Unlike black tea, green tea is made from unfermented leaves and reportedly contains the highest concentration of powerful antioxidants called polyphenols. Antioxidants are substances that scavenge free radicals, which are compounds that negatively affect cells, damage DNA, and may even lead to cell death. Free radicals have been implicated in processes like aging in addition a number of health problems, including cancer and heart disease. Antioxidants such as polyphenols in green tea can destroy free radicals thereby reducing or even preventing some of the damage they cause.
Green tea has been extensively studied in animals, humans, and laboratory experiments. Results from these studies suggest that green tea may help treat the following health conditions:
The antioxidant power of green tea may, according to some studies, be able to help prevent atherosclerosis, particularly coronary artery disease. This information is based upon population-based studies, which are studies that follow large groups of people over time or studies that compare groups of people living in different cultures or with different diets. The mechanism by which green tea reduces the risk of heart disease is still not fully elucidated, but it is claimed that the rate of heart attack decreases by 11% with consumption of 3 cups of tea per day.
Some research shows that green tea lowers total cholesterol and raises HDL ("good") cholesterol in both animals and people. It is postulated from animal studies that polyphenols in green tea may block cholesterol from being absorbed in the intestine and also help the body get rid of cholesterol. In another small study of male smokers, researchers found that green tea significantly reduced blood levels of LDL (“bad”) cholesterol.
Several population-based clinical studies have shown that green as well as black teas may help protect against cancer. For example, cancer rates tend to be low in countries such as Japan where people regularly consume green tea. This however does not constitute conclusive evidence that green tea actually prevents cancer in people.
Bladder cancer. One study compared people with and without bladder cancer; it was found that women who drank black tea and powdered green tea were less likely to develop bladder cancer. A follow-up clinical study by the same group of researchers revealed that people with bladder cancer, namely men who drank green tea, had a more favorable 5-year survival rate than those who did not.
Breast cancer. Clinical studies in animals and lab studies suggest that polyphenols in green tea inhibit the growth of breast cancer cells. It is also claimed that women with early stages of the disease who drank at least 5 cups of tea every day before being diagnosed with cancer were less likely to have relapse after end of treatment. However, women with late stages of breast cancer had little or no improvement from drinking green tea.
Ovarian cancer. In a clinical study done with ovarian cancer patients in China, researchers found that women who drank at least one cup of green tea per day lived longer with the disease than those who didn’t drink green tea; other studies could not replicate the same beneficial effects.
Colorectal cancer. Conflicting results have come out from studies on the effects of green tea on colorectal cancer, as some studies show decreased risk in those who drink the tea, while others show increased risk.
Esophageal cancer. Studies in laboratory animals have found that green tea polyphenols inhibit the growth of esophageal cancer cells. On the other hand, studies in people have produced conflicting findings.
Lung cancer. While green tea polyphenols have been shown to inhibit the growth of human lung cancer cells in the lab, few clinical studies have looked at the link between drinking green tea and lung cancer in people.
Pancreatic cancer. In one large-scale clinical study researchers compared green tea drinkers with non-drinkers and found that those who drank the most tea were less likely to develop pancreatic cancer. This was more prominent in women. Again, this is not clear-cut evidence as other factors might have been responsible for lowering pancreatic cancer risk.
Prostate cancer. Lab studies have shown that green tea extracts prevent the growth of prostate cancer cells in test tubes. In a large clinical study in Southeast China researchers found that the risk of prostate cancer was reduced with increasing frequency, duration and quantity of green tea consumption. Conversely, both green and black tea extracts also stimulated genes that decrease cell sensitivity to chemotherapy drugs.
Skin cancer. The main polyphenol in green tea is epigallocatechin gallate (EGCG). Scientific studies suggest that EGCG and green tea polyphenols have anti-inflammatory and anticancer properties that may help prevent the development and growth of skin tumors.
Stomach cancer. Lab studies have found that green tea polyphenols inhibit the growth of stomach cancer cells in test tubes, but studies in people have produced less reliable findings.
Inflammatory Bowel Disease (IBD)
Green tea may help reduce inflammation associated with Crohn's disease and ulcerative colitis, the two types of IBD.
Traditionally, green tea has been used to control blood sugar levels. Animal studies suggest that green tea may help prevent the development of type 1 diabetes and slow the progression once it has developed. Some small clinical trials have found that taking a green tea extract daily lowered the hemoglobin A1c level in people with borderline diabetes.
Population-based clinical studies have demonstrated that men who drink more than 10 cups of green tea per day are less likely to develop liver problems. Green tea also seems to protect the liver from the damaging effects of toxic substances such as alcohol. Results of a number of animal and human experiments support the idea that one of the polyphenols in green tea, known as catechin, may help treat viral hepatitis. The downside is that 10 cups of green tea a day could cause problems because of the high level of caffeine consumed.
Clinical studies suggest that green tea extract may boost metabolism and help burn fat. Some scientists believe that it’s the catechins that are responsible for the herb's fat-burning effect.
One small study found that drinking green tea helped prevent dental cavities. More studies need to be done. Green tea may also be useful in inflammatory diseases, such as arthritis. Research suggests that green tea may help arthritis by reducing inflammation and slowing the breakdown of cartilage. Chemicals in green tea may also be help treat genital warts and prevent symptoms of colds and flu. Studies also show that drinking green tea is associated with reduced risk of dying from any cause.