Tea, which was probably first brewed as a beverage in China around 2700 BCE, is one of the oldest and (after water) the second most consumed drink in the world. Camellia sinensis is an evergreen shrub with shiny, bright green leaves; strongly scented flowers; and brown-green seed-bearing fruit, used to produce tea. It has more than 1500 cultivars, derived mainly from two varieties: C sinensis var. assamica, an Indian, single-stem plant with large, soft, and short-lived leaves, and C sinensis var. sinensis, a Chinese multiple-stem shrub with smaller leaves that are hardier in cooler temperatures. It grows in generally warm and humid climates, preferably in acidic soils, on sloping hills at elevations of up to 2000 meters.
After becoming popular in Europe, tea was spread widely by the forces of colonialism, and large plantations were established in India, Sri Lanka, Africa, and Indonesia. Today, the biggest tea-producing countries are China (1.9 million tons in 2013, or 38% of the world's total), India (1.2 million tons), Kenya (436,000 tons), and Sri Lanka (343,100 tons). Around the world, many cultures celebrate the drink for its contributions to social cohesiveness, flavor, and potential health benefits. And we drink a lot of tea—4.8 million tons worldwide in 2013; China (1.6 million tons), India (1 million tons), and Turkey (228,000 tons) led the way, whereas Americans drank 127,000 tons. The tea industry was worth an estimated $12.5 billion in the United States in 2017.
What's Your Cup of Tea?
Six types of tea come from the C sinensis plant: white, green, yellow, black, oolong, and pu'erh (Table).
Table. Tea Types[4,5,6,7]
Leaf buds are picked before opening and dried at low temperatures. Least processed of all teas. Most delicate flavors, with highest quantity of antioxidants.
Yin Zhen (Silver Needle)
Bai Mu Dan (White Peony)
Bai Mei (White Eyebrow)
Steamed or heated immediately after plucking to stop leaf oxidation and browning. More intense flavors than white, but milder than black; many vegetal flavors.
Longching (Dragon Well)
A rare variety; dried longer than green tea. Leaves turn yellow and have a grassy flavor. Made in small quantities, making yellow teas expensive and hard to find.
Meng Ding Huang Ya
Jun Shan Yin Zhen
Huo Shan Huang Ya
Oxidized longer for stronger flavor. Processing of leaves lowers flavonoid content. Scented varieties are made with added flavors.
Earl Grey (bergamot)
Partially oxidized; somewhere between a green and black tea. The amount of oxidation determines the flavor.
A strong, earthy tea that is fermented (unlike other teas) to mature after oxidation; compressed into bricks or cakes and buried or shelved for many years before drinking.
Loose-leaf Black Pu'erh
From industrial to artisanal, the production and consumption of tea vary tremendously in scale and execution. According to the International Specialty Tea Association, tea quality depends on numerous factors, including the cultivar and condition of the shrub, expertise, whether the leaves are broken when picked, style of plucking and leaf conformity, harvest date, quality and type of processing, moisture, and oxidation. These factors—not to mention how the tea is brewed—will affect the taste and quality of the beverage and, quite possibly, its therapeutic qualities.
Therapeutic Effects of Tea
The potential health benefits of tea consumption have been the subject of thousands of studies, many of which have examined the role of polyphenols (eg, epigallocatechin gallate in green tea, theaflavins and thearubigins in black tea). Polyphenols (including flavonoids) are a class of phytochemicals believed to give tea (as well as coffee, certain vegetables, fruits, and grains) its antioxidative properties, flavor, color, and smell. These strong antioxidants may reduce the oxidation of low-density lipoprotein (LDL) cholesterol and lower the risk for heart disease. They may also have anticarcinogenic effects stemming from their potential to mediate the oxidation of DNA and by inducing glucuronosyltransferases, thereby helping to eliminate toxicants and carcinogens. Tea polyphenols may also promote favorable intestinal bacterial flora and inhibit reactive oxygen species associated with age-related diseases.
Currently, some of the most promising avenues for research on tea consumption are related to the positive effects of flavonoids on coronary artery disease and stroke. Total intake of flavonoids and flavones is associated with lower risks for fatal cardiovascular disease.
Grassi and colleagues evaluated how flavonoids found in tea may affect flow-mediated dilation and help counter endothelial dysfunction, an early phase in the pathogenesis of atherosclerosis. Although these effects are not completely understood, current evidence suggests that moderate tea drinking may enhance endothelium-dependent vasodilation and help explain the positive benefits of tea on cardiovascular health.
In lab animals, tea has been shown to protect against lead- and cadmium-induced oxidative stress, which appears to support the hypothesis that tea may increase the body's oxidative capacities. Another study examined the effects of tea polyphenols on oxidative stress, and whether the circadian clock could explain the protective effect. Tea polyphenols were shown to ameliorate redox imbalance and mitochondrial dysfunction in hepatocytes.
Ide and colleagues reviewed several well-conducted studies that support the promising antioxidative and anti-inflammatory effects of tea catechins on oxidative stress, considered a key component of the pathologic mechanism underlying Alzheimer disease.
In a large population-based case-control study, Yang and colleagues concluded that drinking very hot tea significantly increased the risk for esophageal squamous cell carcinoma in a sample of high-risk Chinese men, especially in those who also consume alcohol. In a related editorial, Cronin-Fenton posited that it is plausible that the observed increased esophageal cancer risk is attributable to thermal injury caused by drinking very hot tea rather than to the tea per se, and encourages tea drinkers to allow their tea to cool before consuming. The International Agency for Research on Cancer's view is that consuming hot beverages of any type is "probably carcinogenic."
Caffeine exposure from tea. All types of tea derived from the C sinensis plant contain caffeine; however, various factors (eg, tea leaf processing, tea type, brew method, and strength) affect the amount of caffeine in tea. The most widely reported estimates reveal that the lowest amount of caffeine (per 8 oz) is found in the yellow and white teas (30-55 mg) followed by green (35-70 mg), oolong (50-75 mg), and black teas (60-90 mg). The same size cup of coffee contains about 100 mg caffeine. No recent studies have focused on the harms of caffeine exposure from tea per se; rather, concerns about the safety of caffeine to date seem to be tied primarily to the dose of caffeine, regardless of source.
The active compounds in green tea are polyphenols known as catechins. The most abundant polyphenol in green tea is epigallocatechin gallate, which may inhibit the action of the reactive oxygen species molecule, thereby preventing oxidative damage.
Varying amounts of green tea consumption were significantly associated with lower risks for cardiovascular disease, myocardial infarction, stroke, intracerebral hemorrhage, cerebral infarction, and elevated LDL levels. A plausible explanation for the preventive effects on cardiovascular disease involves the antioxidative and anti-inflammatory properties of green tea. Xiang and colleagues in a retrospective study showed that frequently drinking small amounts of green tea was associated with a reduced risk for coronary heart disease in women—but not in men.
Neurodegeneration and Cancer
Roy and Bhat found that green tea polyphenols suppress, disaggregate, and modulate γ-Synuclein fibrillation, which has an important potential role in Parkinson disease. Their research supports the possible beneficial effects of green tea against neurodegeneration, while demonstrating that EGCG-generated oligomers may reduce the viability of neuroblastoma cells but protect breast cancer cells from γ-Syn toxicity.
Schröder and colleagues found that epigallocatechin gallate and quercetin (in extracted forms and as found naturally in green tea) have anticarcinogenic effects on both estrogen receptor-positive and -negative breast cancer cells.
Yang and colleagues reported that green tea polysaccharides decreased microRNA-93—a potential therapeutic target for prostate cancer—and inhibited the growth of prostate cancer cells.
Wang and colleagues investigated the antioxidant effects of green tea polyphenols in preventing hyperuricemia, which leads to preglomerular arteriopathy and chronic kidney disease. They discovered that green tea polyphenols protect against the progression of chronic kidney disease by activating the Jagged1/Notch1-STAT3 pathway.
Whether the consumption of green tea has any true effect on the risk for kidney stones remains unanswered.
In prospective cohorts comprising 58,054 men and 69,166 women, Shu and colleagues found that drinking green tea was associated with a lower risk for self-reported incident kidney stones, an effect that was stronger in men.
In contrast, Wu and colleagues found that tea consumption was a risk factor for the development of kidney stones, although participants in their study drank a combination of green, black, and scented teas.
High levels of calcium oxalate in the urine are associated with a higher risk for kidney stone formation. Green tea contains much lower levels of oxalate and a higher concentration of epigallocatechin gallate, which acts to hinder the formation of kidney stones. (Black tea contains high levels of oxalate and has been shown to increase urinary oxalate concentrations when consumed regularly, prompting recommendations that black tea be eliminated from the diets of people who are prone to form kidney stones).
Other Potential Benefits of Green Tea
Periodontal disease prevention. Green tea promotes periodontal health by reducing inflammation, preventing bone resorption, and inhibiting the growth of certain bacteria associated with periodontal diseases. Gartenmann and colleagues, while noting the heterogeneous nature of the data, found that as an adjunct to scaling and root planing in periodontics, local application of green tea catechin may result in beneficial reduction of probing pocket depth. In addition to its systemic benefits, the discovery of the topical benefits of green tea has led to the marketing of green tea toothpaste.
Stress reduction. L-theanine—a major amino acid in matcha—has been shown to have stress-reducing effects. Matcha also contains an abundance of caffeine, which antagonizes the effects of L-theanine. Therefore, the relative concentrations of these and other components (epigallocatechin gallate and arginine) determine the effectiveness of matcha in stress reduction.
A bioefficacy trial by Imran and colleagues showed a reduction of LDL cholesterol and triglycerides and a significant increase in high-density lipoprotein cholesterol in rats administered black tea polyphenols (theaflavins and thearubigins). The study points to the potential for black tea polyphenols to alleviate hypercholesterolemia and hyperglycemia, with potential benefits in also combating diabetes.
Elevated homocysteine levels are believed to be associated with cardiovascular and cerebrovascular disease. Zhu and colleagues found a significant correlation between drinking black tea and hyperhomocycsteinemia in patients with hypertension. The effect was not found in people who drank oolong or green teas.
Black tea polyphenol (theaflavin-3,3'-digallate [TF3]) in combination with cisplatin has shown a synergistic cytotoxic effect on specific cisplatin-resistant human ovarian cancer cells, indicating that TF3 may have potential as an adjuvant for the treatment of advanced ovarian cancer.
Neurodegenerative Disorders and Vascular Health
Isolated microbial metabolites (L-theanine, CDT-1, and CDT-2) extracted from dark tea helped decrease age-related neurodegenerative disorders in mice.
A long-standing debate about milk and tea is not just about whether the milk should be added to the cup first or vice versa. More than a decade ago, a small study suggested that adding milk to black tea negates the vascular benefits of tea, probably by promoting the formation of complexes with tea catechins. In a small recent study, Ahmad and colleagues also found that adding milk to black tea alters the favorable impact of tea consumption on vascular function and blood pressure—for as-yet unknown reasons.
In a recent study, six breast cancer cell lines were treated with varying concentrations of oolong, green, black, and dark tea extracts. Investigators studied the effect of these tea extracts on cell viability, cell morphology, and DNA damage and cleavage, among other factors. Along with green tea, oolong tea extract induced DNA damage and cleavage; played an inhibitory role in breast cancer cell growth, proliferation, and tumorigenesis; and demonstrated potential as a chemopreventive agent against breast cancer.
No Firm Conclusions Yet
Current evidence points to the many potential benefits of tea drinking—some of which appear to be associated with its antioxidative properties, whereas others may simply coincide with a healthy lifestyle. Nevertheless, the uneven quality of the data and the diverse types of studies make it difficult to draw firm conclusions. Which teas have the strongest health benefits (or risks) and why remains unknown. Researchers still need to address many facets surrounding tea and health, including the ideal brewing methods; the addition of substances; the frequency, amount, and duration of consumption; and whether a tea leaf's health potential declines as it ages. Furthermore, a significant proportion of studies were conducted in Asian populations, where tea drinking is widespread. Future research should include more diverse populations of tea drinkers.