Green tea contains a variety of enzymes, amino acids, carbohydrates, lipids, sterols, polyphenols, carotenoids, tocopherols, vitamins, caffeine and related compounds, phytochemicals, and dietary minerals. Numerous claims have been made for the health benefits of green tea based on chemical composition, in vitro and animal studies, and while preliminary research on many of these claims is promising, many also require further study to evaluate. There is also evidence suggesting consuming large volumes of green tea, and in particularly green tea extracts, may cause oxidative stress and liver toxicity.
A 2012 systematic review concluded the evidence that green tea can prevent cancer "is inadequate and inconclusive" but with some evidence for a reduction in certain types of cancer (breast, prostate, ovarian and endometrial). Green tea may lower blood low-density lipoprotein and total cholesterol levels, though the studies were of short duration and it is not known if these effects result in fewer deaths and evidence does not support green tea's reducing coronary artery disease risk. Several randomized controlled trials suggest green tea can reduce body fat by a small amount for a short time, though it is not certain if the reduction would be meaningful for most people. One study has found that certain catechins found in green tea taken at levels many hundreds of times greater than what could be obtained from even very high tea consumption may actually damage DNA. These results are relevant only for those taking such amounts of the catechins in pharmaceutical formulations. Similar results from unnatural concentrations of other antioxidants including vitamin E and vitamin C have also been demonstrated in human trials. Further, more recent studies and those done in countries outside the US, such as Japan, have found green tea consumption to result in decreased risk of many cancer, cardio-vascular disease, and dementia including Alzheimer's. Green tea may interfere with the anti-cancer drug Bortezomib (Velcade) and other boronic acid-based proteasome inhibitors.