Tuesday April 13 2010
“Eating lots of nuts, fish and poultry while cutting down on red meat and butter could reduce the risk of Alzheimer’s disease,” reported The Times. These findings come from a study of over 2,000 elderly people in New York.
The study looked at the dietary patterns of people aged 65 and over and followed them for an average of four years. It found a lower risk of developing Alzheimer’s in people with a diet that included more salad dressing, nuts, fish, tomatoes, poultry, cruciferous vegetables, fruits and dark and green leafy vegetables, and lower intakes of high-fat dairy products, red meat, offal and butter.
However, this study has some limitations, and these mean it is not possible to say for certain that this dietary pattern itself reduces the risk of Alzheimer’s. Ideally, further studies are needed to confirm these results. Those who wish to try this diet may be encouraged by the fact that it has many of the desired characteristics of a healthy, balanced diet anyway.
Where did the story come from?
The research was carried out by Dr Yian Gu and colleagues from the Taub Institute for Research in Alzheimer’s Disease and the Aging Brain and other research centres in New York. The study was funded by the US National Institute on Aging. It was published in the peer-reviewed medical journal Archives of Neurology.
The Times and BBC News gave accurate and balanced coverage of this story. For example, as the BBC noted, experts have stressed that diet “is not the sole cause or solution where dementia is concerned”.
What kind of research was this?
This prospective cohort study looked at how diet may affect the risk of Alzheimer’s disease in elderly people. The researchers were interested in studying overall patterns of food consumption rather than individual foodstuffs or nutrients. They were chiefly interested in the participants’ overall consumption of several nutrients, which previous research has suggested may affect Alzheimer’s risk. These include: saturated fatty acids (SFA), monounsaturated fatty acids, omega-3 polyunsaturated fatty acids, omega-6 polyunsaturated fatty acids, vitamin E, vitamin B12 and folate. The researchers said this previous research had suggested that a greater intake of SFA or total fats could adversely affect cognitive functions, while increased intake of polyunsaturated and monounsaturated fatty acids, vitamin B12, folate and vitamin E may be related to better cognitive function.
This type of observational study is often the best way to examine how lifestyle choices, such as diet affect health outcomes. This is because it is not usually feasible to assign people randomly to different lifestyles to compare their effects. However, because the groups compared in this study were not randomly selected, they may have differed in ways other than dietary pattern. This ‘confounding’ can affect results. For this reason, this type of study needs to consider any potential confounders.
The collection of data prospectively is one of the strengths of this study, as data collected retrospectively may not be as accurate.
What did the research involve?
The researchers analysed 2,148 elderly people, aged 65 and over, who did not have dementia and who were living within the community in New York. These people were asked about their diet and were classified according to their dietary patterns. They were followed up for an average of 3.9 years to see which of them developed Alzheimer’s disease. The risk of developing Alzheimer’s was then compared between the groups with differing dietary patterns.
These participants were obtained from two previous cohort studies carried out in 1992 and 1999 in New York. In total, 4,166 dementia-free volunteers were enrolled in these studies. However, nearly half of these could not be included in this study because they were missing data on their diets, they died before their first assessment after the start of the study, were otherwise lost to follow-up or developed a non-Alzheimer’s dementia during follow-up.
The volunteers had their medical and neurological history taken, and completed an interview and neurological tests at enrolment. Individuals with dementia were not included in the study. Those who were included had similar assessments every 1.5 years to determine whether they had developed dementia. A consensus diagnosis was made by a panel of experts, including neurologists and neuropsychologists, based on the assessments. The type of dementia was determined based on standard criteria.
Diet was assessed using a food frequency questionnaire that assessed dietary intake over the past year of 61 food items belonging to 30 food groups (such as fruits, legumes and poultry). Nutrient intake was calculated according to the responses to the questionnaire and nutrient contents of a standard portion of the different foods. The researchers analysed the participants’ consumption of the 30 food groups and the seven nutrients of interest to identify dietary patterns, taking into account their overall energy intake. Seven dietary patterns were identified and each individual had a score that indicated how much their diet matched that particular pattern.
The researchers analysed whether any of these seven dietary patterns had an impact on the risk of developing Alzheimer’s disease. They took into account factors that could affect results, including when the person was recruited for the study, age, gender, ethnicity, education, smoking habits, body mass index, overall calorie intake, other medical conditions and which variants of the APOE gene they carried. Further analyses took into account alcohol consumption.
What were the basic results?
During follow-up, 253 of the 2,148 participants (11.8%) developed Alzheimer’s disease.
Of the seven identified dietary patterns, one showed a link with Alzheimer’s risk. This pattern consisted of higher intakes of salad dressing, nuts, fish, tomatoes, poultry, cruciferous vegetables, fruits and dark and green leafy vegetables, and a lower intake of high-fat dairy products, red meat, offal and butter. This represented a diet rich in omega-3 polyunsaturated fatty acids, omega-6 polyunsaturated fatty acids, vitamin E and folate and with lower levels of SFA and vitamin B12. People who showed the greatest adherence to this dietary pattern were 38% less likely to develop Alzheimer’s disease than those who had the least adherence to this pattern (relative risk 0.62, 95% confidence interval 0.43 to 0.89).
Some participants (1,224 people) had their food intakes assessed more than once in the study, with the two assessments occurring an average of five to six years apart. The researchers found that the level of adherence to the dietary pattern described above did not change over time in people who developed dementia (120 people) or those who did not (1,104 people).
How did the researchers interpret the results?
The researchers concluded they have identified a dietary pattern that is “strongly protective against the development of [Alzheimer’s disease]”. They say that their findings support further exploration of dietary patterns with an aim of identifying other food combinations associated with Alzheimer’s disease risk.
This study suggests that a diet higher in salad dressing, nuts, fish, tomatoes, poultry, cruciferous vegetables, fruits, and dark and green leafy vegetables, and with lower intakes of high-fat dairy products, red meat, offal and butter may be associated with a reduced risk of Alzheimer’s. The regular prospective assessment of the study participants for dementia was a strength of this study, but the study also has limitations:
- Diet was assessed by a food frequency questionnaire. Although this is an approved way of assessing diet, some individuals may not have been able to recall accurately what they ate over the past year. The analysis of diet in those who were assessed more than once suggested that adherence to this diet (or recall of dietary pattern) remained stable over the follow-up period. However, the assessment at the start of the study may not have been representative of the participants’ diets earlier in their lives.
- About half of the eligible individuals had to be excluded due to missing information. This could potentially have affected the results, particularly if those who were not included differed significantly from those who were.
- As with all studies of this type, the results may have been affected by factors other than the one of interest. Although the researchers took into account a number of these potential confounders, these adjustments may not have completely removed the effects and there may be other unknown or unmeasured confounders. The authors themselves note that they could not rule out the possibility of residual confounding.
- As dementia develops gradually over time, some of these individuals may already have been in the early stages of dementia at the start of the study. This would mean that the assessment of dietary pattern in these individuals had not preceded the onset of dementia, and therefore their diet could not be affecting their dementia risk. Though this possibility appears less likely due to the fact that the results were not affected if the participants’ cognitive function at enrolment was taken into account, it cannot be ruled out completely.
The limitations to this study mean that it is not possible to say for certain that this dietary pattern reduces the risk of Alzheimer’s. Ideally, further studies would be needed to confirm these results. This dietary pattern has characteristics of a healthy balanced diet, including higher intake of vegetables and lower intake of high-fat products and red meat.