Friday July 18 2008
Tea contains polyphenols, a type of antioxidant
“A couple of cups of black tea a day can significantly cut the risk of dementia,” reported The Sun. The Daily Telegraph also covered the story, saying that a study had found that people who drank two or three cups a day were half as likely to show early signs of dementia as those who rarely or never drank it. The newspaper said that coffee does not have the same effect and that scientists had concluded it was a type of antioxidant in tea called polyphenols which has the effect, rather than caffeine.
In this study, the participants were assessed using a recognised tool for assessing cognitive mental status. However, this tool cannot be used to diagnose dementia and it is not clear how its measurement of changes in information processing relate to the risk of dementia. There could also have been other factors such as subtle differences in education that could have affected both tea drinking and cognition.
Where did the story come from?
Dr Tze-Pin Ng and colleagues from the University of Singapore and the Department of Geriatric Medicine, Alexandra Hospital in Singapore carried out the research. The study was supported by a grant from the Biomedical Research Council, Agency for Science, Technology and Research. The study was published in the peer-reviewed medical journal, the American Journal of Clinical Nutrition.
What kind of scientific study was this?
This was a cross-sectional and longitudinal analysis of data from an ongoing cohort study. Its aim was to investigate if there was a relationship between tea consumption and cognitive decline or impairment. The researchers were mainly interested in the actions of polyphenol compounds, theaflavins and thearubigins, which are formed as a result of enzymatic oxidation and can vary between different types of tea.
To do this, the researchers measured volunteers’ consumption of different types of tea, and tested whether this was linked to changes in their “global cognitive functioning” (such as memory, attention, language, performing actions, and the visual perception of spatial relationships between objects. In particular, they were interested in whether the tea habits reported at the start of the study were linked with the chance of the participants also being “cognitively impaired” and whether these habits were linked with any deterioration in cognitive mental function one to two years later.
This current study used participants from an ongoing cohort study called the Singapore Longitudinal Ageing Study (SLAS). Through a door-to-door census, SLAS identified all available adults over 55 years of age in the southeast region of Singapore. For their study, the researchers excluded people who were less than 55 years old, or who were unable to complete the interview because they were too frail or terminally ill (with stroke or dementia already, for example). After also excluding all non-Chinese participants and people for whom data was missing, the researchers were left with 2501 participants for the first cross-sectional analysis. For the second analysis, the researchers selected the 2194 participants without cognitive impairment and reassessed 1435 of them one to two years later (65.5% of the original sample).
The door-to-door census involved each participant undergoing a Mini-Mental State Examination (MMSE), a widely used tool for assessing cognitive mental status. Although it is often used as a first step in detecting cognitive impairment, it cannot be used for making formal diagnoses of dementia.
Along with the MMSE, the participants were asked about the amount and types of tea that they drank. The researchers restricted their analysis to three main types of tea, black tea (fully fermented), oolong tea (semi-fermented), and green tea (non-fermented). Coffee drinking was also recorded.
The participants’ daily tea consumption was categorised as low, medium or high intake, with the high intake group drinking more than nine cups a day. The MMSE score can range from 0 to 30 and the researchers took any score of 23 or below as indicating cognitive impairment. Reductions of one point on the score over the follow-up period were classed as a decline.
What were the results of the study?
Nearly half of the participants consumed Chinese black or oolong tea and about 40% drank English black tea. Less than 7% drank green tea on a daily basis and 38.1% (954 people) rarely or never drank any tea.
The researchers found that total tea intake was significantly associated with a lower prevalence of cognitive impairment, independent of other risk factors. The cross-sectional analysis showed that those who drank low amounts of tea had almost half the chance, 0.56 (95% CI: 0.40 to 0.78), of scoring 23 or less on the questionnaire (i.e. being ‘cognitively impaired’) than those who never or rarely drank any tea. The chance was even lower in the medium and high intake groups.
In the longitudinal analysis, cognitive decline (defined by the one-point deterioration in MMSE score) was also more common in the tea-drinking groups. However, this was not statistically significant in any of the groups. In contrast, no association between coffee intake and cognitive status was found.
What interpretations did the researchers draw from these results?
The researchers concluded that “regular tea consumption was associated with lower risks of cognitive impairment and decline”.
What does the NHS Knowledge Service make of this study?
This study recorded a large number of variables relating to tea drinking, and also took advantage of data recorded in a previous study to test the links between tea drinking and the rates of cognitive impairment at one point in time as well as its decline over a period of time. It is important to note that in the main longitudinal analysis, the decline over time in cognitive function (defined as one point in the MMSE score) for those who were not cognitively impaired at the beginning of the study, was not significant. Other points to note are that:
- Although this study found that people who drank low amounts of tea had almost half the chance of being “cognitively impaired” than those who did not, it did not make a distinction between dementia and cognitive impairment or decline. Cognition - the mental process of knowing, thinking, learning or judging - naturally declines with age, and for this type of research it is important to assess the nature and extent of what is being defined as abnormal.
- A large number of people dropped out from the longitudinal analysis (35%), and it is possible that those who failed to turn up for their second test were different, i.e. more or less impaired, from those who did turn up. This large number of dropouts is likely to have affected the reliability of this study.
- The authors acknowledge that the use of the MMSE cut-off of 23 or less to identify cognitively impaired subjects may have resulted in the inclusion of some patients with dementia, in whom tea drinking could be underestimated. This may have affected the reliability of the cross-sectional analysis and any links with cognitive impairment.
- As with all observational studies, it is possible that there were some unmeasured factors or factors that were not adequately taken into account in the analysis, such as subtle differences in education or income that is associated with tea drinking and the speed of cognitive decline.
- In this study, the average difference between groups was about 3 points on the 30-point MMSE scale, and the researchers were interested in people whose score reduced by one point over one to two years. It is not clear how such changes in information processing relate to the risk of dementia.
Overall, the limitations of this study indicate that it should be used to guide further research rather than to determine tea-drinking habits.