Antioxidants In Tea And It’s Health Benefits

Subject: Healthcare Research
Pages: 14
Words: 3687
Reading time:
16 min
Study level: College


“Anti” as the word suggest is against or is in resistance to, or counteractive in nature. Hence antioxidant means against the process of oxidation. Technically an “antioxidant” aids in correct oxidation of the cells by fastening themselves with the damaging free radical molecules and finishing the chain of reactions triggered by it. Free radicals begin a chain reaction that damages healthy cells that can cause premature aging, hamper DNA leading to cancer and are considered a leading cause for heart disease, arthritis and other persistent health problems in today’s society. Though there are several antioxidants that are naturally produced by the human body, it has become a necessity to supply additional antioxidants through food and supplementation (Herbal Legacy Newsletter). This paper discusses the antioxidant property of tea and its health benefits.

The USDA conducted research in several fruits and vegetables and concluded the following to be the only top 20 natural foods that are rich in antioxidants: “small red beans, wild blueberries, red kidney beans, pinto beans, cultivated blueberries, cranberries, artichokes, blackberries, prunes, raspberries, strawberries, red delicious apples, Granny Smith apples, Pecans, sweet cherries, black plums, russet potatoes, black beans, plums, gala apples” (Herbal Legacy Newsletter; NewsMax.Com).

Camellia sinensis (the tea plant) is another source of rich antioxidants. The practice of drinking tea to preserve excellent wellbeing among human beings dates back to centuries. For instance, there are literatures saying that the early Buddhist and Chinese healing practices included the herb Camellia sinensis. This herb is known to be effective medicine for supporting good health and long life, additionally it also keep the mind alert and sharp and is also used for treating many ailments, ranging from indigestion to the common cold and flu (Ito En).

Asians have identified the wellbeing and endurance benefits of consumption of tea for centuries. Recent studies are now authenticating these ancient beliefs, citing tea as a main supply of flavonoids, antioxidants that protect the body from the damaging free radicals. Asians drink particularly green tea for its antioxidant power in fighting diseases (

Tea and Its Ingredients

Among all the beverages it is considered that tea is one of the most earliest and well-liked beverages consumed by almost all age groups and in most parts of the world. Tea is consumed in various forms. For instance, majority of the people around the world (75%) consume black tea (Seeram et al. 1599–603). In the America, United Kingdom, and Europe majority of the people prefer black tea as a beverage, where as green tea is the most well-liked tea in Japan and China. Black and green tea consumption is most common when compared to oolong and white tea (Mukhtar and Ahmad 1698S–702S).

Camellia sinensis is the plant from which the tea leaves are harvested. Soon after the harvesting, tea leaves under go various processing such as wilting and oxidation. The main processing of tea leaves is the oxidation process and the enzymatic action that makes the tea leaves dark in colour. This process is also known produce the typical aroma of tea. When the oxidation process is halted by heating at various stages the different types of tea is obtained. For instance, the complete oxidation of tea leaves is required for the formation of black tea. Before the oxidation the leaves have to be wilted, beaten and rolled. On the other hand, green tea is prepared from un-wilted tea leaves that are not oxidized. The incomplete oxidation of wilted and beaten tea leaves make the oolong tea. As there is variation in the processing of the tea leaves, there is also the variation in the antioxidant properties among the different varieties of teas.

There are several components that are present in the tea such as “polyphenols, alkaloids (caffeine, theophylline, and theobromine), amino acids, carbohydrates, proteins, chlorophyll, volatile organic compounds, fluoride, aluminum, minerals, and trace elements” (Cabrera et al. 4427–35). The polyphenols particularly the catechins are the group of chemicals that are responsible for the antioxidant property of tea. The green tea has the highest amount of catechins in them and is comparatively healthier. The most energetic and plentiful catechin in green tea is epigallocatechin-3-gallate (EGCG). Other catechins include Epigallocatechin (EGC), Epicatechin-3-gallate (ECG) and Epicatechin (EC) (Cabrera et al. 79–99).

Epigallocatechin-3-gallate (EGCG)

Epigallocatechin-3-gallate (EGCG)



Epicatechin-3-gallate (ECG)

Epicatechin-3-gallate (ECG)

Epicatechin (EC)

Epicatechin (EC)



According to a few researchers the green and black tea has almost ten times the antioxidants when compared to fruits and vegetables. They also suggest that any formulation that comes from the camellia tea plant is rich in polyphenols. These substqances are known to fight the cell damaging free radicals and detoxify them. As a result they stop the free radical chain reaction that can cause serious problems such as cancers, heart diseases and strokes (Davis). Traditional knowledge combined with the modern scientific studies have added to the health and well being of human beings through simple supplimentations. Today, antioxidants play a major role in Complementary and Alternative Medicine (CAM).

If we compare the amount of polyphenols in the four types of tea, it can be said that black tea contains comparatively low concentrations of catechins when compared to green tea. This makes green tea superior to black tea as an antioxidant. The extensive oxidation of black tea during the processing period increases the concentrations of two complex polyphenols i.e. thearubigins and theaflavins (Mukhtar and Ahmad 1698S–702S). The oolong tea and white tea has a mixture of simple and complex polyphenols but the quantity of these compounds are much less when compared to green tea.

Today, there are several commercial preparations of tea such as iced tea, flavoured teas etc. However, the amount of polyphenols determines the antioxidant property of tea. Iced and ready-to-drink teas have less polyphenol compared to brewed tea. It is also essential to note that the polyphenol concentration vary with the type of tea, the amount used, the brew time, and the temperature. Studies suggest that the highest polyphenol concentration is found in infused or brewed hot tea when compared to instant preparations, and still lesser in iced and ready-to-drink teas (Cabrera et al. 4427–35).

The amount of tea solids (i.e., dried tea leaves and buds) also determines the amount of polyphenols in it. The commercial preparations such as the ready-to-drink teas have fewer amounts of tea solids and therefore lower polyphenol contents. This may be mainly due to the fact that the base ingredient may not be brewed tea. When other liquids or juices are added to the tea, it will result in dilution of the tea solids there by decreasing the antioxidant efficiency of tea (Peterson et al. 397–405). The removal of caffeine or the process of decaffeination results in the reduction of polyphenols in teas.

Green Tea and Its Health Benefits

Scientific studies on different types of teas show that tea contributes to various health benefits. Among the teas, green tea is composed of highly healthy ingredients such as catechins that give the texture to the tea, caffeine contributes to the bitterness, and theanine provides the flavour. Additionally, there are several vitamins and minerals that are present in it (Ito En). The catechins especially the EGCG that is present in green tea is also known to be present in red wine that makes it a healthy drink. Catechins act as a powerful antioxidant that hinders hazardous free radicals in the body.

Caffeine helps in keeping the mind alert and alleviate exhaustion. Theanine which is an amino acid is a mild relaxant. Medical science has identified catechin to be Anti-tumorigensis, Antioxidant, inhabit hypertension, Anti-hypercholestolemia, has hypoglycaemic effect, helps in strengthening capillaries, maintaining the elasticity of skin, and has Antimicrobial activity. It is also known to prevent halitosis (Ito En). Antioxidants fight free radicals and aid in preventing various types of cancer. The caffeine present in the green tea contributes to alertness, eases fatigue and is also known for its diuretic effect.

Theanine the amino acid has an action that is antagonistic to caffeine. It is also known to promote physical sensation of relaxation, inhabit hypertension and improve brain function. Carotene is another ingredient of green tea that is known for its Anti-carcinogenic effect. Y-amino Butyric acid also aid in inhabiting hypertension (Ito En). Flavonoids present in the tea helps in strengthening the blood vessels and prevent halitosis. Vitamin C and vitamin E are well known antioxidants that help the immune system to fight infections and maintain the cells respectively. Additionally, there are also few minerals such as zinc, fluoride and calcium that provide healthy bones and gums (Ito En).

The importance of green tea in Japanese diet dates back to centuries. This was one of the major ingredients in their cuisine that mainly relies in seafood. The importance of green tea was well understood by the Japanese. Studies provide enough evidence that this practice has contributed to the less heart diseases and cancers in the Japanese population. Today with the advancement in the knowledge in this field, the advantage of antioxidants is opened up to almost all parts of the world and tea is one of the most liked beverages by all age groups (Ito En).

White Tea

As mentioned earlier white tea is the tea that undergoes the least oxidation during processing and also because of the higher proportion of immature sprout leaves, this tea least caffeinated. Hence those people, who are conscious of caffeine intake, can opt for white tea on a daily basis. There are many who prefer to take white tea over black or green tea because of low caffeine content and is less processed one. According to few researchers there are almost same catechins in green tea and white tea. Hence the main health benefit of white tea is to prevent heart disease, cancers and stroke, plus serving to take care of diabetes. Additionally, the greater levels of calcium and fluoride aids to maintain strong teeth, gums and bones (Ito En).

Oolong Tea

Oolong Teas is for those who would like to have the benefit from all the range of teas. In other words since the span an oxidation of oolong tea range from 20 to 80%, there is a mixture of black and green tea accordingly there is a variation in the caffeine levels. The catechin levels decrease with the oxidation. As a result they have less antioxidant property when compared to green tea. On the other hand due to the higher oxidation, the polyphenols such as theaflavin and thearubigin levels increase (Ito En). Though less, theaflavin and thearubigin also aid in defending the body against stroke, dementia, heart problems and cancer. Oolong teas also have good digestive properties.

Black Tea

“Black tea is the most oxidised and the processed tea among all the teas” (Ito En). As a result these have the highest caffeine content in it. It is known for the energy boosting capacity. Catechins are comparatively low in black tea but have a high concentration of theaflavins and thearubinins. Studies have shown that black tea is also effective in preventing heart disease, stroke and cancer, and lowering cholesterol (Ito En).

Health Benefits of Pu-Erh Tea

Pu-erh teas are prepared by the fermentation process. The tea leaves are accurately fermented and aged, as a result there are chemical changes in the composition of this tea. Pu-erh tea also contains caffeine but is in low concentrations. It is also known for the greater amounts of flavonoids that in general help to reduce LDL cholesterol and prevent high blood pressure. This tea is entering the market of weight reduction as it shows the ability to break up fats and aid in digestion. Current studies in rats have exhibited weight loss property and also show higher metabolic rate. (Ito En).

Health Benefits of Herbal Tea

Herbal “teas” are not a derivative of Camellia sinensis plant. Therefore, these are not really tea but are infusions that in general do not contain caffeine. This makes them suitable for all age group including children. Herbal teas have a combination of different herbs known for various qualities. The Rooibos plant only grows in South Africa and is known for its high levels of antioxidants and vitamin C and is free of caffeine. Similarly, lavender is traditionally known for its calming, soothing, and soporific effects on mind and body. Chamomile is also another plant that has strong calming powers and is also a natural pain killer. Peppermint also finds its place in herbal tea. These aid in digestion and also take care of problems such as sinus. Though these herbs cannot take the place of Camellia sinensis plant, they do have some health benefits and should not be ignored (Ito En).

Tea and Cancer Prevention

Cancer is the result of free radical damage of the cells. When there is an exposure to carcinogenic substances, free radicals are formed in the body that sets in a chain of reaction that damages the cells and tissues of the body and causes cancer. In other words, the free radicals damage the cells in such a way that their normal functioning is hampered. One of the most common ways to control the free radical formation and stop its damaging chain reaction is by the action of antioxidants. Several studies have shown that tea has high amounts of antioxidants that can prevent cancer. The principal polyphenols in green tea―EGCG, EGC, ECG, and EC―and the theaflavins and thearubigins in black teas comprise antioxidant activity. These chemicals, mainly EGCG and ECG, contain considerable free radical scavenging action and possibly will defend cells from DNA damage (Henning et al. 1558–64).

Laboratory studies provide enough evidence on the action of tea polyphenols in hindering tumour cell propagation. It also induces apoptosis in laboratory and animal studies. There are also studies that point out the inhibition of angiogenesis and tumour cell invasiveness by the catechins present in tea. The polyphenols also have a protective action from the damaging UVB radiation (Lambert and Yang 3262S–67S) and are known to adjust immune system function. This study also finds that green tea activates detoxification enzymes such as glutathione S-transferase and quinone reductase. As a result give good protection against tumour development (Steele et al. 63–67). Though there are several animal studies that show the cancer prevention effects of polyphenols, not many human studies have been carried out. In animal studies these polyphenols have been found to inhibit tumorigenesis in different tissues and organs such as skin, lungs, oral cavity, stomach, intestine, liver, pancreas and mammary glands.

A few epidemiologic studies on the impact of tea in reducing cancer risks are published in the recent years. The results point out that there is an association between tea consumption and cancer risk reduction especially the cancers of the colon, breast, ovary, prostate, and lung (Sun et al. 2143–48; Simons et al. 307–21). However, there is a great variation in results of these studies. This conflicting results may be owing to variables such as diversities in the method of preparation and consumption, the types of tea used for analysis, cultivation methods and variation in genetic variety, the amount of tea consumed etc.

There are also a few clinical trials that have examined the impact of consuming tea in cancer prevention. For instance, two randomized assessments estimated the effects of tea extracts on pre-malignant oral lesions. “The first study was a double-blind interventional experiment concerning 59 people with leukoplakia. This is a recognized predecessor lesion for oral cancer. These 59 people were indiscriminately assigned to receive either 3 grams of a mixed tea product, given both orally and topically, or a placebo on a daily basis for six months. The final outcome of the study showed that in about 38% of the subjects that was partial deterioration of oral lesions. On the other hand in the subjects in the placebo group, only 10 % of partial deterioration of oral lesions was found. Further, it was also found that among those who had an increase in lesion size, the results pointed out that 3% and 7% increase in treatment group and placebo group respectively. The dwindling in mucosal cell proliferation in the treatment group also showed the protective action of tea against oral cancer.” (Li et al. 218–24).

Conversely, in the second study carried out on 39 people with high-risk pre-malignant oral lesions, the results did not show any improvement. These participants were indiscriminately assigned to receive one of three doses of a green tea extract—500 mg/m2 of body surface area, 750 mg/m2, or 1000 mg/m2—or a placebo three times daily for 12 weeks. At the end of 12 weeks there were no significant differences in lesion responses or histology (Tsao et al. 931–41).

The levels of 8-hydroxydeoxyguanosine (8-OHdG) which is a biomarker of oxidative DNA damage was taken into consideration in a clinical research. These are suspected to be the predictor of increased cancer risk. It is found that the amount of 8-OHdG in urine is high in people with lung cancer. It is also the case with human breast, lung, liver, kidney, brain, stomach, and ovarian tumour tissue. In this clinical trial 133 adult heavy smokers were erratically assigned to sip 4 cups of any one of the beverages (decaffeinated green tea, decaffeinated black tea, or water) each day for a period of four months. The results from this study show that there was statistically significant 31 percent decrease in urinary levels of 8-OHdG among the subjects who drank green tea where as the subjects who drank black tea did not find any change in the levels of urinary 8-OHdG (Hakim et al. 3303S–09S).

Similar results were obtained in another study with 124 individuals who were at high risk of liver cancer due to hepatitis B virus infection and aflatoxin exposure. However, in this study 500 mg or 1000 mg of a green tea polyphenol supplement was taken instead of tea (Luo et al. 262–68). There are several such clinical trials with results that indicate a positive impact that green tea catechins and green tea extracts reduce the risk of cancer. Additionally, green tea consumption has been shown to drastically decrease the risk of breast cancer and ovarian cancer in Asian women (Wu et al. 574-79; Zhang et al. 713-18). According to a another population-based case-control study of breast cancer amongst Chinese, Japanese and Filipino women showed that the consumption of green tea reduced the risks of development of breast cancer (Wu et al. 574-79).

Other Sources of Antioxidants

Tea has the highest amounts of antioxidants when compared to coffee or chocolates and therefore has better health outcomes. Though there are studies that point out that coffee has health benefits, the antioxidant benefit comes mainly from tea ( There is some assumption that nutritional flavonoids are present in chocolates especially the epicatechin. These are known to promote good heart health through anti-thrombotic mechanisms and have antioxidant property. However, when there is an addition of milk particularly during the manufacturing process, the antioxidant property is inhabited. This inhibition may be the result of formation of secondary chemical bonds between the flavonoids and proteins present in the milk. This again reduces the antioxidant property of chocolates (Serafini et al. 1013). According to researchers, tea polyphenols are also helpful in stopping the spread of cancer. For instance, there are studies that show that polyphenols have encumbered the capacity of leukemia and liver tumor cells to formulate DNA that is essential in cell reproduction. Consequently, the cancer cells could not multiply and enlarge the tumor ( Researchers also suggest that tea especially the green tea has the highest and varied source of antioxidants when compared to fruits and vegetables.

Adversities of Tea Consumption

While tea is considered as a safe food item by the USFDA, the consumption of up to 1200 mg of EGCG for about four weeks time, has shown some health challenges in healthy individuals. These include intestinal gas, nausea, heartburn, abdominal pain, faintness, headache, and muscle pain (Chow et al. 4627–33). Besides the caffeine content especially in black tea, could also pose health challenges such as tachycardia, palpitations, sleeplessness, agitation, anxiety, tremors, headache, abdominal pain, nausea, vomiting, diarrhoea, and diuresis (Higdon and Frei 101–23).

Though there are minerals such as calcium, fluorides, zinc that are beneficial to health in the tea, there are also some that are not such as aluminum. Aluminum is neurotoxic element present in the tea plants. This is taken up from the soil and is present in varying quantities. It concentration also vary with soil condition, harvesting periods and the water quality. The major health challenges that occur due to high aluminum concentration are osteomalacia and neurodegenerative disorders, particularly in those with renal failure. However, there are not many studies on the bioavailability of aluminum in tea (Cabrera et al. 79–99).

Studies also suggest that the high intake of black and green tea may inhibit iron absorption in the body (Cabrera et al. 79–99). Individuals who are anaemic and those taking iron supplements should not combine tea or coffee along with it. According to a review study in UK, it was found that even though tea drinking restricted the assimilation of non-heme iron from the food, there was not much evidence to conclude that this would have an effect on blood measures (i.e., haemoglobin and ferritin concentrations) of by and large iron status in adults. On the other hand, in the midst of preschool children, statistically considerable interaction was observed between tea drinking and poor iron status (Nelson and Poulter 43–54).


Tea is the most extensively consumed beverage globally, however the form and amount of tea taken varies. The popularity of tea and its variety varies in different parts of the world. Researchers suggest that tea is among the most safest and beneficial beverages in the world. The polyphenolic compounds present in the tea has antioxidant properties that can help the prevention of cancers, heart problems and several other diseases. Regular drinking of tea may consequently improve antioxidant status in vivo and, thus, assist lower risk of certain types of cancer and heart disease. In conclusion, it can be said that tea is one of the healthiest beverage that can be consumed by all age groups.

Work Cited

Benzie, Iris. F. F. and Szeto, Y.T. “Total Antioxidant Capacity of Teas by the Ferric Reducing/Antioxidant Power Assay”. J. Agric. Food Chem. 1999, 47, 633-36.

Cabrera, Carmen., Rafael Giménez., and M. Carmen López. “Determination of tea components with antioxidant activity”. Journal of Agricultural and Food Chemistry 2003; 51(15):4427–35.

Cabrera, Carmen., Reyes Artacho., and Rafael Giménez. “Beneficial effects of green tea―a review”. Journal of the American College of Nutrition 2006; 25(2):79–99.

Chow, H-H. Sherry, Iman A. Hakim, Donna R. Vining, James A. Crowell, James Ranger-Moore, WadeM. Chew, Catherine A. Celaya, Steven R. Rodney, Yukihiko Hara, and David S. Alberts “Effects of dosing condition on the oral bioavailability of green tea catechins after single-dose administration of polyphenon E in healthy individuals”. Clinical Cancer Research 2005; 11(12):4627–33.

Davis Jeanie Lerche, “Antioxidants in Green and Black Tea” WebMD Weight Loss Clinic-Feature. Web.

Hakim Iman A., Robin B. Harris, Sylvia Brown, H-H. Sherry Chow, Sheila Wiseman, Sanjiv Agarwal and Wendy Talbot. “Effect of increased tea consumption on oxidative DNA damage among smokers: A randomized controlled study”. Journal of Nutrition 2003; 133(10):3303S–09S.

Henning Susanne M, Yantao Niu, Nicolas H Lee, Gail D Thames, Rosario R Minutti, Hejing Wang, Vay Liang W Go, and David Heber. “Bioavailability and antioxidant activity of tea flavanols after consumption of green tea, black tea, or a green tea extract supplement”. American Journal of Clinical Nutrition 2004; 80(6):1558–64.

Herbal Legacy Newsletter, “What are antioxidants, free radicals and flavonoids”, 2008. Web.

Higdon Jane V. and Balz Frei. “Coffee and health: A review of recent human research”. Critical Reviews in Food Science and Nutrition 2006; 46(2):101–23. “Drinking tea—Incredible Health Benefits”. 2011. Web.

Ito En,Health Benefits” Ito En (North America) Inc. Brooklyn, NY. 2011. Web.

Lambert Joshua D. and Chung S. Yang. “Mechanisms of cancer prevention by tea constituents”. Journal of Nutrition 2003; 133(10):3262S–67S.

Li Ning, Zheng Sun, Chi Han, Junshi Chen “The chemopreventive effects of tea on human oral precancerous mucosa lesions”. Proceedings from the Society of Experimental Biology and Medicine 1999; 220(4):218–24.

Luo Haitao, Lili Tang, Meng Tang, Madhavi Billam, Tianren Huang, Jiahua Yu, Zhongliang Wei, Yongqiang Liang, Kaibo Wang, Zhen-Quan Zhang, Lisheng Zhang and Jia-Sheng Wang, “Phase IIa chemoprevention trial of green tea polyphenols in high-risk individuals of liver cancer: Modulation of urinary excretion of green tea polyphenols and 8-hydroxydeoxyguanosine”. Carcinogenesis 2006; 27(2):262–68.

Mukhtar Hasan and Nihal Ahmad. “Tea polyphenols: Prevention of cancer and optimizing health”. American Journal of Clinical Nutrition 2000; 71(6 Suppl):1698S–702S.

Nelson Michael and Poulter J. “Impact of tea drinking on iron status in the UK: A review”. Journal of Human Nutrition and Dietetics 2004; 17(1):43–54.

NewsMax.Com. “Strawberries Among Super Foods For Super Health” 2006. Web.

Peterson, J. Dwyer J., Jacques P., Rand W., Prior R.and. Chui K.Tea variety and brewing techniques influence flavonoid content of black tea”. Journal of Food Composition and Analysis 2004; 17(3–4):397–405.

Seeram Navindra P., Susanne M. Henning, Yantao Niu, Rupo Lee, H. Samuel Scheuller, and David Heber “Catechin and caffeine content of green tea dietary supplements and correlation with antioxidant capacity”. Journal of Agricultural and Food Chemistry 2006; 54(5):1599–603.

Serafini Mauro, Rossana Bugianesi, Giuseppe Maiani, Silvia Valtuena, Simone De Santis, Alan Crozier. “Plasma antioxidants from chocolate”. Nature 2003. VOL 424.: 1013.

Simons Colinda C. J. M., Lina J. Leurs, Matty P. Weijenberg, Leo J. Schouten, R. Alexandra Goldbohm, Piet A. van den Brandt. “Fluid intake and colorectal cancer risk in the Netherlands Cohort Study”. Nutrition and Cancer 2010; 62(3):307–21.

Steele Vernon E., Gary J. Kelloff, Douglas Balentine, Charles W. Boone, Rajendra Mehta, Donya Bagheri, Caroline C. Sigman, Songyun Zhu and Sheela Sharma. “Comparative chemopreventive mechanisms of green tea, black tea and selected polyphenol extracts measured by in vitro bioassays”. Carcinogenesis 2000; 21(1):63–7.

Sun Can-Lan, Jian-Min Yuan, Woon Puay Koh, Hin-Peng Lee, Mimi C. Yu. “Green tea and black tea consumption in relation to colorectal cancer risk: The Singapore Chinese Health Study”. Carcinogenesis 2007; 28(10):2143–48. “Tea vs Coffee – A Brief Overview”. 2011. Web.

Tsao Anne S., Diane Liu, Jack Martin, Xi-ming Tang, J. Jack Lee, Adel K. El-Naggar, Ignacio Wistuba, Kirk S. Culotta, Li Mao,Ann Gillenwater, Yuko M. Sagesaka, Waun K. Hong and Vassiliki Papadimitrakopoulou. “Phase II randomized, placebo-controlled trial of green tea extract in patients with high-risk oral premalignant lesions”. Cancer Prevention Research 2009; 2(11):931–41.

Wu Anna H., Mimi C. Yu , Chiu-Chen Tseng , Jean Hankin , Malcolm C. Pike. “Green tea and risk of breast cancer in Asian Americans”. International Journal of Cancer. Sept. 10 106(4) :574-79.

Zhang Min, Colin W. Binns, and Andy H. Lee. “Tea consumption and ovarian cancer risk: a case control study in China”. Cancer Epidemiol Biomarkers Prev. 2002;11:713-18.