Tea not proven to 'shield you against dementia'
"It's tea time! How at least two cups a day can shield you from dementia," reports the Mail Online. This rather optimistic headline reports on a Singaporean study of around 900 Chinese people aged 55 and above.
The study searched for a potential link between tea consumption and development of dementia. It found the risks of dementia were halved in tea-drinkers. However, when breaking the results down further, the links were only significant for women who drank three to four cups a day and in drinkers who carry a particular dementia-risk gene.
Despite the relatively large sample size, only 72 people developed dementia. But breaking down this number further according to tea intake leaves only small groups for analysis. And the smaller the sample size, the bigger the risk that pure chance affected the results.
Also, despite adjusting for other health and lifestyle factors that could be influencing the link, it is always difficult to isolate the direct effect of tea drinking.
The researchers suggest that promotion of tea drinking could have benefits for the brain but they also point out that further studies are needed to confirm the results found in their study.
There is currently no guaranteed method(s) of preventing dementia, but a useful maxim is "what is good for the heart is also good for the brain". Regular exercise, a healthy diet, moderate alcohol consumption and avoiding smoking can help lower dementia risk.
Where did the story come from?
The study was carried out by researchers from the National University of Singapore. It was funded by the Biomedical Research Council; the Agency for Science, Technology and Research; the Virtual Institute for the Study of Ageing; and the Alice Lim Memorial Fund.
The study was published in the peer-reviewed Journal of Nutrition, Health and Aging.
The Mail Online's headline was overly optimistic: it took the 50% figure at face value. The website noted the small sample size, but the limitations of the study were not discussed.
What kind of research was this?
This was an analysis of data from a prospective cohort study of Chinese older adults, which aimed to investigate whether there's a link between tea consumption and dementia.
In Chinese culture, the consumption of strong tea is considered to enhance brain-based skills such as memory and alertness in the short-term. However, the regular consumption is also thought to have long-term benefits, which several studies have previously looked into.
With this background, the authors of this research wanted to further test the hypothesis that tea drinkers are less likely to develop brain disorders such as dementia, when compared to non-drinkers.
The researchers also wanted to see whether the association was different between males and females, and in people carrying a high-risk variant of the apolipoprotein (APOE) gene – studies have suggested that people are more likely to develop Alzheimer's if they carry this gene type.
Cohort studies are valuable for testing the association between an exposure and outcome, and although they aren't always able to prove cause and effect, can give a good indication of any potential links.
As mentioned by the study authors, a randomised-controlled trial (RCT) would be one of the best ways to further test a hypothesis such as this. However, it is not easy to recruit sufficient people, randomise them to a tea drinking pattern they have to stick to, and then follow them for long enough to look at the effect on cognitive outcomes.
What did the research involve?
This analysis used data from the Singapore Longitudinal Ageing Studies (SLAS), which studied ageing and health in Singaporeans aged 55 years and above. It recruited 2,808 participants. Baseline data was collected from 2003 to 2005, and follow-up of neurocognitive disorders was conducted from 2006 to 2010.
The SLAS study collected information on cognitive function, having assessed this using a version of the Mini-Mental State Examination (MMSE) at baseline and during follow-up assessments. The MMSE is a well respected method of testing a range of cognitive abilities.
An MMSE score of 26 and above was defined as being "normal". During follow-up, adults who had a score of less than 26 or a decline in the MMSE score of one or more were further assessed using the Clinical Dementia Rating (CDR).
Participants were asked about their tea consumption habits through a questionnaire given at baseline and during follow-up. The survey categorised tea into: "Ceylon/English" tea; "Chinese" tea and "Green" tea.
Frequency of tea consumption was coded as:
- 0 – never or rarely
- 1 – less than one cup/week
- 2 – more than one cup/week but less than one cup/day
- 3 – one-two cups/day
- 4 – three or more cups/day
The level of tea consumption was categorised into four groups:
- 0 – none
- 1-2 – low
- 3-4 – medium
- 5 or more – high
This study assessed data from a group of 957 SLAS participants who had an MMSE score of 26 or more at baseline. Among these people, 72 (7.5%) developed a neurocognitive disorder (dementia) during follow-up.
The researchers analysed the data for differences between tea drinkers and non-drinkers. The model was adjusted for numerous potential confounders, including:
- alcohol consumption
- body mass index (BMI)
- heart diseases
- dietary intake
- presence of the APOE ε4 gene (the high risk variant)
What were the basic results?
69% of participants in this analysis were tea consumers at baseline. Out of the 660 tea drinkers, 39 individuals (5.9%) developed dementia; out of the 297 non-drinkers, there were 33 incident cases (11.1%).
The analysis indicated that tea drinkers had a 50% reduced chance of developing dementia during follow-up (odds ratio [OR] 0.50, 95% confidence interval [CI]: 0.28 to 0.87). Those who drank a medium amount of tea had 64% reduced risk (OR 0.36, 95% CI: 0.16 to 0.78). Results were not statistically significant for either low and high levels of tea intake, which slightly confuses the picture.
By gender, tea drinking gave reduced risk in women (OR 0.32, 95% CI: 0.15 to 0.69) but links were not statistically significant in men.
Tea drinking appeared to give protection in high-risk APOE carriers (OR 0.14, 95% CI: 0.02 to 0.93), but did not have a significant effect in non-carriers (OR 0.56, 95% CI: 0.30 to 1.04).
How did the researchers interpret the results?
The researchers concluded: "Our data suggest that a simple lifestyle measure such as tea drinking can reduce a person's risk of developing neurocognitive disorders in late life.
"Together with earlier reports on cognitive benefits of tea drinking, our study supports the promotion of tea drinking as a simple, culturally acceptable and cheap preventive measure among other known protective interventions such as participating in physical, social and cognitive activities."
This analysis of data from a prospective cohort study of Chinese older adults looked at a potential link between tea consumption and development of dementia. It found that tea drinkers who took part in the study were less likely to develop dementia compared to non-drinkers. The links were observed specifically in women tea drinkers, and in drinkers who carry the APOE ε4 gene that has been linked with Alzheimer's development.
This was a well-designed cohort study which controlled for numerous potential confounders in its analysis. However, there are a number of things to bear in mind, many of which have been pointed out by the authors:
- Not all of the links with tea consumption were significant. When analysed by intake, only those drinking three or four cups a day had apparent protection, with no link for greater or lesser amounts. However, as they say, this could be down to the small number of people who developed dementia during the study. Breaking down this number according to tea intake leaves only small samples for analysis. Large scale studies would need to be conducted to confirm these findings.
- While significant links were found specifically for women and for carriers of the high-risk variant of the APOE gene, we shouldn't draw strong conclusions at this stage. We don't know the reason for this link – it could be influenced by other health and lifestyle factors that haven't been taken into account.
- This study was conducted in people of Chinese ethnicity and therefore cannot necessarily be generalised to other populations. People of different cultures and ethnicities could have different susceptibilities to medical conditions, and could also have different tea drinking patterns.
- The study defined development of dementia as scoring above a particular level on the Clinical Dementia Scale, but we cannot necessarily apply the findings to particular types of dementia, like Alzheimer’s or vascular dementia.
The findings of this study warrant further exploration, although high quality randomised controlled trials may be difficult. This study, on its own, does not prove that drinking tea will stop you from getting dementia.
To reduce your risk of developing dementia and other serious health conditions, it's recommended that you:
- eat a healthy diet
- maintain a healthy weight
- exercise regularly
- don't drink too much alcohol
- stop smoking (if you smoke)
Links To The Headlines
It's tea time! How at least two cups a day can shield you from dementia. Mail Online, April 7 2017
Links To Science
Feng L, Chong MS, Lim WS, et al. Tea consumption reduces the incidence of neurocognitive disorders: Findings from the Singapore longitudinal aging study. The Journal of Nutrition, Health and Aging. Published online January 15 2016