Many of us would like to keep our brains sharp as we get older. That has helped create a growing market for “brain-health” supplements. These include fish-oil capsules and postbiotics, the beneficial, non-living compounds and byproducts produced by “good” gut bacteria (probiotics) when they digest dietary fibre or prebiotics.

But assessing these products is complicated.

A study found that omega-3 supplement use was associated with faster cognitive decline in older adults. Researchers analysed five years of data from the Alzheimer’s Disease Neuroimaging Initiative and found a more rapid decline on several cognitive measures among people who reported taking the supplements than among matched non-users. Brain scans also showed lower glucose metabolism in vulnerable areas, meaning those brain regions appeared to be using less of their main energy source.

The study cannot show that the supplements caused the differences. It was observational: the researchers compared people who chose to take omega-3 supplements with similar people who did not. They did not assign participants to supplements or placebo, so other differences between the groups may have influenced the results.

The findings do not show that eating fish is harmful. Previous omega-3 research has produced mixed results, and supplements should not be treated as interchangeable with eating fish as part of a balanced diet. A 2023 meta-analysis of longitudinal studies found an association between dietary omega-3 intake and a lower risk of dementia or cognitive decline.

Another recent study illustrates a different aspect of the problem. In a randomised........

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