Doubt cast on the benefits of omega-3 for the brain
"The great superfood U-turn," claims the Mail Online today, suggesting that scientists have shown that "feasting on salmon and nuts may not preserve brainpower after all".
The news is based on a study of more than 2,000 older women. The researchers looked at the relationship between their blood levels of two omega-3 fatty acids and their performance in tests of thinking and memory skills. These tests were repeated every year for several years.
The study found no difference in cognitive skills at the start of the study between women with high and low levels of these fats in their blood, and no difference between the two groups in how quickly their thinking skills changed over time.
There are quite a few problems with this study, not least, that it measured blood levels of omega-3 fats only once, at the start of the study. It is possible that blood levels changed over the years if women changed their diets or started or stopped taking fish oil supplements.
There is little hard evidence that foods high in omega-3 fatty acids boost cognitive function or protect against conditions such as dementia. The best evidence about the health benefits of omega-3 fats suggests that they protect the heart rather than the brain. Some research shows that eating oily fish, which is a rich source of omega-3 fats, may help prevent heart disease.
Where did the story come from?
The study was carried out by researchers from a number of US institutions including the University of Iowa, the University of South Dakota and Wake Forest School of Medicine, in the US. It was partly funded by the US National Heart, Lung and Blood Institute.
The study was published in the peer-reviewed medical journal Neurology.
The Mail’s claim that the results are a “U-turn” is misleading as it implies that there was a previous consensus of opinion. While some observational studies have suggested that omega-3 fats may help halt age-related cognitive decline, this has never been proven.
What kind of research was this?
This was a retrospective cohort study of more than 2,000 older women. It tested whether higher blood levels of two omega-3 fatty acids were associated with a protective effect on memory and thinking skills. The two fatty acids under study were docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), both of which are found in oily fish. Other omega-3 fatty acids, such as alpha linoleic acid, are also found in many nuts.
This type of study enables researchers to follow large groups of people for many years, and to look at any associations between lifestyle and health. But this type of study is not able to prove that high blood levels of omega-3 fats can protect against a decline in thinking or memory skills. A randomised controlled trial would be needed for this.
Also, the study was retrospective (a secondary analysis of another study), which means the results should be viewed with some caution. Recall bias or inaccurate reporting of symptoms could have affected the results.
The researchers point out that previous studies have suggested that increasing omega-3 dietary intake may prevent or delay age-related mental decline.
What did the research involve?
The study involved more than 2,000 women aged 65 to 80 who had been involved in part of a large randomised clinical trial of hormone therapy, called the Women’s Health Initiative Study of Cognitive Aging.
Researchers used data collected during the study, which started in 1999 and was designed to look at the possible effect of hormone therapy on cognition. This original study showed that hormone therapy had a negative effect on mental function.
Researchers took blood samples from 2,208 women at the start of the original study and these were isolated, frozen and stored. Researchers measured levels of both DHA and EPA in the women’s red blood cells (RBC). They divided the women into three groups (or “tertiles”), depending on their blood levels of DHA and EPA.
The women were given annual tests of memory and thinking skills at the start of the study and annually. The tests looked at performance in seven “cognitive domains”.
These were:
- fine motor speed – the ability to co-ordinate “body and mind”, for example, by catching a ball
- spatial ability – the ability to recognise a 2D or 3D environment and interact with it
- visual memory
- verbal memory
- verbal knowledge – the ability to recognise spoken information and respond
- verbal fluency
- working memory – how much information the mind can hold and access at any one time
They also collected a range of other information from participants about their health, lifestyle, ethnicity, income, diet and exercise.
For this study researchers assessed the relationship between blood levels of DHA and EPA and:
- the results of their cognitive tests at baseline (the start of the study)
- the rate of changes in cognitive ability over time
The researchers adjusted their findings for other factors (confounders) that might affect the results, such as health and lifestyle.
What were the basic results?
The women were followed up on average for 5.9 years. The researchers found:
- no significant cognitive differences between women in the high and low DHA and EPA tertiles at the time of the first annual cognitive test
- no significant differences between the high and low DHA and EPA tertiles in the rate of cognitive change over time
How did the researchers interpret the results?
The researchers say the findings are consistent with previous controlled trials showing that omega-3 supplements did not slow down cognitive ageing. However, some previous observational studies have suggested they might do so.
Conclusion
This study investigated the possible benefits of a diet high in omega-3 fatty acids on the brain. It has several limitations:
- One problem is that it only measured blood levels of omega-3 fatty acids at the start of the study. These might have changed over time if the women changed their diets or started – or stopped – taking omega-3 supplements. Also, the first cognitive tests were administered on average three years after the blood samples were taken.
- It’s important to note that the researchers did not measure women’s dietary intake of omega-3 fats, only blood levels, although the authors say these have been found to correlate with dietary habits.
- Finally, the fact that this was retrospective, secondary analysis of previous research means the results should be viewed with caution.
The research on omega-3 fatty acids remains inconclusive, as is the case with many so-called superfoods.
While it remains unclear whether a diet rich in omega-3 fats may help protect against dementia or related conditions, some research has found omega-3 fats may help protect against heart disease. A healthy diet should include at least two portions of fish a week, including one of oily fish.
It is interesting to note that this study is a relatively rare example of a study published in a high profile journal that has produced a negative finding. This should help combat the problem of publication bias.
