Populations in different parts of the world have different beverage preferences, with Asian countries preferring green tea and Western countries preferring black tea and coffee. Polyphenols are plant based chemicals that may have health benefits for people, and black tea, green tea, and coffee each contain polyphenols. The researchers of this study wanted to evaluate whether the consumption of beverages rich in polyphenols and caffeine, specifically green tea, coffee, and black tea, would be protective and delay the onset of dementia.1
The researchers determined that consumption of green tea was associated with a reduced risk for dementia or mild cognitive impairment (MCI). However, consumption of black tea or coffee did not produce these same protective results.
The baseline measures for the study were taken between 2007 and 2008 in Nakajima, Japan. The purpose of the study was to evaluate possible relationships between lifestyle factors and dementia in the elderly Japanese populace. This research summary is part of a much larger study, called the Nakajima Project, which aimed to evaluate the health of the 2,845 people over the age of 60 years who were legally residing in Nakajima. The baseline survey was a self-administered questionnaire that assessed sociodemographic data (including age, sex, and education), past medical history (hypertension, hyperlipidemia, and diabetes mellitus), smoking habits, physical activities and hobbies, and green tea, coffee, and black tea consumption. A blood sample was also collected from each participant, with vitamin C and ApoE status determined for each participant. The frequency of green tea, black tea and coffee consumption was self-reported as 0, 1, 2, 3, 4, 5, or 6 times/week or every day. The researchers for this study divided these results into the following three categories: zero (no consumption), 1–6 days/week, and every day. Each participant was given a Mini-Mental State Examination (MMSE) to determine cognitive function, with a higher score indicating higher cognitive function (maximum possible score =30). Cognitive function was also evaluated with the Clinical Dementia Rating (CDR) with possible scores ranging from 0 (no cognitive or functional deficits) to 3 (severe dementia). The CDR evaluates memory, orientation, judgment and problem solving, function in community affairs, home and hobbies, and personal care.
In 2011 and 2013 all willing participants were interviewed for a follow-up to determine if their cognitive function or health status had changed over time. The same questionnaires and neurological tests were administered at follow-up as had been given at baseline. Seven of the participants had been moved to long-term care facilities or hospitals, and the researchers received permission from their families to visit and administer the same examinations to them as well. Any participants who were deceased or moved away from Nakajima before the follow-up were excluded from the analysis.
Of the 2,845 potential study participants over the age of 60 residing in Nakajima in 2007, 982 voluntarily participated in the brain-function examination. The researchers excluded 217 participants from the analysis because of dementia, MCI, or failure to complete the cognitive tests, and another 42 were excluded because they had a MMSE score of <24 at baseline. After these exclusions were made, 723 participants were judged to have normal cognitive function at the initial baseline survey. However, the number of participants for the follow-up survey was lower (n=490) because 55 died, 5 moved, 167 did not repeat cognitive testing at follow-up, and 6 incompletely answered the beverage-frequency questionnaire at the initial baseline survey. Hence, the population included in statistical analysis was the 490 participants who completed both the baseline and follow-up surveys and met all selection criteria regarding cognitive performance.
Green tea consumption was evaluated and it was determined that 195 participants drank moderate amounts of green tea (1–6 days per week), 157 drank green tea every day, and 138 did not drink green tea. For coffee intake, 212 participants drank coffee every day, 180 drank moderate amounts (1–6 days per week), and 98 did not drink coffee. Black tea consumption was not as evenly divided, with 404 participants reporting they did not consume black tea and 86 reported consuming black tea at least 1 day per week. There were only 6 participants who reported drinking black tea daily, so the researchers grouped the participants drinking black tea for 1–6 days/week and every day for statistical analysis
When evaluating the baseline test results, researchers determined the participants who consumed larger amounts of green tea were younger, had more formal education, had higher MMSE scores, and had higher scores for current physical activities and/or hobbies. The baseline evaluation also revealed that more frequent consumption of coffee was associated with lower age, higher ascorbic acid levels (vitamin C), and higher scores for current physical activities and/or hobbies. Also, the baseline results revealed that the study participants who did not consume black tea had fewer years of formal education and tended to have lower MMSE scores than those who consumed black tea more than 1 day per week.
The researchers identified 26 participants with dementia and 64 with MCI during their follow-up evaluations. The researchers identified that more frequent consumption of green tea was associated with a lower incidence of dementia and MCI. Meanwhile, no association was found between the incidences of dementia or MCI and coffee or black tea consumption. The researchers found that green tea consumption was inversely associated with the incidence of dementia, meaning the more days green tea was consumed the lower the incidence of dementia or MCI, for both age and gender adjusted models. This inverse association remained present after adjustment for physical activities and hobbies as potential confounders that were also associated with green tea consumption.
Due to a small sample size, the researchers were not able to assess the association of dementia and MCI with the combined frequency of green tea, coffee, and black tea consumption. Also, the study evaluated frequency of beverage consumption, but not quantity of consumption. Hence, the researchers were unable to determine a dosing relationship between quantity of beverage consumed and cognitive decline. The researchers also identified that further studies utilizing neuroimaging and neuropathology would be helpful to determine the cause of the dementia and MCI and the effects of the green tea consumption on the brain.
Overall, the study results indicated that higher green tea consumption was associated with a lower incidence of cognitive decline (dementia or MCI) in this elderly Japanese population. They suggest that green tea consumption may be beneficial for reducing the risk of cognitive decline and warrants further research in a larger population, looking at quantity of green tea consumed and including imaging to evaluate the effects on the brain.
- Noguchi-Shinohara M, Yuki S, Dohmoto C, et al. Consumption of Green Tea, but Not Black Tea or Coffee, Is Associated with Reduced Risk of Cognitive Decline. PLoS One. 2014; 9(5): e96013 (pp. 1-8).