There are catechins and thermogenic properties in green and oolong teas that can increase metabolism and speed up fat oxidation. One cup of green tea can increase metabolism by 4 percent for 1 and 1/2 hours. One cup of oolong tea can increase metabolism by 10 percent for 1 and 1/2 hours.
Green tea is also an antioxidant that is rich in compounds called polyphenols that reduce the growth of new blood vessels that feed tumors. It is also a good detoxifier that activates enzymes in the liver that eliminate toxins from the body. It has even been shown to increase the effect of radiation on cancer cells to help cancer patients. It prevents the formation of free radicals that can cause diseases, such as cancer, stroke, and heart disease.