change with age. Supplements rarely carry comparable age-specific instructions, even though they contain biologically active ingredients.

They also are not reviewed like prescription drugs. The Food and Drug Administration generally does not approve dietary supplements for safety and effectiveness before they are marketed. Manufacturers are responsible for ensuring that their products are safe and accurately labeled.

What should you do now?

Researchers are not recommending that everyone stop taking vitamin D, fish oil or glucosamine. Stopping a supplement abruptly could create its own problems. Instead, use the new findings as a reason to review what is in your medicine cabinet:

Make a list of every vitamin, herb, powder and supplement you take, including dosages.

Show the list to your physician or pharmacist, especially if you take prescription medications.

Ask what specific benefit each supplement is supposed to provide—and whether evidence supports it.

For vitamin D, ask whether a blood test is appropriate before taking or increasing a dose.

Don’t assume that “natural” means risk-free or that more is better.

Consider whether food can provide the desired nutrient instead. Fatty fish, eggs and fortified dairy or plant-based products, for example, can supply omega-3s or vitamin D.

The emerging research does not prove that these familiar supplements damage the aging brain. But it does deliver a useful reminder: A bottle sold without a prescription can still have powerful effects—and those effects may change as we do.