Black tea is made by adding hot water to the cured leaves of the Camellia sinensis plant. Green tea leaves are unoxidized, whereas black tea leaves have been allowed to oxidize. "Tea" refers to black tea for purposes of this web page. Green tea and herbal teas such as yerba maté and chamomile, mint, sage, and Essiac™ tea are covered in other web pages. Tea has been shown to have antioxidant and anti-inflammatory properties. Tea may improve glucose metabolism and has been shown to support weight loss. Tea consumption has been found to be associated with increased bone mineral density and may help prevent osteoporotic fractures in older women.
Tea contains theaflavins, catechins, quercetin and other polyphenols that are thought to be responsible for its anti-cancer activities. Tea consumption has been found to be associated with reduced risks of squamous cell carcinoma of the skin, head and neck cancers, and adult leukemia, as well as gastric, endometrial, colorectal, and prostate cancer. On the other hand, maternal tea drinking during pregnancy has been found to be associated with increased risk of childhood central nervous system tumors.
Breast cancer-related effects of drinking black tea
Laboratory studies have found that tea extracts or tea polyphenols suppress breast cancer. For example, tea theaflavins have been shown to inhibit proliferation of MCF-7 human estrogen receptor positive (ER+) breast cancer cells, as well as ER- cells, and HER2/neu-overexpressing (HER2+) cells. Tea extract consisting predominantly of theaflavins has been shown to delay mammary tumor development, reduce tumor burden and prolong survival in Tag mice, which develop estrogen independent (ER-) mammary tumors.
The majority of epidemiological studies have found no association or a modest negative association between tea intake and risk of breast cancer. Stronger associations have been found for subsets of women and breast cancer types:
- Drinking black tea has been found to be associated with a protective effect for those with lobular breast cancer.
- Women under 50 who consumed three or more cups of tea per day had a 37% lower risk of breast cancer than women under 50 reporting no tea consumption in one study. This inverse association was found for in situ and invasive breast cancer, and for ductal as well as lobular breast cancer.
- An inverse relationship between caffeine intake and breast cancer was found among postmenopausal women in one large study.
- A 2015 Chinese study reported an increased risk of breast cancer in postmenopausal women who were regular black tea drinkers.
- Two large Swedish studies have reported that black tea may be positively associated with risk of ER+/PR+ breast cancer.
- A meta-analysis of five cohort studies found a small increase in breast cancer risk associated with black tea consumption; the authors concluded that the data suggested a possible late-stage, promotional effect of black tea on breast carcinogenesis.
- Another meta-analysis reported that consumption of at least three cups of black tea per day was a significant risk factor for breast cancer.
- A study of the relationship between tea intake and circulating steroid hormone levels found that while higher green tea consumption was associated with lower estrone, estradiol and androstenedione levels, higher black tea consumption was associated with higher plasma levels of these hormones. The potentially differing effects of black and green tea on circulating estrogen levels could influence the impact of tea consumption on breast cancer risk.
- A large Netherlands prospective study found that breast cancer incidence was increased among consumers of five or more cups of tea per day. However, the association was not strictly statistically significant ( RR = 1.3 (95% confidence interval = 0.9-2.0)) and no dose-response relationship was observed.
Black tea versus green tea and breast cancer
Studies comparing black tea to green tea consumption have consistently found that green tea is associated with lower risk of breast cancer than black tea. One possible explanation is that green tea polyphenols include mostly catechins (up to 40% of dry weight); whereas black tea polyphenols include more than 20% thearubigins (formed during oxidation of the tea leaves), up to 10% catechins, and approximately 5% theaflavins. Catechins have been found to possess more powerful anti-cancer properties than thearubigins. As mentioned above, green tea also appears to have more favorable impact on circulating estrogens than black tea. Given that green tea has more consistent anti-cancer properties than black tea and that black tea apparently can promote breast cancer under some circumstances (although these circumstances are not well understood), we conclude that green tea is preferable for those with breast cancer, survivors and those at high risk for breast cancer.
Drinking very hot black tea (or very hot tea of any type) should be avoided. Rates of esophageal cancer are high in areas of the world where tea normally is consumed at high temperatures. In one study, the risk of esophageal cancer was found to be eight times higher for those who regularly drank very hot tea compared to those who drank warm or lukewarm tea. Note that it is the hot temperature that contributes to esophageal cancer, not the compounds found in tea.
Tea consumption decreases the bioavailability of folic acid in the diet by reducing the level of intestinal absorption and interfering with the folate pathway. This folate cycle inhibition is thought to be one mechanism by which green tea protects against breast cancer. Similarly, tea inhibits iron absorption and high levels of iron in benign breast tissue has been found to be associated with increased risk of subsequent breast cancer. Supplements containing folate or iron should not be taken with tea by those who require additional folate or iron.
Prepared tea contains less than half the caffeine of the same amount of drip coffee. However, high caffeine intake may increase the risk of benign breast disease, particularly atypical hyperplasia, which is associated with increased breast cancer risk. Those sensitive to caffeine or with benign breast disease should take steps to limit their caffeine intake.
Below are links to recent studies concerning this food. For a more complete list of studies, please click on black tea.