Reference
Prospective cohort study of tea consumption and risk of digestive system cancers: results from the Shanghai Women’s Health Study (SWHS). Am J Clin Nutr. 2012;96: 1056-1063.
Design
The Shanghai Women’s Health Study is a population-based prospective cohort study on middle-aged and older women.
Participants
74,941 women aged 40–70 were recruited from 7 urban areas around Shanghai, China from December 1996 through May 2000, with a participation rate of 92.7%. Criteria for exclusion included regular smoking or alcohol consumption at any time (n=3,513), loss to follow-up (n=7), or missing or implausible data (n=2,111), for a final sample of 69,310.
Study Medication and Dosage
Those women who had been drinking tea regularly (at least 3 times/week for at least 6 months) were included in the “regular tea drinker” category (n=19,382), which was approximately 28% of the cohort. Of the regular tea drinkers, 88% drank green tea only; 5.1% drank a combination of green, black, and scented teas; 3.5% drank only scented tea (such as jasmine); 1.1% drank only black tea; 0.7% drank only oolong; and 1.4% drank other types of tea.
Outcome Measures
The baseline survey included an in-person interview, self-administered questionnaire, and anthropometric measurements. Information was collected on demographics, diet, lifestyle habits, menstrual and reproductive history, occupational history, and some health and lifestyle information about their spouses. Follow-up includes an in-person interview every 2–3 years; 3 of these have been conducted so far. Data on vital status and cancer diagnoses were also obtained through the population-based Shanghai cancer and vital statistics registries and were confirmed via home visits and medical chart review. For the purposes of this article, data was included for stomach, esophageal, colon, rectal, pancreatic, liver, and gallbladder/bile duct cancers.
Key Findings
After a mean follow-up of 11 years, 1,255 digestive system cancers occurred. In comparison to women who reported never having been regular tea drinkers, regular tea intake was associated with a 17% reduction in risk for all digestive system cancers combined (HR: 0.86, 95% CI). This reduction increased as amount and years of consumption increased: Women who consumed at least 150 g of tea per month (2–3 cups/day) had a 21% reduced risk for digestive system cancers. Those who reported tea consumption for at least 20 years had a 27% reduced risk for all digestive cancers and a 29% reduced risk for colorectal cancer. Women who reported drinking tea at both the baseline and follow-up surveys had reduced risk for all digestive cancers combined (HR: 0.74, 95% CI), cancers of the stomach and esophagus combined, and colorectal cancer. For those who reported drinking tea only at baseline or at 1 follow up, there was no associated risk reduction. There was no risk reduction of liver or pancreatic cancer with any level of tea consumption.
Practice Implications
This is the latest addition to the growing body of research on the effect of green tea on cancer risk, particularly cancers of the digestive tract. In particular, the SWHS found an inverse association between long-term consumption of tea (primarily green tea) and cancers of the esophagus, stomach, and colorectal areas. This study did not find this association for liver or pancreatic cancers. There have been a number of studies, meta-analyses and epidemiologic reviews on tea consumption and incident gastrointestinal (GI) cancers with mixed results. This study gives us more information to compare with the available literature to further understand the relationship between tea consumption and GI cancer prevention.
One important note to remember is that the SWHS and similar studies are looking at a lifestyle habit of green tea consumption rather than supplementation with freeze-dried tea or extracted subfractions of green tea, such as epigallocatechin-3-gallate (EGCG). Although many in vitro studies have shown catechins such as EGCG to have cancer preventative properties,1–7 the SWHS has the benefit of showing the long-term effect of humans making specific food choices. One result we see from this study is better results with greater consumption over a longer period of time. Especially in the case of colorectal cancer, cancer prevention from drinking tea is most effective with a lifetime of exposure. Additionally, there are studies that advocate for consumption of unfractionated green tea based on the notion that there are multiple compounds contained within green tea that may work synergistically.8
SWHS and similar studies are looking at a lifestyle habit of green tea consumption rather than supplementation with freeze-dried tea or extracted subfractions of green tea, such as epigallocatechin-3-gallate.
One variable related to whether green tea drinking is found to have an inverse relationship to GI cancers is where the studies were performed. In general, Chinese and US studies have shown an inverse relationship between green tea consumption and GI cancer risk,9–14 while studies from Japan, Korea, and Singapore have shown mixed results.15–19 Across all of these regions the species of plant does not differ. Green tea is always Camellia sinensis; however, the processing of the leaves differs significantly by region. Japanese green tea is generally steam-processed, while Chinese green tea is dry roasted. It is possible that the dry roasting preparation better preserves cancer preventative constituents than steaming. Another study specifically found preventative effects from unfermented green tea on squamous cell carcinomas of the esophagus.20 Fermented green teas, such as Pu Erh, are popular in Tibet and Southern China, whereas most of the Chinese studies have been performed in Shanghai, which is in the far west side of China.
Another important variable to note in these studies is gender. While generally the Japanese studies have found no cancer prevention benefit overall from green tea, some have found an inverse relation between tea consumption and cancer risk in women only.21,22 The question exists then if there is a difference between the protective effect of green tea in men and women or if there are other factors that account for this difference.
According to the authors of this study, in Asian men, drinking alcohol and smoking cigarettes frequently accompanies green tea consumption. So those men who drink tea more frequently also generally have higher tobacco and alcohol consumption. Most Chinese women neither drink nor smoke, which eliminates these possible confounders from the current study. However, one Chinese study found green tea to reduce gastric cancer risk in men and women who were also heavy drinkers.23 Results from the Shanghai Men’s Health Study showed the same protective effect of green tea consumption on colorectal cancer, but only in nonsmokers.24 These two studies suggest that consumption of green tea of Chinese origin is protective for cancers of the GI tract for both men and women, but smoking negates this protective effect. Interestingly, one of the meta-analyses from Korea that found no protective effect was conducted at a smoking cessation facility, which begs the question of whether their data was on smokers only.25 Another less obvious factor is the temperature at which people chose to drink their tea. Especially in drinkers and smokers, drinking tea at high temperatures (above 70º C) significantly increases the risk of esophageal and gastric cancers.26–28
Several other lifestyle factors related to this study are also important to consider. In the SWHS, regular tea drinkers were younger, more highly educated, and more likely to have a professional occupation. In addition to being nonsmokers and nondrinkers, tea drinkers in this study also reported other habits that were possibly cancer preventative such as greater physical activity and fruit and vegetable intake. However, they were also more likely to be overweight and to report a family history of digestive system cancers and diabetes. All of these confounders were accounted for in the data but could possibly affect the overall outcome.
Summary
We can take away several points from the SWHS within the context of the other work that has been published on the subject: Regular drinking of Chinese processed green tea at a temperature under 70º C has an inverse relation to the risk of developing cancers of the esophagus, stomach, and colon. This protective effect increases with time and amount of tea consumed each day. Alcohol consumption does not seem to affect this, but smoking diminishes or eliminates this benefit. Advising patients, especially those with a family history or other risk factors for cancers of the digestive tract, to adopt the ritual of drinking green tea daily is a sensible preventative decision.
For more research involving integrative oncology, click here.
