Ng TP, Feng L, Niti M, Kua EH, Yap KB. Tea consumption and cognitive impairment and decline in older Chinese adults. Am J Clin Nutr. Jul 2008;88(1): 224-231.

Tea (Camellia sinensis) is associated with many beneficial effects, including cardioprotective, anti-cancer, and neuroprotective properties. In this study the authors have analyzed data on tea consumption and cognitive decline and impairment in older Chinese adults. Cognitive impairment is an early warning sign of Alzheimer's disease. The three main types of tea are black tea (from Chinese grown tea or from British tea cultivated in India, Sri Lanka, and Africa; full fermented/oxidized), oolong tea (partially fermented), and green tea (non-fermented) which is picked and quickly heated by steaming or pan frying. Black tea accounts for roughly 72% of the world's total tea production.

The researchers analyzed cross-sectional and longitudinal data concerning a cohort of Chinese adults over the age of 55 participating in the Singapore Longitudinal Aging Studies (SLAS). The subjects completed a Chinese language version of the Mini-Mental State Examination (MMSE) at baseline at and 1-2 years later (median: 16 months). Information on the subjects' consumption of Ceylon or English black tea (consumed with milk), Chinese black or oolong tea (consumed without milk), and green tea, and data on diet, health status, and other risk factors were gathered. The frequency of tea consumption was coded on a scale of 0-6, from "never to rarely" to 10 cups or more per day. Coffee intake and cognitive decline was also analyzed for comparison.

There were 2,501 subjects in the baseline survey, and 1,438 subjects without cognitive impairment at baseline participated in the follow-up survey. The majority of the participants drank oolong or Chinese black tea, while fewer participants drank green tea or English black tea. In the univariate analysis, there was a significantly lower incidence of cognitive impairment in high frequency drinkers of all types of tea (P<0.001). In the multivariate cross-sectional analysis, a higher level of tea consumption was linked to a lower risk of cognitive impairment in 4 hierarchical logistic regression models that adjusted for confounding factors including age, body mass index, physical activities, and more (P<0.001 for all odds ratio [OR]: 0.14-0.37). Consumption of black tea or oolong tea only was associated with significantly lower OR of cognitive impairment in the cross-sectional analysis (OR: 0.55, P<0.001) and cognitive decline in the longitudinal analysis (OR: 0.69, P<0.02). There were very few green tea drinkers; nevertheless green tea consumption was significantly associated with a lower risk of cognitive impairment (OR. 0.42, P=0.001), but not cognitive decline, when compared to non-tea drinkers. Daily consumption of black or oolong tea was linked to lower risk of cognitive impairment (OR: 0.46, P<0.001) and decline (OR: 0.68, P=0.017) compared to non-tea drinkers. There were no associations between coffee intake and cognitive impairment or decline.

The authors conclude that regularly drinking tea, particularly black or oolong tea, is associated with a lower risk of cognitive impairment and decline. There are many chemical constituents in both green and black tea that could contribute to this effect, and "it is possible that the cognitive protective effect of tea is not due to a single compound but rather to the synergistic effects of several or many of its chemical components." The researchers comment that, because there was no association between coffee intake and cognitive impairment or decline in this study, it is less likely that caffeine contributes to the cognitive protective effects. They also state that more research is needed to determine if there is a link between tea consumption and a lower risk of vascular dementia and/or Alzheimer's disease.