Penn State Department of Family and Community Medicine, Penn State College of Medicine, Hershey (Dr. Malone); Torrance Memorial Physician Network, Calif (Dr. Tsai) malm0001@hotmail.com
The authors reported no potential conflict of interest relevant to this article.
Most tea leaves come from the Camellia sinensis bush, but green and black tea are processed differently to produce different end products.23 It is estimated that green tea accounts for approximately a quarter of all tea consumption, and is most commonly consumed in Asian countries.23 The health-promoting effects of green tea are mainly attributed to its polyphenol content.24 While there are many types of tea due to how they are processed, green tea has the highest concentration of polyphenols, including catechins, which are powerful antioxidants.23,24 Green tea has been used in traditional Chinese and Indian medicine to control bleeding, improve digestion, and promote overall health.23
Dementia. Green tea polyphenols may enhance cognition and may protect against the development of dementia. In-vitro studies have shown that green tea reduces hydrogen peroxide and beta-amyloid peptides, which are significant in the development of Alzheimer’s disease.25 A 12-subject double-blind study found green tea increased working memory and had an impact on frontoparietal brain connections.26 Furthermore, a cohort study with 13,645 Japanese participants over a 5-year period found that frequent green tea consumption (>5 cups per day) was associated with a lower risk of dementa.27 Additional studies are needed, but green tea may be useful in the treatment or prevention of dementia in the future.
Coronary artery disease.In one study, greentea plasma and urinary concentrations were associated with plasma biomarkers of cardiovascular disease and diabetes.28In one review, the consumption of green tea was associated with a statistically significant reduction in low-density lipoprotein cholesterol.29 Furthermore, a 2015 systematic review and meta-analysis of prospective observational studies concluded that increased tea consumption (of any type) is associated with a reduced risk of coronary heart disease, cardiac death, stroke, and total mortality.30
Cancer. Many studies have shown that green tea may reduce the risk of cancer development, although epidemiologic evidence is inconsistent. Studies have shown that cancer rates tend to be lower in those who consume higher levels of green tea.31,32 Whether this can be attributed solely to green tea remains debatable. Several other studies have shown that polyphenols in green tea can inhibit the growth of cancer cells, but the exact mechanisms by which tea interacts with cancerous cells is unknown.23
Many studies have shown that green tea may reduce the risk of cancer development, although epidemiologic evidence is inconsistent.
Several population-based studies have been performed, mostly in Japan, which showed green tea consumption reduced the risk of developing cancer. Fewer prostate cancer cases have been reported in men who consume green tea.33 While studies have been performed to determine whether green tea has effects on pancreatic, esophageal, ovarian, breast, bladder, and colorectal cancer, the evidence remains inadequate.32
Diabetes.Green tea has been shown in several studies to have a beneficial effect on diabetes. A retrospective Japanese cohort study showed that those who consumed green tea were one-third less likely to develop type 2 diabetes mellitus.34 A 10-year study from Taiwan found lower body fat and smaller waist circumference in those who consumed green tea regularly.35 A 2014 meta-analysis and systematic review of tea (any type) consumption and the risk of diabetes concluded that 3 cups or more of tea per day was associated with a lower risk of diabetes.36 Another meta-analysis that included 17 RCTs and that focused on green tea concluded that green tea improves glucose control and A1C values.37
Adverse effects
There have been concerns about potential hepatotoxicity induced bygreen teaintake.38 However, a systematic review of 34 RCTs on liver-related adverse events from green tea showed only a slight elevation in liver function tests; no serious liver-related adverse events were reported.38 Thisreview suggested that liver-related adverse events after intake ofgreen teaextracts are rare.38
A meta-analysis that included 17 RCTs found that green tea improves glucose control and A1C values.
Consuming green tea in the diet may lower the risk of adverse effects since the concentration consumed is generally much lower than that found in extracts.
Contraindications to drinking green tea are few. Individuals with caffeine sensitivities could experienceinsomnia,anxiety, irritability, orupset stomach. Additionally, patients who are taking anticoagulation drugs, such as warfarin, should avoid green tea due to itsvitaminK content, which can counter the effects of warfarin. Pregnant or breastfeeding women, those with heart problems orhigh blood pressure, kidney or liver problems, stomach ulcers, or anxiety disorders should use caution with green tea consumption.
The bottom line
Green tea consumption in the diet appears to be safe and may have beneficial effects on weight, diabetes mellitus risk, cancer risk, dementia, and cardiovascular risk. Patients may want to consider drinking green tea as part of a healthy diet, in combination with exercise.
