Green tea (Camellia sinensis, Theaceae) leaf contains catechin polyphenols that are reported to have neuroprotective effects in various assays, including the reduction of oxidative stress and inflammation, inhibition of beta-amyloid aggregation, and prevention of apoptosis. Epidemiological studies have reported a correlation between green tea consumption and decreased risk of neurodegenerative disease and cognitive impairment. The purpose of this systematic review was to evaluate the association between green tea beverage intake and risk of dementia, Alzheimer’s disease (AD), and cognitive impairment.
PubMed was searched from inception through August 23, 2018. Included articles met these criteria: (1) English-language article published after 1966, (2) included free-living (non-institutionalized) populations, (3) reported original data on the relationship between green tea intake and risk of dementia, AD, mild cognitive impairment (MCI), or cognitive impairment, and (4) were cohort, case-cohort, nested case-control, case-control, or cross-sectional studies. Studies that evaluated green tea in combination with other factors such as metabolic enzyme activity or other dietary factors were excluded. Reference lists of included articles also were screened. The reporting quality of each included study was assessed using the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) checklist. The methodological quality was judged by various other factors.
A total of 81 articles were located, and eight articles met the eligibility criteria. A meta-analysis was precluded due to the high degree of heterogeneity among studies and limited number of high-quality studies. Therefore, only a qualitative assessment was conducted. The eight included articles reported the results of three cohort longitudinal studies conducted in Germany (n = 1) and Japan (n = 2) and five cross-sectional observational studies conducted in China (n = 2), Taiwan (n = 1), and Japan (n = 2). Together the studies included a total of 37,578 participants (723 to 13,645 participants per study). The categorization of green tea consumption varied significantly among studies and included: (1) < 1 cup/day, 1-2 cups/day, 3-4 cups/day, or ≥ 5 cups/day compared with no consumption; (2) consumption versus no consumption; (3) 1-6 days/week compared with no consumption; (4) 1-6 days/week or > 7 times/week compared with no consumption; and (5) ≤ 3 cups/week, 4-6 cups/week, 1 cup/day, or ≥ 2 cups/day.
The STROBE assessments indicated one study was high quality, two were medium quality, and five were low quality. The only high-quality study reported that high green tea intake was significantly associated with a low hazard ratio for dementia. The results of the two medium-quality studies were conflicting; one study found green tea consumption was not correlated with AD risk, while the second study found green tea intake was associated with a significantly lower risk of MCI or dementia (although the odds ratio of developing only dementia did not change with green tea consumption). The five low-quality studies reported: (1) green tea consumption decreased the odds ratio of MCI only in men aged 55 to 65 years old; (2) green tea consumption had no effect on the odds ratio for cognitive impairment; (3) green tea consumption decreased the risk of cognitive impairment (two studies); and (4) green tea consumption significantly lowered the odds ratio for all-cause dementia. Safety was not assessed.
Overall, six of the eight studies reported a preventive effect associated with green tea consumption. Based on this evidence, the authors concluded that “it is possible that green tea intake might prevent the incidence of dementia, AD, MCI, or cognitive impairment.” They point out that the inconsistent study results may be due to heterogeneous classification of green tea consumption.
Limitations of this review are that only one database was searched, only articles published in English were included, few studies were eligible, and many of the included studies were poor quality. The authors noted that to draw a solid conclusion regarding the impact of green tea consumption on the risk of developing dementia, AD, MCI, or cognitive impairment, further data from “well-designed and well-conducted cohort studies are well founded and required.”