Wednesday June 13 2012
Current evidence does not prove that fish oil prevents dementia
“Fish oils don’t help ward off dementia,” BBC News has reported. The broadcaster said that new research may show taking fish-oil supplements to ward off dementia “could be a waste of time”.
The news is based on an evidence review pulling together high-quality scientific studies on omega-3 fish supplements, which are used by thousands of people in a bid to prevent mental decline. Researchers were able to draw on the results of three large, high-quality studies on cognitive decline (a symptom often seen during dementia) but none on the development of dementia itself. Together, the studies featured over 3,500 participants aged 60 and over, who were assigned either an omega-3 supplement (in the form of a pill or margarine spread) or a dummy “placebo” pill. Researchers used various types of tests to assess their cognitive performance and then tracked them for a period of 6, 12 and 40 months.
The results showed that those taking omega-3 fish oil supplements scored no better in memory and mental performance tests at the end of the trial than those given dummy pills. However, the review also called for further studies of longer duration, which could better detect changes in cognitive function over a longer period of time.
Despite these results, the authors stress that omega 3 may have other benefits and that fish is an important part of a healthy diet. This study does not change NHS recommendations to eat two portions of fish a week, including one portion of oily fish such as mackerel, salmon or sardines.
Where did the story come from?
The study was carried out by researchers from the London School of Hygiene and Tropical Medicine and Tan Tock Seng Hospital in Singapore. No sources of support were reported as the study article reported that all the authors “contributed to this review in their own time”. It was published in the Cochrane Database of Systematic Reviews.
Dementia is a progressive illness that mainly affects older people. Symptoms include loss of memory, language problems, difficulties in performing activities of daily living and psychological changes. Evidence from observational studies suggested that people may receive some protection from cognitive decline and dementia by consuming greater levels of fish oils containing a substance called “omega 3”. Omega 3 is a type of long-chain polyunsaturated fat which is reported to be essential for brain development. Many oily fish, such as salmon, mackerel, herring and sardines, are rich sources of omega 3.
The media’s reporting of this research generally focused on dementia, reporting for example that fish oils “offer no protection against dementia” (The Daily Telegraph). The reviewers conducting this research explicitly said that they found no suitable trials directly addressing whether omega 3 prevented dementia, and so such claims about dementia are not supported by the review. However, the review did find there was no evidence of omega 3 preventing cognitive decline (a symptom of dementia), so it is easy to see how the media somewhat blurred the boundaries of these closely related findings.
What kind of research was this?
Fish oil’s potential protective effect has been tested in recent randomised controlled trials, and this review aimed to pool the evidence from these trials into a single, larger analysis. It set out to assess the effects of omega-3 fatty acid supplementation on the prevention of dementia and cognitive decline in cognitively healthy older people.
A systematic review of high-quality randomised controlled studies is the most appropriate study design to address this issue. Systematic reviews aim to identify and summarise the findings of all high-quality research carried out on the particular topic of interest, regardless of whether or not they support a particular theory. Cochrane systematic reviews are particularly well known for their high-quality methods and clear reporting style, and so the conclusions of such reviews are generally considered as one of the highest level of evidence available on a topic.
What did the research involve?
Researchers searched electronic databases for all high-quality research studies relevant to the research question, which was whether omega-3 supplements can prevent dementia and cognitive decline in healthy older people.
The review included only randomised controlled trials – the gold standard of tests – of omega-3 fatty acid supplements. To be eligible for inclusion, studies had to give supplements for a minimum of six months to participants aged 60 and over who were free from dementia or cognitive impairment at the beginning of the study.
The review authors then extracted data on new cases of dementia, cognitive function, safety and adherence to the course of supplements. They did so from the published studies or by contacting the authors of the studies directly for additional information.
Three trials were deemed suitable and included in the review. None of the studies examined the effect of omega 3 on new cases of dementia, instead looking at cognitive decline. The authors reported that all three studies included in the review were of high methodological quality. The studies involved a total of 3,536 participants and the durations of the interventions in the three trials were 6, 24 and 40 months.
In two of the studies, participants were given gel capsule supplements containing omega 3 (the “intervention” treatment) or capsules containing olive or sunflower oil (the “placebo” treatment) for six and 24 months. In the third study, participants received either omega 3 containing margarine spread (intervention) or margarine spread without omega 3 (placebo) for a period of 40 months.
Cognitive function was assessed using a variety of standard methods, including word learning, verbal fluency tests and a widely used assessment method called the mini-mental state examination (MMSE). The analysis of the results was appropriate.
What were the basic results?
Cognitive-function data for the full length of the trials was available for 3,536 participants (ranging from six to 40 months).
In two of the studies (involving a total of 3,221 participants) there was no significant difference between the omega 3 and placebo group in mini-mental state examination score at final follow-up (24 or 40 months of intervention); the MMSE scores participants’ cognitive performance on a scale of 0-30, and the two treatments produced a difference of 0.07 points.
Two of the studies, involving 1,043 participants, featured other tests of cognitive function such as word learning, number identification and verbal fluency. Their results showed no beneficial effect of omega-3 supplementation. Participants in both the intervention and control groups experienced either small or no cognitive declines during the studies.
The main reported side effect of omega-3 supplementation was mild gastrointestinal problems. Overall, minor adverse events were reported by fewer than 15% of participants, and reports were balanced between intervention groups. Adherence to the intervention was on average over 90% among people who completed the trials.
How did the researchers interpret the results?
The researchers concluded that “direct evidence on the effect of omega 3 on incident [new cases] dementia is lacking”. From the available evidence they concluded there “is no benefit to cognitive function from omega-3 supplementation in cognitively healthy people over 60”.
The researchers added, however, that studies of longer duration are required to determine whether omega-3 supplementation delays cognitive decline in older people.
This review, summarising the findings of three high-quality randomised controlled trials involving 3,536 participants, found that there was no benefit to cognitive function from omega-3 supplementation in cognitively healthy people over 60 when assessed over a period of between six and 40 months.
This was a high-quality systematic review that included three high-quality RCTs, and so the conclusions are likely to be reliable and an accurate reflection of current research. However, the following limitations should be considered:
- There was no evidence from randomised control trials on whether omega-3 supplementation affected new cases of dementia, only cognitive decline. Hence, this potential effect on dementia remains unknown, and media reports saying that omega 3 does not prevent dementia have misrepresented the conclusions of this study.
- The included RCTs had a maximum follow-up period of 40 months, and during the follow-up participants in both the intervention and placebo groups experienced little or no cognitive decline. This may suggest that the follow-up period was too short to detect any significant cognitive decline and therefore any potential protective effect of omega 3. As the authors of the review highlighted, studies with a longer follow-up period are needed to assess whether omega-3 supplementation yields any benefits in cognitive functioning after this period.
- The review focused on omega-3 supplementation for the prevention of dementia and cognitive decline, and as such does not provide any further insight into the effect of omega-3 on prevention of physical illnesses or disease.
- The effect of taking omega-3 supplements for more than 40 months (the maximum time assessed in the three RCTs) is also not addressed by this review as there was a lack of evidence testing this effect. Furthermore, the research looked at people aged 60 and above, and therefore does not tell us about taking fish oil supplements earlier in life.
Despite their results, the authors stress that omega 3 may have other benefits and that fish is an important part of a healthy diet. This study does not change NHS recommendations to eat two portions of fish a week, including one portion of oily fish such as mackerel, salmon or sardines.
Analysis by Bazian