- a standard Mediterranean-type diet
- the Dietary Approach to Stop Hypertension diet (DASH) – designed to reduce blood pressure
- Mediterranean-DASH Intervention for Neurodegenerative Delay (MIND) – this combines elements of the Mediterranean diet and the DASH diet
The researchers say they found the greatest effect from the MIND diet, which is rich in green leafy vegetables, wholegrains, nuts and berries, even if people didn't follow it closely. Participants who did stick rigorously to the MIND diet were 52% less likely to be diagnosed with Alzheimer's disease.
This large observational study can't show that the diets protected against Alzheimer's, only that there seems to be a link between eating a healthy diet and a lower risk of getting Alzheimer's disease. The three diets weren't compared directly, so we can't be sure which one is best.
The study provides further evidence that eating a healthy diet may reduce the chances of developing Alzheimer's disease.
Where did the story come from?The study was carried out by researchers from Rush University Medical Center in Chicago and Harvard School of Public Health in Boston, and was funded by grants from the US National Institute on Aging.
It was published in the peer-reviewed medical journal Alzheimer's & Dementia.
The Mail Online reported the study accurately for the most part, although it did not say that this type of study cannot prove causation. Strangely, it repeatedly said that the MIND diet called for a daily salad, although salad was not mentioned specifically in the study.
What kind of research was this?This was a large prospective cohort study of older people who were taking part in a long-running study of memory and ageing. It aimed to see whether people whose food consumption was closest to one of three types of healthy diet were less likely to be diagnosed with Alzheimer's disease during the course of the study.
As this was an observational study, it cannot prove that the diet protected against Alzheimer's disease or other types of dementia. A randomised controlled trial would be needed for that.
What did the research involve?Researchers worked with volunteers living in retirement communities and public housing in Chicago. They were asked to complete a questionnaire to assess their diet. They all had annual neurological examinations for an average of four to five years, which checked for Alzheimer's disease.
Researchers adjusted the results to take account of other factors that can affect Alzheimer's risk. They then looked for links between Alzheimer's diagnosis and people's diets.
At the start of the study, the researchers decided to assess three types of diet:
- The Dietary Approach to Stop Hypertension (DASH) has been used to reduce blood pressure and stroke risk. It includes total grains and wholegrains, fruit, vegetables, dairy products, meat and fish, nuts and legumes, but restricts fat, sweets and salt.
- The Mediterranean diet (MEDdiet) is often recommended for heart health. It includes olive oil, wholegrains, vegetables, potatoes, fruit, fish, nuts and legumes, and moderate wine, but restricts full-fat dairy products and red meat.
- The Mediterranean-DASH Intervention for Neurodegenerative Delay (MIND) diet is a new diet developed by the researchers with elements from the DASH and MEDdiet, and also includes foods thought to protect the brain. It includes olive oil, wholegrains, green leafy vegetables, other vegetables, berries, fish, poultry, beans and nuts, and a daily glass of wine, but restricts red meat and meat products, fast or fried food, cheese, butter, pastries and sweets.
They then looked at whether people in the high-scoring groups for each diet were less likely to be diagnosed with Alzheimer's disease during the average 4.5 years of follow-up, compared with people in the low-scoring groups.
People diagnosed with other types of dementia, such as dementia with Lewy bodies or vascular dementia, were not included as Alzheimer's cases.
The researchers did a good job of checking for other factors that could affect Alzheimer's risk. This included testing for a type of gene (APOE) that raises the risk of Alzheimer's, as well as asking about people's education level, whether they took part in cognitively stimulating activities such as playing games and reading, how much physical activity they got, their body mass index (BMI), whether they had symptoms of depression, and their medical history.
What were the basic results?During the study, there were 144 cases of Alzheimer's disease among the 923 people taking part.
People with the highest scores in all three diets were less likely to be diagnosed with Alzheimer's disease than people with the lowest scores.
The link was slightly stronger for the MIND and MEDdiet than the DASH diet. People who had the highest scores on the MIND diet were 52% less likely to be diagnosed with Alzheimer's disease (hazard ratio [HR] 0.48, 95% confidence interval [CI] 0.29 to 0.79).
People who had moderate scores for the MIND diet were also less likely to be diagnosed with Alzheimer's than those with the lowest scores, but the link was not as strong (HR 0.64, 95% CI 0.42 to 0.97). Moderate scores on the DASH and MEDdiet did not show a statistically significant reduction in risk.
How did the researchers interpret the results?The researchers said their results showed that "even modest adherence" to the MIND diet "may have substantial benefits" for preventing Alzheimer's disease.
They say that while the DASH and MEDdiet also showed positive results, "only the highest concordance" with those diets was linked to the prevention of Alzheimer's disease.
They go on to speculate that the dairy and low-salt recommendations in DASH, while useful for reducing blood pressure, may not be particularly relevant to brain health.
They concluded that, "High-quality diets such as the Mediterranean and DASH diets can be modified ... to provide better protection against dementia."
The study found
people who ate a healthy diet – with plenty of green vegetables,
wholegrains, legumes and less red meat – may be less likely to get
Alzheimer's disease. However, we should be wary of saying that their
diet actually protected them from Alzheimer's, as it is a complex
disease with many potential causes.
The main limitation is that observational studies cannot prove causation, even when researchers take care, as they did here, to include factors that we know affect disease risk. It's also notable that the researchers excluded dementia, other than Alzheimer's disease, from their calculations.
It would be interesting to see the effect of these diets on other types of dementia, too, especially as the DASH diet protects against hypertension, which can be a cause of vascular dementia. This was not taken into consideration when the authors concluded that low dairy and salt may not be needed for brain health (though they still remain part of a healthy, balanced diet).
Another limitation is that the food frequency questionnaire may not have completely captured people's adherence to the three diets. For example, people were asked about how often they ate strawberries, not about other types of berries. This could underestimate the effect of berry consumption in the diet.
Experts already think a healthy lifestyle can help lower the risk of getting dementia. Recommendations include eating a healthy diet, keeping to a healthy weight, exercising regularly, not smoking, drinking in moderation, and keeping blood pressure healthy. The question is: what type of healthy diet is best?
This study suggests the MIND diet may be better at lowering the risk of Alzheimer's disease than two other healthy diets. However, the study did not compare the effect of the diets directly.
We also don't know which foods in the diets might make the difference. The best advice may be to follow a healthy balanced diet, without worrying too much about exactly which foods might protect your brain.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.