Wednesday August 6 2008
Salmon is a good source
‘Three oily fish meals a week can cut memory loss by 25%’, reported the Daily Mail. It said researchers found that eating oily fish (baked or steamed, not fried) can reduce the harmful brain lesions that can cause Alzheimer’s disease. The research involved looking at brain scans of over 2,000 people, and seeing how changes in the brain were associated with eating oily fish in the diet.
Although this is a study of a large group of people, it has several limitations, including how fish consumption was assessed, and by its design cannot provide conclusive evidence that eating oily fish prevents brain changes. In addition, the links found between risk of areas of infarct in the brain (areas starved of oxygen) and fish consumption were not statistically significant. This study did not assess how the changes seen on brain imaging relate to any memory change or to cognitive brain function in the person. Although Omega-3 or ‘essential fatty acids’ as found in oily fish are known to be an important part of a healthy balanced diet, this study does not bear out the claim that they protect memory or brain function.
Where did the story come from?
Dr Jyrki Virtanen and colleagues from the University of Kuopio, Finland, carried out the research. Funding was provided by the National Heart, Lung and Blood Institute, the National Institute of Neurological Disorders and Stroke, the Finnish Cultural Foundation, and several other Finnish foundations. The study was published in the peer-reviewed medical journal: Neurology.
What kind of scientific study was this?
The aim of this cohort study was to investigate the association between fish consumption and brain abnormalities. The researchers used participants who were already involved in the ongoing Cardiovascular Health Study (CHS), a prospective cohort study of 5,888 adults in the United States. All participants were aged 65 or older when they enrolled between 1989 and 1990.
At the beginning of the study, the participants had all undergone extensive clinical assessments and completed questionnaires, with diagnoses of coronary heart disease, stroke, high blood pressure, or diabetes being noted. Their diets were assessed using a pictorial version of the food frequency questionnaire, which asked how frequently they had consumed certain foods during the past year. At their first assessment when they enrolled, they were asked how much tuna fish, ‘other broiled or baked fish’ or ‘fried fish or fish sandwiches’ they ate. When diets were assessed again in 1995-1996, they were asked how much canned tuna fish, dark-meat fish (mackerel, salmon, sardines, bluefish, swordfish) or other white fish they consumed. At this assessment, they were not asked about fried fish. The researchers estimated the participants’ nutrient intake and omega 3 fatty acid intake from the questionnaire responses.
The CHS participants were invited to have MRI brain scans between 1991 and 1994. A total of 3,660 (62%) agreed. Those who agreed tended to be slightly younger and healthier than those who did not. All the participants were again invited to have a scan five years later, at which point 2,313 were scanned. There was a total of 2,116 participants who received both scans (36% of total cohort) and these people were reported to be healthier than those who only received the first scan, with lower prevalence of chronic diseases and smoking. When the scans were analysed, attention was given to areas of brain infarct (areas that have been starved of oxygen). People who have had a stroke have these, but in this study the infarcts were termed “subclinical”, as they were not associated with any known clinical effects in the person. Other structures in the brain were also examined, including the ventricles (brain cavities continuous with the spinal cord), brain sulci (brain folds) and white matter (nerve fibres). These latter three structures were given a grade (details of the grading system not provided in the report).
The researchers carried out cross-sectional statistical analyses to see how dietary intake affected the risk of brain infarcts or ventricular, sulcal or white matter grades seen on brain imaging. This considered the timing of the food questionnaires to roughly correspond to the timing of the MRI scans. After confirming that results were similar, they then compared diet intake at the first questionnaire to the second brain scan. They excluded from their evaluations those people who had a history of stroke or mini-stroke (TIA), those with prior brain haemorrhage and those with incomplete information on fish consumption. Analyses were adjusted for other potential medical and lifestyle confounders.
What were the results of the study?
After exclusions, 2,465 subjects were left at the first scan, 1,663 left at the second scan, and 1,124 were left with both scans available for analysis. Of the participants who had the first scan, 23% had evident subclinical infarcts. The researchers also found that 23% of the participants who had the second scan had infarcts.
After taking into account various confounding factors, there were no significant associations between fish consumption of any type or frequency and risk of subclinical infarcts on brain scan. The 26% reduction in risk reported by the study from eating ‘tuna or other fish’ three times per week (compared to eating it less than once a month), was not significant (95% CI 0.54 to 1.01). There was no link between ventricular and sulcal grade and fish consumption, but there was correlation seen between lower white matter grade and higher tuna and other fish consumption.
The researchers also found that other social and lifestyle factors, such as sex, education and fruit and vegetable intake, was associated with type of fish intake (i.e. frequency of tuna or other fish consumption, and frequency of fried fish consumption).
What interpretations did the researchers draw from these results?
The researchers concluded that modest consumption of tuna and other fish, but not fried fish, is linked with a lower prevalence of subclinical infarcts and white matter abnormalities on brain imaging.
What does the NHS Knowledge Service make of this study?
This is a study of a very large cohort of people; however, it has been over interpreted by the newspapers and does not demonstrate that oily fish, or any other type of fish, protects against memory loss, risk of Alzheimer’s dementia, or risk of any other type of dementia. This is borne out by the following points:
- None of the links between risk of subclinical infarct and fish consumption of any type were statistically significant.
- The presence of ‘subclinical infarcts’ does not necessarily relate to any change in memory or cognitive function in the person, and these were not tested by the study.
- Subclinical infarcts are also not a feature of Alzheimer’s disease (a condition characterised by having unknown cause). The brain ventricles are known to become enlarged in people with Alzheimer’s, but there was no link seen between ventricular grade and oily fish in this study. Other changes known to be associated with Alzheimer’s, such as neurofibrillary tangles and brain plaques, were not examined.
- Fish consumption was assessed by a person’s recall of how much fish they had eaten over the past year. There are several limitations to this. Although this was assessed at two separate time periods, it cannot be assumed that consumption remained the same. There is also likely to be some errors in the participants estimations of their normal consumption, and portion sizes are subjective and the method of assessing this is not specifically reported in this study. Finally, although examples are given of the fish groupings asked about, the method of grouping used in the analyses of ‘tuna and other fish’ or ‘fried fish’ are extremely broad and cannot be assumed to relate to oily fish or any other particular type of fish without further information.
- There are likely to be a large number of confounders that will affect brain change, and although the researchers have considered many, there may be others.
- Only a small proportion of the total participants received both scans (36%), and the researchers reported that these people were younger and healthier than those receiving only first scans or not scanned at all. The results may have been different again if all the participants could have been scanned.
- There may be some difference in detection of infarcts and grading of ventricular, sulcal and white matter abnormalities, between different observers.
Omega-3 or ‘essential fatty acids’, such as those found in oily fish, are known to be an important part of a healthy balanced diet. However, this particular study does not prove that they protect memory or brain function.
Sir Muir Gray adds...
Impressive evidence, but still not strong enough to persuade me to eat oily fish three times a week.