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 controlled trial, researchers found that taking a daily multivitamin for two years was associated with a modest slowing of changes in two measures of biological ageing known as epigenetic clocks, which estimate ageing-related changes by looking at chemical marks on DNA. The authors stressed that more research is needed to establish whether these small changes translate into meaningful health benefits.
Together, the studies highlight a central problem in supplement research. A product may affect the brain or body without producing an obvious improvement in everyday functioning. A measurable change in the body may show that something is happening, but not whether it matters for memory, wellbeing or daily life.
Looking beneath the test score
One approach is to ask whether people perform better on cognitive tests after taking a supplement. Digital cognitive assessments can provide a practical way to measure aspects of cognitive function. But, like any measure, they need to be validated and interpreted carefully.
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Cognitive tests can also be blunt instruments, particularly when researchers are studying healthy adults who are already functioning well. A person can achieve the same score twice while their brain is working differently underneath. Stable performance may sometimes reflect the brain’s ability to adapt or compensate.
Researchers studying healthy cognitive ageing are increasingly interested in how the brain maintains performance over time.
Our ongoing better brain trial is designed to explore whether supplements and other nutrition-based products can change the way the brain supports cognitive performance, even when improvements are not immediately visible on a standard test. The findings are still being analysed and have not yet been published in a peer-reviewed journal.
In our randomised, double-blind, placebo-controlled study, adults aged over 55 took supplement combinations or matched placebos each day for 60 days. They completed tasks examining working memory and attention while their brain activity was recorded using electroencephalography (EEG).
EEG is a non-invasive method that uses sensors on the scalp to measure tiny changes in electrical activity associated with the brain’s responses during a task. It does not read thoughts.
Participants completed an n-back task, in which they responded when a letter matched one shown earlier. They also completed the Stroop task, which requires people to focus on one feature while ignoring a conflicting cue, such as identifying the ink colour of the word “red” when the word itself is printed in blue.
Researchers can compare scores and reaction times with tiny changes in brain activity known as event-related potentials (ERP). These changes happen in the milliseconds after someone sees or responds to something. ERP measures can help researchers investigate cognitive processing during tasks. They may reveal subtle shifts that test scores alone miss.
Researchers still need to establish whether a change is reliable, whether it can be replicated and whether it is associated with an outcome that matters in people’s lives.
Not just the brain
Nutrition affects the whole body, and the body communicates with the brain through metabolism, inflammation, blood vessels, hormones and the gut-brain axis, the two-way communication between the digestive system and the brain.
The better brain trial also uses metabolomics, a technique that measures small molecules in biological samples such as blood or urine. These molecules can provide clues about biological processes. Metabolomics may also help researchers explore whether people respond differently to the same intervention.
Studies often report an average result across a group, but an average can conceal variation. This technique may identify patterns worth investigating in larger trials. It cannot, on its own, prove that a supplement improves memory or protects the brain.
A cognitive-test score can show whether a person completed a task more quickly or accurately. EEG can provide information about the brain activity associated with that performance. Metabolomics can reveal changes elsewhere in the body.
Each method captures part of the picture. A supplement should not be marketed as supporting memory simply because it changes a biomarker. Researchers need to know whether an effect is robust, whether it lasts and whether it translates into a meaningful benefit.
For consumers, caution is key. Supplements should not be treated as a shortcut to brain health. Evidence reviewed by the 2024 Lancet Commission on dementia prevention supports the importance of physical activity, social contact and management of long-term health risks such as high blood pressure.
In a market where claims can easily run ahead of evidence, measuring more does not remove uncertainty. But it can help researchers ask better questions and explain their findings more honestly.
