People have been drinking tea for thousands of years, but only in the last couple of decades have we begun to document the potential health benefits of this ancient beverage. Both black and green tea are made from the same plant, but more of the original substances endure in the less-processed green form. Green tea contains high levels of substances called polyphenols, known to possess strong antioxidant, anticarcinogenic, antitumorigenic, and even antibiotic properties.
A growing body of evidence in both human and animal studies suggests that regular consumption of green tea can reduce the incidence of a variety of cancers, including colon, pancreatic, and stomach cancers. Green tea might also help prevent heart disease.
Based on the widely publicized results of observational studies, as well as basic research on its constituents, green tea has become popular as a daily drink for cancer prevention. There is some evidence that it may be helpful for preventing heart disease as well. However, the evidence used to arrive at these conclusions is weak, and not all studies agree but some studies have found evidence that it is ineffective.
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 and sunburn. Unlike normal sunscreen preparations, green tea does not physically block ultraviolet light. Rather, it seems to protect cells from damage. Because it works by such a different mechanism of action, green tea might offer synergistic benefits if combined with standard sunscreens.
A small double-blind placebo-controlled trial found extremely weak evidence that green tea chew candy might reduce gum inflammation in individuals with periodontal disease (gingivitis).
Green tea has also been proposed as means of preventing liver disease, but the evidence for this use remains unconvincing.
Green tea is often recommended for weight loss, but there is little scientific backing for this use.
Very preliminary evidence suggests that black tea, which is quite similar but not identical to green tea, may help protect against osteoporosis and may help prevent atherosclerosis.
Studies suggest that 3 cups of green tea daily provide protection against 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 to 150 mg 3 times daily of a green tea extract standardized to contain 80% total polyphenols and 50% epigallocatechin gallate. Whether these extracts work as well as the real thing remains unknown.
As a widely consumed beverage, green tea is generally regarded as safe. It does contain caffeine, although at a lower level than black tea or coffee, and can therefore cause insomnia, nervousness, and the other well-known symptoms of excess caffeine intake. Green tea should not be given to infants and young children.