Although tea is believed to have been around for 50,000 years, the first known report of its use for its medicinal value was during the Han Dynasty (206 BC to 220 AD) where the Materia medica (Ben Cao Jing) listed tea as “an antidote to herbal poisons, as a cure for swelling and abscesses in the head, and as a sleep inhibitor.” Traditional Chinese and Indian medicine used the tea as a stimulant, astringent and diuretic, and to improve cardiovascular health.
All about tea
From the perennial shrub, Camellia sinensis, tea comes in three main varieties: green, black and oolong. Green tea―including varieties such as Matcha, Gunpowder, Hyson, Sencha and Dragon Well―is the only one of the three main types that doesn’t undergo fermentation, as upon harvesting, the leaves are immediately steamed or pan-fired to prevent fermentation.
As such, green tea leaves have been found to have the highest levels of polyphenols, which are powerful antioxidants and potential disease fighters. The slight bitter flavor in green tea comes from the catechins, a subgroup of polyphenols. The primary catechin compounds found in green tea include gallaogatechin (GC), epicatechin (EC), epigallocatechin (EGC), epicatechin gallate (ECG) and epigallocatechin gallate (EGCG). EGCG is thought to be the most potent physiologically and is more frequently studied. By weight, catechins account for 15% to 30% of dried green tea leaves, as opposed to 8% to 20% of oolong and 3% to 10% of black tea.
An average cup contains between 50 to 150 mg of polyphenols. Decaffeinated varieties also contain concentrated amounts of polyphenols. Two to three cups per day is suggested for health benefits (for approximately 240 to 320 mg of polyphenols) or 100 to 750 mg per day of green tea extract. Green tea extract is composed mostly of catechins (specifically EGCG), and can be found in tablet, capsule or liquid form.
Green tea also contains alkaloids, including caffeine, theobromine and theophylline, that provide stimulant and possibly thermogenic effects. Vitamin C, various energy-metabolizing B-vitamins, vitamin E and fluoride can also be found in green tea.
Medication and supplement interactions with green tea are well documented, and a physician should be consulted before taking green tea supplements in any form or drinking green tea. Counterindications of consumption include cardiovascular, renal and psychological disorders. Drug interactions include, but are not limited to, beta-blockers, blood thinners (including aspirin), chemotherapy medications, oral contraceptives and momoamine oxidase inhibitors (MAOIs). Children and pregnant and lactating women should avoid green tea supplements. During pregnancy and lactation, caffeine intake should be monitored, as caffeine is known to cross the placenta, and can draw out iron in infants, leading to iron deficiency.
Studying health benefits
Throughout history, green tea has been consumed for its presumed medicinal effects. Today, extensive studies indicate that green tea may play a role in reducing risk of cardiovascular disease and cancer, promoting weight loss, and improving cognitive functions.
A 2006 study published in the Journal of the American Medical Association (296(10):1,255-1,265) investigated the association between green-tea consumption and mortality rate in Japanese adults due to various diseases, including cardiovascular disease. An initial cohort of 40,530 followed over 11 years indicated an inverse relationship between green tea consumption and mortality rate from cardiovascular disease, especially for stroke mortality. A second study published the same year in Nutrition Research (26(11):604-607) examined the effects of green tea supplementation on serum lipid profiles. Significant blood lipid changes resulted, including an average 8.9% decrease of low-density lipoprotein cholesterol in 90% of the subjects, and a 4% increase of high-density lipoprotein cholesterol in 69% of the subjects. Although numerous studies link green tea’s antioxidant properties to cardiovascular health, in May 2006 FDA concluded that there was insufficient credible evidence to support the health claim that green tea or green tea extract help reduce the risk of heart disease.
An association between green tea and tumor growth prevention in the lungs, breast, skin and prostate has been documented, as well. A 2009 study published online in Cancer Prevention Research found that men who took polyphenon E (green tea extract) supplements prior to radical prostatectomy had significant reductions in serum levels of hepatocyte growth factor, vascular endothelial growth factor and prostate specific antigen. Other large population studies have linked green tea to a reduction in other cancers, including stomach, colon and esophageal.
Several studies have linked weight and fat loss to the thermogenic components found in green tea, namely EGCG and caffeine. A study published in International Journal of Obesity & Related Metabolic Disorders (2000; 24(2):252-258) found that green tea extract may increase energy expenditure and fat oxidation, while a second study, published in The American Journal of Clinical Nutrition (1999; 70(6):1,040-1,045), found that men who took green tea extract containing EGCG and caffeine burned about 80 calories more per day compared with those who did not.
Improved cognitive function has also been suggested. A study published in The American Journal of Clinical Nutrition (2006; 83(2):355-361) examined green tea consumption among 1,003 Japanese subjects over 70 years of age and found high consumption was associated with a lower prevalence of cognitive impairment. New research also shows green tea may help prevent dental caries, inhibit viral action, and preserve food.
Toby Amidor is a registered dietitian with 10 years experience in the field as a culinary school educator, food safety consultant and clinical practitioner. She has consulted for various online sites and is currently the expert for healthyeats.com. She can be reached at firstname.lastname@example.org.