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The rise and fall of fish oils

by on 2013/10/30

If you were asked what types of food might protect or enhance your brainpower, fish is likely to be high on your list. Fish is often called brain food. It’s easy to locate reports citing the link between eating more fish and the maintenance of thinking skills; for example BBC GoodFood, WebMD and The Huffington Post were just the top three hits when I searched for “brain food”. Oily fish are reported as being the best for a healthy brain because they are rich in omega-3 fatty acids. The omega-3s are a component of the membranes of the brain’s grey matter (the thinking cells) and might also help with the transmission of signals between brain cells. However, a recent study reported no association between the levels of omega-3s and cognitive ability in a large sample of women, or with change in their cognitive ability over almost 6 years.

What about the actual research?

The study analysed data from 2,157 women enrolled in a larger study which has examined effects of different hormone replacement therapies. For the current study, however, blood samples were available and these were used to assess levels of two omega-3 fatty acids. The participants completed a series of cognitive assessments – tests of their mental skills – about 3 years after the samples were taken when the women were all aged between 65 and 84. They were followed-up for an average of 6 years, retaking the cognitive assessments every year.

The researchers, led by PhD student Eric Ammann and Professor Jennifer Robinson, both from the Department of Epidemiology at the University of Iowa, reported that there was no association between the levels of omega-3s and cognitive ability at the first assessment (whether looking at the various tests separately or as an overall composite score) after considering a number of factors that might confound the results, such as educational level, physical activity, body mass index and disease burden, for example. Furthermore, levels of omega-3s were not associated with the degree of change observed in cognitive abilities over the follow-up.

The study had a good sample size, and assessed a range of cognitive abilities. It’s important to make clear that the study did not measure fish intake, rather the researchers directly assessed omega-3 levels in blood samples, which gives a good indication of “intake over the 1 to 2 months before measurement”. Oily fish (and nuts) are good sources of omega-3s so when reported in the media, these foodstuffs will generally be mentioned.

What did the media say?

The study was reported across number of media outlets, for example under the headline of “The great superfood U-turn” in the Daily Mail. The coverage in the Daily Mail actually does a good job of briefly summarising the findings of the study and providing direct quotes from the research team. It also makes clear that there are other health benefits of having fish in your diet. The headline, however, might lead readers to question what to do regarding eating fish, now it’s no longer beneficial to the brain. Evidence of fish actually being good for the brain is less consistent than might be suggested by the links at the top of this post. Ammann and his colleagues noted this in their study: some studies find a benefit of omega-3s for cognitive abilities and others don’t, but in studies where people are randomly assigned to getting omega-3s or not, the evidence of a benefit is generally not there.

The bottom line.

It’s important to remember that our knowledge of what is good or bad for you comes from a whole body of research over a number of years rather than one study giving the final answer. With the current study, it is important to see how it fits within the current literature. In general, the current study is consistent with the lack of strong evidence for the cognitive benefits of omega-3s. In fact, a good summary of this evidence comes from the Cochrane Collaboration, a group that specifically searches for and summarises groups of studies linking a given factor with a health outcome. Their summary of omega-3 trials doesn’t suggest any protection against cognitive decline.

As with all reports in this area, just because omega-3s don’t appear to have benefits for the ageing brain, it says nothing of the other health benefits linked to eating a balanced diet including fish. The Cochrane Collaboration summary therefore suggests: “Fish is an important part of a healthy diet and we would still support the recommendation to eat two portions a week, including one portion of oily fish”.

Ammann, E. M., Pottala, J. V., Harris, W. S., Espeland, M. A., Wallace, R., Denburg, N. L., Carnahan, R. M., & Robinson, J. G. (2013). Omega-3 fatty acids and domain-specific cognitive aging. Neurology, 81, 1484-1491. doi: 10.1212/WNL.0b013e3182a9584c

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