Overview
People have been drinking tea for thousands of years, and more recently a number of potential health benefits have been attributed to this ancient beverage. Black tea and green tea are made from the same plant, but a higher level of the original substances endure in the less-processed green form.
Uses and Applications
Green tea contains high levels of substances called catechin polyphenols, known
to possess strong antioxidant, anticarcinogenic, antitumorigenic, and even
antibiotic properties. Based on these findings and on observational studies, green
tea has become popular as a daily drink for preventing cancer and heart disease.
However, some observational trials failed to find indications of benefit with
green tea. Furthermore, only double-blind, placebo-controlled
studies can prove a treatment effective, and there is little
direct evidence of this type of study regarding green tea and cancer or heart
disease prevention.
One such study found that green teas produced short-term improvements in cholesterol profile, with the benefits disappearing after four weeks. More positive results were seen in a study that evaluated a form of green tea enriched with the substance theaflavin, which is found in black tea. In this fairly large (more than two hundred participants), three-month study, the use of the tea product resulted in significant, ongoing reductions in LDL (bad) cholesterol compared with placebo. A green tea extract enhanced with catechins has also shown promise for reducing LDL levels, according to one somewhat flawed double-blind study. However, a study involving catechin-enhanced green tea in Japanese children was less convincing.
Preliminary studies suggest that certain green tea polyphenols may help prevent skin cancer if they are applied directly to the skin. In addition, there is some evidence that green tea constituents might help protect the skin from sun damage. Unlike normal sunscreen preparations, green tea does not physically block ultraviolet light. Rather, it seems to protect cells from some of the damage caused by ultraviolet light. Because it works by such a different mechanism of action, green tea might offer synergistic benefits if combined with standard sunscreens. However, in an eight-week double-blind, placebo-controlled study of forty women who already had symptoms of aging skin, the combined use of oral green tea and a topical green tea cream failed to prove more effective than placebo. Some possible benefits were seen in the microscopic evaluation of the skin condition.
Topical green tea extracts have also shown some promise for the treatment of cervical dysplasia, while oral green tea extracts might reduce the risk of prostate cancer, according to a small pilot study. Combining the results of thirteen observational studies, researchers found conflicting evidence for green tea’s effect on the risk of stomach cancer. In a Japanese pilot study, green tea extract supplements lowered the risk of recurrent colorectal polyps. In a review of nine observational studies involving more than 5,600 cases of breast cancer, researchers failed to find reliable evidence for a reduction in the incidence of breast cancer. However, they did find weak evidence for a decrease in breast cancer recurrence among women who consumed more than three cups of green tea daily
On a completely different note, one study tested the effectiveness of gargling
with green tea catechins as a means of preventing influenza. In this double-blind,
placebo-controlled study, 124 residents of a Japanese nursing home gargled with
green tea catechins or placebo for three months. All participants received
standard influenza vaccine. The results showed that residents who gargled with the
tea extract were less likely to develop influenza than those using the placebo. In
addition, another double-blind study found preliminary evidence that oral
consumption of a green tea extract might help prevent colds and flu.
A small double-blind, placebo-controlled trial found weak evidence that green tea chew candy might reduce gum inflammation in persons with periodontal disease (gingivitis). Green tea extract has also shown some promise for treating borderline diabetes. However, one double-blind study failed to find that a combined extract of black and green tea was helpful for controlling blood sugar levels in people with type 2 diabetes. Green tea also has been proposed as a means of preventing liver disease, but the evidence remains unconvincing.
Green tea is sometimes recommended for weight loss on the basis of rather
theoretical evidence that it speeds up metabolism. However, there is little direct
scientific backing for this use. If green tea does increase metabolism, the effect
is extremely small. One study conducted in Thailand reported weight-loss benefits
with green tea, as did a second study of oolong tea enriched with green tea
extracts. However, a Dutch study failed to find green tea helpful for preventing
weight regain after weight loss. In another study, the use of green tea failed to
produce significant weight loss in overweight women with polycystic ovary
syndrome. Green tea extract enriched with catechins has done
somewhat better, enhancing weight loss in one substantial but flawed trial.
However, a study in overweight Japanese children did not support the effectiveness
of green tea catechins for weight reduction. Similar results were obtained in
another placebo-controlled trial involving seventy-eight overweight women after
twelve weeks of treatment.
One preliminary study found some evidence that green tea cream may be helpful
for the skin condition rosacea. The results of another study
weakly hint that green tea extracts taken orally might reduce symptoms of
benign
prostatic hyperplasia. One study found that inhaled tea
catechins could reduce levels of resistant staph carried in the sputum of disabled
elderly persons. One should not, however, attempt to inhale green tea
products.
Dosage
Studies weakly suggest that three cups of green tea daily might provide protection from cancer. However, because not everyone wants to take the time to drink green tea, manufacturers have offered extracts that can be taken in pill form. A typical dosage is 100 milligrams (mg) to 150 mg three times daily of a green tea extract standardized to contain 80 percent total polyphenols and 50 percent epigallocatechin gallate. Whether these extracts offer any benefit remains unknown. Furthermore, there are growing concerns about liver toxicity with the use of green tea extracts. In an analysis performed in 2006, some tested green tea products were found to be contaminated with lead.
Safety Issues
As a widely consumed beverage, green tea is generally regarded as safe. It does
contain caffeine, at perhaps a slightly lower level than black tea,
and can therefore cause insomnia, nervousness, and the other well-known symptoms
of excess caffeine intake.
Green tea extracts, however, may not be safe. There are a growing number of
case reports in which the use of a concentrated green tea extract was associated
with liver inflammation. In most cases, liver problems disappeared after the
extract was discontinued, but in two cases, permanent liver failure ensued,
requiring liver
transplantation. While it is not certain that the green tea
extract caused the liver problems, or how it might do so, these reports do raise
significant concerns about the use of green tea extracts, especially by those with
liver disease or those who are prone to it.
Green tea should not be given to infants and young children. There are theoretical concerns that high dosages of epigallocatechin gallate might be unsafe for pregnant women.
Dried green tea leaf contains significant levels of vitamin K on a per-weight basis. On this basis, it has been stated that people using blood thinners in the warfarin (Coumadin) family should avoid green tea, because vitamin K antagonizes the effect of those drugs. However, green tea taken as a beverage provides such small amounts of the vitamin that the risk seems minimal for normal consumption. There is one case report of problems that developed in a person on warfarin who consumed as much as 1 gallon of green tea daily.
Important interactions. The caffeine in green tea could cause
serious problems in persons who are taking monoamine oxidase
inhibitors. One should avoid drinking large quantities of
green tea if also taking warfarin. Finally, green tea may decrease the absorption
of folic
acid into the bloodstream.
Bibliography
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Hsu, C. H., et al. “Effect of Green Tea Extract on Obese Women.” Clinical Nutrition 27 (2008): 363-370.
Katiyar, S. K., N. Ahmad, and H. Mukhtar. “Green Tea and Skin.” Archives of Dermatology 136 (2000): 989-994.
Liu, J., J. Xing, and Y. Fei. “Green Tea (Camellia sinensis) and Cancer Prevention.” Chinese Medicine 3 (2008): 12.
Mackenzie, T., L. Leary, and W. B. Brooks. “The Effect of an Extract of Green and Black Tea on Glucose Control in Adults with Type 2 Diabetes Mellitus.” Metabolism 56 (2007): 1340-1344.
Matsuyama, T., et al. “Catechin Safely Improved Higher Levels of Fatness, Blood Pressure, and Cholesterol in Children.” Obesity 16 (2008): 1338-1348.
Myung, S. K., et al. “Green Tea Consumption and Risk of Stomach Cancer.” International Journal of Cancer 124 (2009): 670-677.
Ogunleye, A. A., F. Xue, and K. B. Michels. “Green Tea Consumption and Breast Cancer Risk or Recurrence.” Breast Cancer Research and Treatment 119 (2010): 477-484.
Rowe, C. A., et al. “Specific Formulation of Camellia sinensis Prevents Cold and Flu Symptoms and Enhances T Cell Function.” Journal of the American College of Nutrition 26 (2007): 445-452.
Sarma, D. N., et al. “Safety of Green Tea Extracts: A Systematic Review by the U.S. Pharmacopeia.” Drug Safety 31 (2008): 469-484.
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