It is often said that that fish is 'brain food', and you may have read the speculation that omega-3 in the diet can help reduce your risk of dementia by improving heart and brain health.

There is good evidence that eating fish, which contains omega-3, is good for your health. There have been reports that it may reduce the risk of developing dementia, especially when it is eaten as part of a healthy diet. However, there isn't very strong evidence that omega-3 itself is behind this benefit.

What is Omega-3? Why might it be good for our brains?

Omega-3 is a kind of fat found in cell membranes (the protective 'skin' that surrounds cells). It is made in our bodies, but very slowly, so we mostly get it from our diet. Oily fish, for example mackerel, tuna, herring, and salmon, have especially high levels of omega-3.

Omega-3 is important for our brain throughout life, from early cognitive development in foetuses to learning and memory in adults. Brain cells with high levels of omega-3 in their membranes are thought to be better at communicating with other cells, an important process for brain function.

When omega-3 is taken up by the body, some of it is broken down into other molecules that have important roles in the brain. Some are found to reduce the body's immune response, while others are thought to be involved in protecting cells from a harmful process called oxidative stress. Research has indicated that the immune response and oxidative stress in the brain may contribute towards the development of Alzheimer's disease.

Omega-3, fish and dementia

Studies showing the potential benefits of omega-3 for the brain are one thing, but does that mean more omega-3 in the diet really affects the risk of developing dementia? This question has been investigated in two ways. Some studies have looked at what people with and without dementia eat and how often and then followed them to see whether diet affected dementia risk. Other studies were clinical trials, where some participants are given omega-3 supplements and others a placebo, and their risk of dementia is compared.

Studies looking at the link between fish consumption and the risk of dementia have produced mixed results. One study that followed 2,233 older people for five or six years found that eating fish twice a week could reduce dementia risk by 41% compared to groups eating fish once a month. However, another study looking at fish in the diet in 5,395 people over ten years found there was no change in dementia risk depending on the amount of fish consumed.

Clinical trials have also studied the effect of omega-3 on its own in the form of supplements but these studies have been small. For example, a study that gave supplements to 171 people affected by dementia for 18 months found that they had no cognitive improvement compared to a group not given supplements over the same period. Another study looked at giving supplements to people who were healthy but were beginning to show some symptoms of dementia, such as difficulty with memory recall. This study involved 437 people and indicated that after 24 weeks of taking the supplement, some kinds of memory recall and learning were improved. This could suggest that taking omega-3 supplements early on in dementia development may improve symptoms, but that in the later stages omega-3 has no effect.

The research evidence on fish oils to date is contradictory - fish are probably good for your brain but omega-3 on its own may not be. One explanation for this could be that there are other nutrients in fish that may play a role in risk reduction. Another is that people who eat fish may have a healthier diet overall, for example a Mediterranean diet. Further studies are needed to continue taking apart the effects of different nutrients in fish, and to indicate what processes in the brain they influence and how. There are also many different ways to eat fish. Eating fried fish and chips three times a week is unlikely to provide the same health benefits as eating baked salmon and salad. We need to think about fish as part of a whole diet.

From the available evidence, it's likely that eating fish regularly as part of a balanced diet can improve your risk of age-related cognitive decline as well as other aspects of your health. However, the jury's still out on omega-3.