Wednesday November 30 2011
Eating fish could protect against Alzheimer's, study says
“Fish could protect against Alzheimer’s,” reported The Daily Telegraph. Several newspapers said that people who eat baked or grilled fish once a week are up to five times less likely to develop Alzheimer’s disease.
The story is based on an abstract of a study that examined the association between the amount of fish eaten and the size of certain brain structures 10 years later. Researchers also looked at whether the size of these structures was associated with a risk of loss of brain function (cognitive decline) over five years.
While the media widely reported that the research looked at Alzheimer’s disease, the abstract did not report findings on Alzheimer’s specifically, only on cognitive decline.
The research suggested that eating baked or grilled fish at least once a week is good for brain function and is associated with a reduced risk of cognitive decline, which is often a precursor to Alzheimer’s disease.
Only limited detail on the methods and results of this study is currently available. Until more information is published, it is not possible to say whether this study suggests that eating fish has a meaningful impact on cognitive decline or the risk of developing Alzheimer’s disease.
Where did the story come from?
The study was carried out by researchers from the University of Pittsburgh. No information on funding was provided in the abstract. The research is being presented at the 2011 meeting of the Radiological Society of North America on November 30.
Studies presented as conference abstracts have not yet undergone the full peer-review process required for publication in a peer-reviewed journal. The results are often preliminary, and may differ when all the data have been collected and analysed once the study has been completed. Therefore, the results should be interpreted cautiously.
The media reported this research appropriately, given the limited information available. The Daily Telegraph outlined some of the limitations of the study and reported that previous research did not show any association between the fatty acids found in oily fish and future risk of dementia.
The media widely reported that this research looked at Alzheimer’s disease. However, the conference abstract did not report any findings on Alzheimer’s specifically, only on cognitive decline. Further results for Alzheimer’s from this study may be described at the conference.
What kind of research was this?
This cohort study examined the link between fish consumption, brain structure and cognitive decline later in life. Only limited information on the methods and results of this study is available from the conference abstract and related press release.
The conference abstract focussed mainly on the methods and results of part of the study, which looked at the association between fish consumption and the volume of grey matter in the brain.
Grey matter is the part of the brain that includes the main bodies of the nerve cells, and a reduction in grey matter volume suggests that brain cells are shrinking. This measurement was chosen as it is thought to be related to the maintenance of brain health and cognitive performance. The research focused on areas of the brain that are responsible for memory and learning, and are particularly affected by Alzheimer’s disease.
A cohort study is an appropriate design for examining the association between two factors. The researchers collected information on diet and fish consumption prospectively, at the beginning of the study. This helps ensure that any brain changes seen occurred after the recorded fish consumption, rather than before.
What did the research involve?
The researchers recruited 260 mentally healthy individuals from a long-running cohort study. At the start of the study, each participant completed a questionnaire to determine how much fish they ate each week, as well as the cooking methods used to prepare the fish. A brain-imaging technique was used at this time to record the size of key brain structures.
Ten years later, the participants underwent another brain scan, and grey matter volume was measured. The researchers then determined the association between fish consumption and the size of brain structures. Their analysis took into account several factors that may have affected the results, including age, gender, race, education, obesity (measured as waist-to-hip ratio), and physical activity levels.
Once the effect of fish consumption on the size of certain brain structures was determined, the researchers used a statistical model to assess the risk of cognitive decline over five years. This model included several factors including age, gender, education, race and genetic risk factors for Alzheimer’s disease. Little information was provided in the abstract on this aspect of the study. It was unclear how the participants were assessed for cognitive decline, or whether they were assessed for Alzheimer’s.
What were the basic results?
The press release for the study stated that 163 participants consumed fish on a weekly basis, with most of them eating fish one to four times a week.
Eating baked or grilled fish at least once a week was associated with larger grey matter volume 10 years later in certain brain structures that are important in memory and learning. These structures included the hippocampus, precuneus, posterior cingulate and orbital frontal cortex.
The researchers found that larger volumes of the hippocampus, orbital frontal cortex and posterior cingulate were associated with a five-fold reduced risk of cognitive decline. The researchers found no association between the consumption of fried fish and grey matter volume or cognitive decline.
How did the researchers interpret the results?
The researchers concluded that “this work suggests that dietary habits may reduce risk for cognitive decline and dementia by exerting beneficial effects on brain structure.”
This research has examined the association between eating fish and brain structures and the subsequent risk of cognitive decline. It is not possible as yet to fully appraise this study and draw firm conclusions as so far it has only been presented at a conference and not in a peer-reviewed publication. This means that little information has been presented on the methods and results of the study.
Before drawing conclusions on the link between consumption of fish and risk of developing Alzheimer’s disease, it is important to note that:
- The conference abstract focused on cognitive decline as the outcome of interest. It is unclear from the abstract how this was measured, and whether or not the researchers specifically examined the development of Alzheimer’s.
- Information on consumption of fish was collected at the beginning of the study, but not during the 10-year follow-up. Eating habits could have changed during that time. For example, participants who were originally classified as not eating fish could have increased their consumption during the follow-up. Likewise, those who were classified as eating fish could have decreased their consumption. This potential for misclassification could affect the results of the study.
- The conference abstract described the change in risk as “five-fold” but did not give any indication of what proportion of people had cognitive decline. If the risk of cognitive decline in non-fish eaters was very low, then a five-fold difference may not be very large in real terms.
- While the researchers took into account some factors, other than fish consumption, that could have affected the results, it is still possible that other factors may account for the relationship seen.
Until further information on this study is published, it is not possible to say whether the study suggests that eating fish has a meaningful impact on cognitive decline or the risk of developing Alzheimer’s disease.