Dietary supplements can harm the brain in old age

The key rule of the hierarchy of evidence is that randomized controlled trials are at the top: in them, one group of participants is given the supplement, while another is given a placebo. This approach makes it possible to more reliably distinguish the real cause from a random association. A striking example is the large VITAL study, which began in 2009: it observed 3,400 elderly people and found no benefit or harm from taking vitamin D or fish oil for cognitive functions.

Observational studies, in which scientists simply record what people are already taking, provide a completely different picture. Such studies often involve many hidden factors: for example, those who take fish oil may eat almost no fish, while those who take glucosamine often suffer from osteoarthritis — and these characteristics in themselves affect brain health. In addition, people recall their habits differently, the observation periods are often short, and changes in the cognitive sphere are difficult to measure.

The issue of vitamin D stands apart. It belongs to the fat‑soluble vitamins: unlike water‑soluble ones, the body cannot quickly eliminate its excess, and the substance accumulates in tissues. With age, metabolism changes, and this fact becomes particularly important. According to experts, an excess of vitamin D can theoretically intensify the processes leading to the formation of amyloid plaques — protein accumulations that are associated with Alzheimer’s disease. Another problem is the huge variation in dosages in supplements: one capsule may contain 1,000 IU, while another may contain as much as 5,000 IU. At the same time, the upper safe limit for adults is 4,000 IU (100 mcg) per day, meaning that a single serving may already exceed the recommended limit. If you add a multivitamin to this, the total dose will be even higher, and an excess can be toxic and may interact with certain medications.

The data on omega‑3 and fish oil appear contradictory. On the one hand, observational studies have found a link between fish and omega‑3 consumption and a lower risk of cognitive decline, and long‑term supplementation has been associated with a 64 % reduction in the risk of Alzheimer’s disease. On the other hand, high doses of omega‑3 in the form of supplements may be associated with atrial fibrillation — a heart rhythm disorder that in itself increases the risk of cognitive problems. Randomized trials have not confirmed the harm of fish oil for healthy older adults, and in some patients with mild cognitive impairment, individual studies even showed benefits. Omega‑3 levels can be checked with a blood test — this helps make more informed decisions.

Glucosamine deserves special attention. It is involved in the formation of glycan chains — chains of sugar molecules that can accumulate in the body. In addition to metabolic issues, there are also quite practical risks: glucosamine can interact with warfarin, a blood‑thinning medication, which means it may pose a real danger when taken together.

Experts agree that it is best to obtain nutrients from food: a balanced diet provides a complex of substances in natural proportions. To support cognitive health, the Mediterranean diet or the MIND diet is particularly recommended — a plant‑based eating pattern specifically designed to preserve brain function. At the same time, no vitamin or dietary supplement has been proven to prevent or treat Alzheimer’s disease. If a person has a confirmed deficiency and actually feels better on the supplement, they can continue taking it, but only in consultation with a doctor. It is important to carefully monitor dosages and composition: check exactly how much of the substance is contained in one serving, and do not exceed the upper safe threshold. It should also be taken into account that the response to supplements varies greatly from person to person.