Diet and dementia

Eating foods rich in omega-3 oils could “reduce the risk of suffering Alzheimer’s and other forms of dementia” reported The Guardian . Omega-3 oils can be found in “rapeseed, flaxseed and walnut oil”, the newspaper said. The Daily Telegraph said that “eating oily fish once a week can cut the risk of developing Alzheimer’s by a third”. The newspapers also report that eating fruit and vegetables every day can reduce risk of developing dementia by about 30%.

The story is based on a study that showed that a diet high in fruit, vegetables and oily fish appeared to reduce the risk of dementia. However, the design of the study means that definite conclusions about the effect of diet on the risk of dementia cannot be drawn. This is not the first time that fish oils have been linked to improvements in brain function. In particular, the finding about omega-3 oils, which has been picked up by the newspapers, was not statistically significant, and therefore the confidence that it shows a true effect is limited.

Where did the story come from?

Dr Pascale Barberger-Gateau and colleagues from the Three City Study Group carried out this research in France. The study was funded by the Fondation pour la Recherche Medicale, Caisse Nationale Maladie des Travailleurs Salaries, Direction Generale de la Sante, Mutuelle Generale de l’Education Nationale, Institut de la Longevite, Regional Councils of Aquitaine and Bourgogne, Fondation de France, the Ministry of Research-INSERM Programme, and the French National Agency for Research. It was published in the peer-reviewed medical journal: Neurology .

What kind of scientific study was this?

This was a prospective cohort study called the Three City study. The researchers recruited 8,085 adults aged 65 and over who did not have dementia, in three regions in France. At enrolment, they gave participants brief questionnaires about the types of food they ate, and how often they ate these foods. The food types assessed included fish, fruits and vegetables, and fats used in cooking.

Then researchers then followed up these participants over four years to see if they developed dementia. To test for dementia, the researchers gave the participants a set of neuropsychological tests, and those who were suspected of having dementia went on to be examined by a neurologist. A group of independent neurologists reviewed all the available information before deciding if a person had dementia, or probable or possible Alzheimer’s disease, according to standard criteria.

The researchers compared the risk of developing dementia between people who ate different amounts of the food types assessed. Researchers also took into account other factors that might affect the risk of developing dementia, such as age, marital status, educational level, smoking, high blood pressure, diabetes, high cholesterol, and body mass index. Researchers also considered whether participants carried a variant of the ApoE gene, which is known to make people more susceptible to developing Alzheimer’s disease.

What were the results of the study?

Over the course of the study, 281 people (3.5%) developed dementia. The researchers found that eating fruit and vegetables every day reduced risk of dementia by about 28%. They found that eating fish once a week reduced risk of Alzheimer’s disease by about 35%, and any dementia in general by 40% among those who did not have a specific genetic predisposition to developing Alzheimer’s disease (those without the ApoE gene).

Although there was a reduction in the risk of dementia with regular use of omega-3 rich oils (such as walnut or soya oil), this reduction was not large enough to be statistically significant. People who were not genetically predisposed to Alzheimer’s, who ate high levels of omega-6 rich oils (such as sunflower and rapeseed oils) but not omega-3 rich oils or fish doubled their risk of dementia. There was no association between the consumption of saturated fat, such as butter, goose or duck fat, and the risk of dementia.

What interpretations did the researchers draw from these results?

Researchers concluded that regular consumption of fish, omega-3 fish oils, and fruit and vegetables “may decrease the risk of dementia and Alzheimer disease”, especially among people who do not have a genetic predisposition to Alzheimer’s disease.

What does the NHS Knowledge Service make of this study?

This was a relatively large and well-conducted cohort study, however, there are some limitations to bear in mind when it comes to interpretation:

• As with all cohort studies, there may be confounding factors, which are responsible for the results seen, rather than the specific factors investigated. Authors of this study have taken into account some of the potential confounding factors, but it is impossible to eliminate them all.
• Participants were asked about their food consumption when they enrolled. Their answers may not have been representative of their lifetime consumption of these foods, or of their consumption during the follow-up period.
• Even though the overall numbers of people in this study was high, the numbers of people in some of the groups compared would have been relatively low.
• This study conducted multiple analyses, and this increases the possibility of finding a statistically significant result by chance. Some of the results, for example, the association between fish consumption or omega-6 consumption and dementia, were only significant when results were analysed in a particular way, with specific adjustments to the results and only in people without a genetic predisposition to Alzheimer’s. This reduces the confidence that these results are robust.
• It is worth bearing in mind that the absolute risk of developing dementia in this study was relatively low.
• Diagnoses of Alzheimer’s disease can only be confirmed after death on autopsy. No one in this study had died and been autopsied, therefore the results may be affected if people were misdiagnosed.

The message that people should eat more fish, fruit and vegetables is one that has been promoted strongly in recent years, because it is associated with a number of benefits. Although this study may not conclusively prove that doing so will reduce your chances of developing dementia, there are still many reasons to choose this type of diet.  

Sir Muir Gray adds...

Alzheimer’s is one of the diseases I fear most, but I won’t change my diet based on this study.