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1 From the Division of Epidemiology, Departments of Public Health and Forensic Medicine (SK, AH, KO, TS, and IT), Geriatric and Respiratory Medicine (TM and SE), and Psychiatry (SA), the Division of Medicine and Science in Sports and Exercise, Departments of Functional Medical Science (RN) and Geriatric and Complementary Medicine (HA), Tohoku University Graduate School of Medicine, Sendai, Japan
2 Supported by grants for scientific research (13557031) and for Japan Society for the Promotion of Science (JSPS) research (1410301) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan, by a research grant (2002) from the Japan Atherosclerosis Prevention Fund, by a Health Science Grant on Health Services (H13-kenko-008), and by a Comprehensive Research on Aging and Health grant (H13-choju-007, H13-choju-023) from the Ministry of Health, Labour and Welfare of Japan. 3 Address reprint requests to S Kuriyama, Division of Epidemiology, Department of Public Health and Forensic Medicine, Tohoku University Graduate School of Medicine, 2-1, Seiryo-machi, Aoba-ku, Sendai, 980-8575, Japan. E-mail: kuriyama-thk{at}umin.ac.jp.
ABSTRACT
Background: Although considerable experimental and animal evidence shows that green tea may possess potent activities of neuroprotection, neurorescue, and amyloid precursor protein processing that may lead to cognitive enhancement, no human data are available.
Objective: The objective was to examine the association between green tea consumption and cognitive function in humans.
Design: We analyzed cross-sectional data from a community-based Comprehensive Geriatric Assessment (CGA) conducted in 2002. The subjects were 1003 Japanese subjects aged 70 y. They completed a self-administered questionnaire that included questions about the frequency of green tea consumption. We evaluated cognitive function by using the Mini-Mental State Examination with cutoffs of <28, <26, and <24 and calculated multivariate-adjusted odds ratios (ORs) of cognitive impairment.
Results: Higher consumption of green tea was associated with a lower prevalence of cognitive impairment. At the <26 cutoff, after adjustment for potential confounders, the ORs for the cognitive impairment associated with different frequencies of green tea consumption were 1.00 (reference) for 3 cups/wk, 0.62 (95% CI: 0.33, 1.19) for 4–6 cups/wk or 1 cup/d, and 0.46 (95% CI: 0.30, 0.72) for 2 cups/d (P for trend = 0.0006). Corresponding ORs were 1.00 (reference), 0.60 (95% CI: 0.35, 1.02), and 0.87 (95% CI: 0.55, 1.38) (P for trend = 0.33) for black or oolong tea and 1.00 (reference), 1.16 (95% CI: 0.78, 1.73), and 1.03 (95% CI: 0.59, 1.80) (P for trend = 0.70) for coffee. The results were essentially the same at cutoffs of <28 and <24.
Conclusion: A higher consumption of green tea is associated with a lower prevalence of cognitive impairment in humans.
Key Words: Cognitive function • elderly • green tea • Japanese • Mini-Mental State Examination
INTRODUCTION
Dementia is a rapidly growing public health concern as a result of aging of the population (1, 2). In developed countries, dementia has a reported prevalence of 1.5% at age 65 y, doubling every 4 y to reach 30% at age 80 y (1). Environmental factors associated with the risk of Alzheimer disease (AD), a common cause of dementia, remain largely undefined, although several risk factors for vascular dementia have been identified (1, 3–6).
Experimental and animal studies have shown that tea and tea polyphenols (which include catechins and their derivatives), particularly those from green tea, may possess potent neuroprotective activity that can help to ameliorate neurodegenerative diseases such as AD and Parkinson disease (PD) (7). Green tea catechins, especially (–)-epigallocatechin-3-gallate (EGCG), formerly thought to be simple radical scavengers, are now considered to invoke a spectrum of cellular mechanisms related to neuroprotective as well as neurorescue activities (8–10). One of these mechanisms includes protective effects against ß-amyloid (Aß)–induced neurotoxicity by enhancing the release of the nonamyloidogenic soluble form of amyloid precursor protein (APP) (8). Aß protein is formed by proteolytic cleavage of APP (11) and is the main constituent of the neuritic plaques that are the physiologic hallmark of AD (12). In addition, EGCG was shown to have neuroprotective activity in a mice model of PD (13), and an epidemiologic study indicated that the risk of PD was reduced if tea consumption was 2 cups/d (14). Despite this considerable evidence that tea, especially green tea, can protect against neurodegenerative diseases, to our knowledge, no data are available on any association between green tea intake and dementia or cognitive impairment in humans.
We therefore designed this cross-sectional analysis to investigate the association between consumption of green tea and cognitive function in elderly Japanese subjects, among whom green tea was widely consumed. We considered it important to search for modifiable factors underlying cognitive impairment because early detection and management of cognitive decline contribute to the prevention of dementia rather than to treatment (15, 16).
SUBJECTS AND METHODS
Study population
The Tsurugaya Project was a community-based Comprehensive Geriatric Assessment (CGA) conducted among elderly Japanese subjects living in Tsurugaya district, a suburban area of Sendai City in northern Japan, between July and October 2002 (17, 18). CGA is a structured approach to measuring the physical, mental, and social functioning of elderly people to assess early deterioration that may result in the need for long-term care and to promote healthy aging (19, 20).
At the time of the study, 2730 people aged 70 y were living in the Tsurugaya district. We sent letters to all of these people and invited them to participate in the health survey. Of those invited, 1198 participated in the survey and 1178 (43.2%) gave written informed consent to be included in the analysis. The study protocol was approved by the institutional review board of Tohoku University Graduate School of Medicine.
Data about consumption of green tea, black or oolong tea, and coffee and cognitive function were obtained from 1151 of the subjects who gave written informed consent. We excluded 148 subjects with missing data on body weight, height, blood glucose concentrations, blood pressure values, or depressive symptoms (described in Measurements). Thus, data from 1003 subjects contributed to the final analyses.
Measurements
The questionnaire in the CGA included items about the frequency of recent consumption of 5 beverages (green tea, black or oolong tea, coffee, cola or juice, 100% fresh vegetable juice) and 55 items about food intake during the previous month. The frequency of consumption of green tea was divided into 8 categories: never, <1 cup (0.1 L)/wk, 1 cup/wk, 2–3 cups/wk, 4–6 cups/wk, 1 cup/d, 2–3 cups/d, and 4 cups/d. In the study region, the volume of a typical cup of green tea is 100 mL. We grouped the subjects into 3 categories according to their beverage consumption: 3cups/wk, 4–6 cups/wk or 1 cup/d, and 2 cups/d.
The questionnaire in the CGA also included 1) demographic characteristics (age, sex, and duration of education); 2) social factors (visiting friends); 3) lifestyle habits (smoking, alcohol use, and physical activity); and 4) physical health [history of chronic medical conditions such as stroke or myocardial infarction, regular intake of supplements and medication, and self-rated health (excellent, good, normal, poor, or very poor)].
Cognitive function was tested by using the Japanese language version of the 30-point Mini-Mental State Examination (MMSE) (21). The test was administered by specially trained research assistants. The MMSE includes questions on orientation to time and place, registration, attention and calculation, recall, language, and visual construction. This screening test was originally created for a clinical setting (21) and is used extensively in epidemiologic studies (22). Higher MMSE scores indicate higher cognitive function, and the maximum score is 30 points. The analyses were conducted by using 3 cutoff points to define different levels of cognitive impairment. The initial cutoff point was <26, because a score of <26 points on the MMSE generally indicates cognitive impairment (23). The second was <28, which we regarded as slight cognitive impairment, and the third was <24, which we regarded as relatively severe cognitive impairment. In the initial analyses, the group with cutoff points of <26 included subjects with cutoff points of <24, and the group with cutoff points of <28 included subjects with cutoff points of <26 and <24. In further analyses, we reanalyzed the data by using cutoff points of <26 or <28 after excluding subjects with a MMSE score of <24.
Data were obtained about 1) body mass index (BMI; in kg/m2; as calculated from participants' measured weight and height); 2) the presence or absence of diabetes mellitus, defined as a nonfasting blood glucose concentration 140 mg/dL or a history of diabetes mellitus; 3) the presence or absence of hypertension, defined as a self-measured systolic blood pressure 135 mm Hg (measured at home) or a history of hypertension; 4) the presence or absence of depressive symptoms, as assessed by using the Japanese version of the 30-item Geriatric Depression Scale (24); and 5) physical functioning status, assessed by using the 6-item physical functioning status measure of the Medical Outcomes Study (MOS) Short-form General Health Survey (lower MOS scores indicate lower physical functioning status) (25).
Statistical analysis
The subjects' characteristics according to categories of green tea consumption were compared by using analysis of variance or chi-squared test, as appropriate. We used multivariate logistic regression analysis to calculate odds ratios (ORs) for cognitive impairment relative to the consumption frequencies of green tea or other beverages, with the lowest frequency category (3 cups/wk) treated as the reference group. Trend tests were performed by including the ordinal variable in a linear regression analysis. In these analyses, we regarded the following data as covariates: age (continuous variable); sex; consumption of green tea (3 cups/wk, 4–6 cups/wk or 1 cup/d, 2 cups/d; when calculating the ORs for consumption of black or oolong tea or coffee); consumption of black or oolong tea (3 cups/wk, 4–6 cups/wk or 1 cup/d, 2 cups/d; when calculating the ORs for consumption of green tea or for coffee); consumption of coffee (3 cups/wk, 4–6 cups/wk or 1 cup/d, 2 cups/d; when calculating the ORs for the consumption of green tea or black or oolong tea); BMI (<18.5, 18.5–24.9, 25.0–29.9, 30.0); diabetes mellitus (presence or absence); hypertension (presence or absence); history of stroke (presence or absence); history of myocardial infarction (presence or absence); depressive symptoms (Geriatric Depression Scale scores of <11 or 11); duration of education (12 y or >12 y); living with a spouse (yes or no); self-rated health (excellent or good or other); visiting friends (yes or no); physical functioning status (MOS scores of 0–1, 2–4, or 5–6); energy intake (continuous variable); intake of nondietary vitamin C or E including supplement vitamin C, supplement vitamin E, prescribed vitamin C, and prescribed vitamin E (yes or no); consumption of fish (<1 time/wk, 1–6 times/wk, or 1 time/d); consumption of green or yellow vegetables (<1 time/wk, 1–6 times/wk, or 1 time/d); mild leisure-time physical activity such as walking (yes or no); vigorous leisure-time physical activity such as tennis or jogging (yes or no); smoking (never, former, currently smoking <20 cigarettes/d, and currently smoking 20 cigarettes/d); and use of alcohol (never, former, and currently drinking).
Interactions between consumption of green tea and all confounders were tested through the addition of cross-product terms to the regression model. All statistical analyses were performed with the use of SAS software, version 9.1 (26). All the statistical tests that we report were two-sided. A P value of < 0.05 was accepted as statistically significant.
RESULTS
The subjects' characteristics according to categories of green tea consumption are shown in Table 1. Of the subjects, 16.9% consumed 3 cups green tea/wk, 10.8% consumed 4–6 cups/wk or 1 cup/d, and 72.3% consumed 2cups/d. The mean ± SD overall MMSE score was 27.4 ± 2.7. The prevalence of cognitive impairment decreased with increasing consumption of green tea for every cutoff point (P for the cutoff points of <28, <26, <24 = 0.06, 0.002, 0.09, respectively). Subjects who consumed 2 cups green tea/d were more likely to be women, have better self-rated health (P = 0.06), visit friends, have more total energy intake, consume green or yellow vegetables, never have smoked, and never have used alcohol (P = 0.10). They were less likely to consume black or oolong tea or coffee, have a history of stroke or myocardial infarction (P = 0.06), have depressive symptoms, and have limited physical functioning status (P = 0.06). No apparent associations were observed among mean age, BMI, presence or absence of diabetes mellitus or hypertension, duration of education, living with a spouse, intake of nondietary antioxidants, consumption of fish, or mild and vigorous leisure-time activities and frequency of green tea consumption.
View this table:
TABLE 1. Characteristics of the study subjects according to categories of green tea consumption1
Statistically significant inverse associations were observed between green tea consumption and cognitive impairment (Table 2). With the use of the <26 MMSE score cutoff point, the crude ORs of cognitive impairment associated with the different frequencies of green tea consumption were 1.00 (reference) for 3 cups/wk, 0.63 (95% CI: 0.35, 1.15) for 4–6 cups/wk or 1 cup/d, and 0.50 (95% CI: 0.33, 0.74) for 2 cups/d. We included a variety of potential confounders in our multivariate logistic models; however, the results did not change substantially even after adjustment for these variables. The results for MMSE score cutoff points of <28 and <24 were essentially the same as those for the <26 cutoff point.
View this table:
TABLE 2. Odds ratios (ORs) and 95% CIs from logistic regression models for the association between consumption of green tea and cognitive impairment1
In the final model used to investigate the association between different frequencies of green tea consumption and cognitive impairment, we chose the following data as covariates according to their relative contribution to the model outlined in Table 2 and their clinical importance: age, sex, consumption of green tea (when calculating ORs for consumption of black or oolong tea or coffee), consumption of black or oolong tea (when calculating ORs for consumption of green tea or coffee), consumption of coffee (when calculating ORs for consumption of green tea or black or oolong tea), presence or absence of diabetes mellitus, presence or absence of hypertension, history of stroke, depressive symptoms, duration of education, visiting friends, energy intake, intake of nondietary vitamin C or E, and consumption of fish. The ORs (95% CIs) in the final model (using a cutoff point of <26) and corresponding ORs (95% CIs) for consumption of black or oolong tea or coffee are shown in Figure 1. The multivariate ORs according to frequencies of green tea consumption were 1.00 (reference) for 3 cups/wk, 0.62 (95% CI: 0.33, 1.19) for 4–6 cups/wk or 1 cup/d, and 0.46 (95% CI: 0.30, 0.72) for 2 cups/d. In contrast, a weak or null association was observed between intake of black or oolong tea or coffee and the prevalence of cognitive impairment. The ORs for black or oolong tea were 1.00 (reference), 0.60 (95% CI: 0.35, 1.02), and 0.87 (95% CI: 0.55, 1.38), whereas those for coffee were 1.00 (reference), 1.16 (95% CI: 0.78, 1.73), and 1.03 (95% CI: 0.59, 1.80). When cutoff points of <28 or <24 were used, the results for the final model were similar to those for the <26 cutoff point (data not shown). We were unable to examine the associations between cola or juice and 100% fresh vegetable juice and cognitive impairment because an insufficient number of subjects consumed these beverages. Tests for interaction between consumption of green tea and all confounders in the final models were not statistically significant.
FIGURE 1.. Odds ratios (ORs) for the association between different frequencies of beverage consumption and cognitive impairment. The bars indicate adjusted ORs for the association between different beverage consumption frequencies and cognitive impairment, respectively; error bars represent the corresponding 95% CIs. Multivariate logistic regression analysis was used to calculate ORs for cognitive impairment relative to the consumption frequencies of green tea or other beverages, with the lowest frequency category (3 cups/wk) treated as the reference group. Trend tests were performed by including the ordinal variable in a linear regression analysis. The ORs and 95% CIs for the ORs were adjusted for age, sex, green tea consumption (when calculating ORs for black or oolong tea or coffee consumption), black or oolong tea consumption (when calculating ORs for green tea or coffee consumption), coffee consumption (when calculating ORs for green tea or black or oolong tea consumption), presence or absence of diabetes mellitus, presence or absence of hypertension, history of stroke, depressive symptoms, duration of education, visiting friends, energy intake, intake of nondietary vitamin C or E, and fish consumption. Cognitive impairment was defined as a Mini-Mental State Examination score <26. *P < 0.001. 1 cup = 0.1 L.
We repeated the analysis after expanding the highest category of green tea consumption in the final model. With a cutoff point of <26, the ORs for the different frequencies of green tea consumption were 1.00 (reference) for 3 cups/wk, 0.62 (95% CI: 0.33, 1.19) for 4–6 cups/wk or 1 cup/d, 0.42 (95% CI: 0.25, 0.71) for 2–3 cups/d (n = 258), and 0.49 (95% CI: 0.30, 0.79) for 4 cups/d (n = 467) (P for trend = 0.004). With a cutoff point of <28, the corresponding ORs were 1.00 (reference), 0.80 (95% CI: 0.48, 1.34), 0.59 (95% CI: 0.39, 0.90), and 0.67 (95% CI: 0.45, 0.98) (P for trend = 0.04). With a cutoff point of <24, the corresponding ORs were 1.00 (reference), 0.77 (95% CI: 0.32, 1.86), 0.54 (95% CI: 0.26, 1.10), and 0.50 (95% CI: 0.26, 0.98) (P for trend = 0.04).
We also repeated the analysis for the final model after excluding subjects with relatively severe cognitive impairment (MMSE score < 24; n = 74). The results did not change substantially. With a cutoff point of <26, the ORs for the different frequencies of green tea consumption were 1.00 (reference) for 3cups/wk, 0.55 (95% CI: 0.24, 1.27) for 4–6 cups/wk or 1 cup/d, and 0.44 (95% CI: 0.25, 0.78) for 2 cups/d (P for trend = 0.006). With a cutoff point of <28, the corresponding ORs were 1.00 (reference), 0.82 (95% CI: 0.47, 1.41), and 0.68 (95% CI: 0.46, 1.00) (P for trend = 0.05).
DISCUSSION
Our study showed inverse dose-response relations between consumption of green tea and the prevalence of cognitive impairment. In contrast, a weak or null relation between consumption of black or oolong tea or coffee and cognitive impairment was observed. To our knowledge, this is the first study to examine the association between consumption of green tea and cognitive function in humans.
Our study had several methodologic strengths. We recruited subjects from the general population, and a substantial variation was observed in the consumption of green tea among our subjects. We conducted a CGA that allowed us to carefully consider cardiovascular risk factors, which were causes of vascular dementia. Our study had a reasonably large sample size, which gave us the opportunity to test the association between consumption of green tea and various grades of cognitive impairment (from slight to relatively severe).
Several methodologic limitations should be considered in the interpretation of our results. First, our study had a cross-sectional design; therefore, no temporal relation between consumption of green tea and cognitive function can be inferred.
Second, our observational study design does not allow us to fully exclude the possibility of residual confounding by unmeasured factors. For example, healthier and more active individuals might have more opportunities to consume green tea. Among the Japanese, green tea is often consumed as a social activity, and this in itself may contribute to maintaining higher cognitive function (27). However, we controlled for many potential confounders, and the findings were robust to adjustments for these confounders.
Finally, because functional impairments of daily living were not fully assessed here, we cannot diagnose the presence or absence of dementia or the subtype of dementia syndromes, but we did evaluate cognitive impairment by using MMSE scores. However, cognitive decline is generally regarded as a core symptom of dementia. Furthermore, reduced cognition may be a key predictor of the development of dementia and may be considered a preclinical marker of the early stages of dementia (15, 16). Therefore, we believe that our data provide a useful clue to effective preventive interventions for dementia.
Green tea polyphenols, especially EGCG, might explain the observed association with improved cognitive function (7–10). Green tea is much richer in catechins than other beverages; Khokhar et al (28) reported that green tea contains 67.5 mg catechins/100 mL, whereas black tea contains only 15.5 mg/100 mL. The weak or null relations observed between consumption of black or oolong tea or coffee and cognitive impairment might reflect the important neuroprotective effects of catechins described in numerous experimental and animal studies (7–10). EGCG is brain permeable (29–31), and its neuroprotective and neurorescue effects were explained in terms of various mechanisms in addition to its well-established antioxidant and iron-chelating properties (7). These properties include modulation of cell survival and cell cycle genes (9) and promotion of neurite outgrowth activity (10). Furthermore, Levites et al (8) have shown that EGCG exerts neuroprotective and neurorescue effects against Aß toxicity by regulating the secretory processing of nonamyloidogenic APP through the protein kinase C pathway. In addition to the above-mentioned experimental and animal evidence, recent epidemiologic studies have suggested that red wine, which is also rich in polyphenols, may be associated with reduced risk of dementia (32, 33).
In addition to polyphenols, green tea contains vitamin C, caffeine, and other nutrients (34). Intake of vitamin C accompanied by high consumption of green tea might contribute to the observed association (3–6). Green tea contains 6 mg vitamin C/100 mL (10 g tea leaf/430 mL water, 90 °C, 1 min) (34) and is, in fact, the most common source of vitamin C (13.6%) among the population in our study region (35). Therefore, we cannot exclude a possible effect of vitamin C in the green tea on cognitive function. However, our results were not substantially changed even after adjustment for intake of nondietary vitamin C or E, indicating that the effects of vitamin C may be small. The contribution of caffeine to higher cognitive function also appears to be small because of the null relation observed between consumption of coffee and cognitive impairment. Green tea contains 0.02 g caffeine/100 mL (10 g tea leaf/430 mL water, 90 °C, 1 min), whereas coffee contains 0.06 g caffeine/100 mL (10 g coffee powder/150 mL water, 100 °C) (34). Nutrients in green tea other than polyphenols, vitamin C, and caffeine remain to be studied.
In conclusion, the present results suggest that higher consumption of green tea is associated with lower prevalence of cognitive impairment in humans. The results might partly explain the relatively lower prevalence of dementia, especially AD, in Japan than in Europe and North America (1). Given the high prevalence, worldwide rapid increase, and clinical significance of dementia (1, 2), any association between the intake of green tea, a drink with little toxicity and no calorific value, and cognitive function could have considerable clinical and public health relevance. The results of this cross-sectional study generate a new hypothesis and warrant further investigation.
ACKNOWLEDGMENTS
We thank all the participants of the Tsurugaya Project.
SK, AH, KO, and TS participated in the study design, data acquisition, data analysis, data interpretation, preparation of the written report, and final review of the report. TM, SE, SA, RN, HA, and IT participated in the study design, data acquisition, data interpretation, and final review of the report. None of the authors had any conflict of interest.
REFERENCES