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There are predominant polyphenols in green tea that have antioxidant activity. Scientists think that  polyphenols found in the tea, especially EGCG and ECG, contain free radicals, which have the ability to protect cells from DNA damages caused by the reactions of oxygen species.They also have the ability to inhibit the abnormal growth of body cells, protect against  ultraviolet (UV) radiation impact, and enhance the function of the immune system.Additionally, green tea capable of activating enzymes responsible for detoxification (removing toxins from the body), such as glutathione S-transferase and quinone reductase, which in turn help protect against tumor development.Dr, David Servan-Schreiber, MD, PhD, got cancer and recovered after many times of treatment. So, he decided to learn everything that he could about a lifestyle by which he can prevent and fight cancer.After 25 years, he knew that he had the way to fight and prevent cancer in his book, Anticancer: A New Way of Life.The first food that he mentioned in his book is green tea. After that he mentions the spice turmeric and then mushrooms. Consequently, the first section is called “Green Tea Blocks Tissue Invasion and angiogenesis.”Studies that link tea to cancer preventionAccording to many recent studies on animals, tea polyphenols have the ability to inhibit the formation of cancer cells at different organs, including the skin, lung, liver, esophagus, stomach, small intestine, and mammary gland.On the other hand, the results of human studies have been inconclusive.1- The link between tea (tea polyphenols) and premalignant lesionThe first study evaluated the effects of tea polyphenols on premalignant oral lesions. But, it was a double-blind intervention trial.It involved 59 participants with leukoplakia (a putative precursor lesion for oral cancer).The participants were divided into two groups. The first group was assigned to receive 3 grams of a mixed tea product orally and topically. In contrast the other group was assigned to receive 3 grams of placebo.After 6 months, 38 percent of the participants in the first group had partial regression of the oral lesions compared with only 10 % of the participants in the placebo group.Furthermore, the participants in the first group had a fewer increase in lesion size (3%) than in the placebo group (7%).On the other hand, there was a second trial involved 39 people with high-risk premalignant oral lesions. Some of them received 500 mg of green tea per square meter of body surface area (mg/m2), and the others received placebo three times daily for 12 weeks.At the end of the trial, there were no differences in lesion responses between the groups.2- The link between tea and 8-OHDGAdditionally, there were researchers held trials to examine the effect of tea on the level of of 8-hydroxydeoxyguanosine (8-OHdG) in urine.8-HODG is a biomarker of oxidative DNA damage that may be a predictor of increased cancer risk.They found that the amount of 8-OHdG in urine is higher in persons with many types of cancer than in healthy subjects. Also, there was a higher content of 8-OHdG in many tissues than adjacent nontumor tissue.There were about 130 heavy smokers. In the trial, they had to receive 4 cups of one of the following beverages every day for 4 months: decaffeinated green tea, black tea, or water.After the 4 months, those who drank green tea had a significant decrease (31%) in urinary levels of 8-OHdG. In contrast, the other 2 groups had no change in urinary 8-OHdG levels.Although this trial indicates that green tea polyphenols can sufficiently reduce urinary 8-OHdG levels, it is unclear if reduced 8-OHdG levels are associated with reduced cancer risk.3- the effect of tea polyphenols on the level of pepsinogen in serumAnother trial involved about 160 individuals with high serum pepsinogen levels to examine the effect of polyphenols in green tea on serum pepsinogen levels.Serum pepsinogen is considered to be an indicator of increased risk for stomach cancer because it is a biomarker of gastric atrophy.Half of The participants received one capsule of tea polyphenols, and the others received six capsules daily for 1 year.Each capsule was the equivalent of about 2 cups of tea.The results was that there are no changes in serum pepsinogen levels in either two group.Eventually, we can say that the evidences regarding the benefits of green tea consumption in relation to cancer is not enough at present.The adverse effect of green teaTea is recognized as safe by the Food and Drug Administration (FDA). The adverse effects reported in some studies included, dizziness, headache, excess intestinal gas, nausea, heartburn, stomach ache, abdominal pain, and muscle pain.Many scientists think that The tea as any other caffeinated beverages (such as coffee and colas) may potentially cause adverse effects, including tachycardia, insomnia, uncomfort, nervousness, abdominal pain, nausea, vomiting, diarrhea, and diuresis.However, there is not enough evidences of health risks for adults who consume moderate amounts of caffeine (300 to 400 mg a day).A review was held by Health Canada suggested that moderate amount of caffeine intakes (up to 400 mg per day or 6 mg per kilogram body weight) was not associated with adverse effects in healthy adults.Additionally, The amount of caffeine content is higher in black teas (64 to 112 mg per 8 FL Oz serving) followed by Oolong tea (29 to 53 mg per 8 FL Oz serving). But, Green and white teas contain slightly less caffeine (24 to 39 mg per 8 FL Oz serving).Generally, caffeine doses of less than 3.0 mg per kg body weight have no adverse effects on kids. However, Higher doses have a bad impact in some behavioral effects, such as nervousness, anxiety as well as sleep disturbances.