"Drinking tea and red wine with plenty of kale could slash the risk of Alzheimer's disease," reports the Mail Online.
If you think this claim sounds familiar, that's because it is. For decades, scientists have been investigating the possible effects of chemicals called flavonols, which are found in tea, red wine, fruit and green vegetables.
We know that foods such as fruit and vegetables are an important part of a healthy diet as there is strong evidence they can reduce the risk of heart disease, stroke and some types of cancer. But we do not know if we could replicate the health effects if we just consumed flavonols separately, as supplements.
Previous research found that the evidence about red wine is inconsistent. Plus, any possible benefit does not outweigh the risks if you drink alcohol above the recommended limit.
This study looked at the diets of 921 people in the US, with an average age of 81, and followed them up for 6 years, on average, with annual checks. During the study almost 24% developed Alzheimer's disease. The researchers found that those who ate diets highest in flavonols had a 40% lower chance of developing Alzheimer's disease compared to those with diets lowest in flavonols.
The study cannot prove that flavonols have specifically contributed to disease risk, as other health or lifestyle factors or other nutrients in the foods may still be having an influence. But regardless of this, it adds weight to what we previously knew – that eating plenty of fruits and vegetables may help protect against dementia. Other things that may decrease your risk of dementia include being active, not smoking, and (despite headlines to the contrary) not drinking too much alcohol.
Where did the story come from?
The researchers who carried out the study were from Rush University Medical Center and Tufts University, both in the US. The study was funded by the US National Institutes for Health and the USDA Agricultural Research Service. It was published in the peer-reviewed journal Neurology, on an open-access basis, so it's free to read online.
Despite the usual headline extolling the virtues of red wine, the Mail Online's coverage does include commentary from experts making it clear that the study is not conclusive.
What kind of research was this?
This was a cohort study. Like other observational studies, cohort studies are a good way to look for links between factors, such as diet, and outcomes, such as Alzheimer's disease. However, they cannot demonstrate that dietary factors directly lead to the outcomes. Other factors (such as other chemicals found in the foods studied, or people's general lifestyle) may be involved.
What did the research involve?
This analysis, beginning in 2004, included 921 volunteers from the Rush Memory and Aging Project, an ongoing study of older adults living in retirement communities and public housing in Chicago, US, that's been running since 1997. None of the participants had Alzheimer's disease at recruitment and all had completed questionnaires about their diet over the previous 12 months and attended at least 2 annual follow-up assessments. Their average age was 81.2 years and the majority (75%) were women.
The food questionnaire looked at what people ate, then calculated the average number of flavonols they ate each week. The researchers looked at 4 types of flavonols:
- kaempferol – found in kale, beans, tea and spinach
- quercetin – found in tomatoes, kale, apples and tea
- myricetin – found in tea, wine, kale, oranges and tomatoes
- isorhamnetin – found in pears, olive oil, wine and tomato sauce
People were followed up for an average of 6.1 years and had at least 2 clinical assessments during that time where Alzheimer's disease was assessed. This included having their brain function assessed using 19 cognitive tests and being assessed by specialist doctors who made the diagnosis according to standard criteria.
Researchers adjusted their results to take account of several factors known to affect the risk of Alzheimer's disease, including:
- education level
- participation in activities that stimulate the brain
- physical activity
- presence of a genetic mutation APOE4, which raises the risk of Alzheimer's disease
What were the basic results?
Of the 921 people recruited, 220 developed Alzheimer's disease (24%) during the average follow-up of 6.1 years.
The amount of flavonols in people's diet varied from an average 5.3mg a day for those eating least, to an average 15.3mg for those eating most. An expert quoted in the Mail Online report said most UK adults consume around 30mg a day.
The researchers found that people eating the most flavonols in their study had the lowest chance of developing Alzheimer's disease.
- people eating diets with the most flavonol content had a 48% lower risk of Alzheimer's disease (hazard ratio (HR) 0.52, 95% confidence interval (CI) 0.33 to 0.84) compared to those eating diets with the least flavonol content
- the results did not change much when the researchers also adjusted the figures for whether people had cardiovascular disease, or for the presence of other nutrients linked with Alzheimer's (such as omega-3 fatty acids)
How did the researchers interpret the results?
The researchers said the findings "suggest that dietary intake of flavonols may reduce the risk of developing Alzheimer's dementia".
They added: "Although there is more work to be done, the associations that we observed are promising and deserve further study."
We know that eating a healthy, balanced diet with plenty of fruits and vegetables helps to keep the body and brain healthy as we age. Whether flavonols are the key to this, or other chemicals found in food, or combinations of foods, perhaps does not matter, as long as we continue to eat a balanced diet.
The study has some limitations. Firstly, as it is an observational study it cannot show cause and effect. Many different health and lifestyle factors may influence the risk of Alzheimer's disease and it's not possible to take account of them all.
The people who volunteered to take part in the study were mostly white, college-educated and from one area of the US. We do not know if the same results would be found in other groups of people. People reported their diet through food questionnaires, which means they may not be wholly accurate. This may lead to inaccuracy in estimating flavonol intake.
Also, as the study included people who were already in old age, we cannot conclude that a higher intake of flavonols, or fruit and vegetables in general throughout life, decreases dementia risk.
The study adds weight to previous findings that a healthy diet containing plenty of plant-based foods is good for general health and may also reduce dementia risk. However, as the cause of Alzheimer's is not completely clear (aside from age and certain genetic factors), there are no guarantees.
Other things that may reduce the risk of dementia include:
- keeping your cardiovascular system healthy through regular exercise, not smoking, keeping alcohol to a minimum and having regular blood pressure checks
- staying mentally active through reading, learning foreign languages or playing musical instruments
- staying socially active through volunteering in your local community, taking part in group activities, trying new hobbies and maintaining an active social life
Find out more about preventing Alzheimer's disease.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
Mail Online, 29 January 2020
Links to the science
Neurology. Published online 29 January 2020