Green tea as a preventive measure
Green tea is a natural beverage made from the leaves of the Camellia sinensis plant, known for its potential health benefits, particularly as a preventive measure against various diseases. Rich in catechin polyphenols, green tea is touted for its antioxidant and potential anticancer properties. It has been suggested as a preventive agent for conditions such as cancer, cardiovascular diseases, diabetes, and gingivitis, with some studies indicating positive effects on cholesterol levels and immune function. However, while observational studies have shown promise, the evidence from double-blind, placebo-controlled trials remains inconclusive for many of these benefits.
Additionally, topical applications of green tea extracts might protect against skin damage from UV radiation and conditions like rosacea. Despite its popularity, there are safety concerns regarding concentrated green tea extracts, which have been linked to liver toxicity and other health issues. Regular consumption of green tea as a beverage is generally considered safe for most individuals, although caution is advised for certain populations, including those on blood thinners or with existing liver issues. Overall, while green tea may offer various health benefits, further research is necessary to establish its effectiveness as a preventive measure.
Green tea as a preventive measure
- DEFINITION: Natural plant product consumed for specific health benefits.
- PRINCIPAL PROPOSED USES: Cancer prevention, gingivitis
- OTHER PROPOSED USES: Cardiovascular health, cognitive function, diabetes, heart disease prevention, high cholesterol, influenza prevention, liver disease prevention, rosacea, sun damage protection, weight loss
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.
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 a 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 has also 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 older individuals with disabilities. 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. A 2020 update of a 2012 study by the National Institute of Diabetes and Digestive and Kidney Diseases linked the ingestion of green tea extract or large amounts of green tea to acute liver injury, including the possibility of liver failure.
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 leaves contain 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.
The temperature of green tea when consumed as a beverage should be monitored, as some studies indicate drinking hot tea can lead to an increased risk of esophageal cancer.
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.
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