Green Tea Prevents Colorectal Cancer in Women

Abstract & commentary

By Donald Brown, ND. Dr. Brown is Founder and Director, Natural Product Research Consultants, Inc.; he serves on the Advisory Board of the American Botanical Council and the President's Advisory Board, Bastyr University, Seattle, WA; and is an Advisor to the Office of Dietary Supplements at the National Institutes of Health; he is a consultant for Nature's Way, Inc.

Source: Yang G, et al. Prospective cohort study of green tea consumption and colorectal cancer risk in women. Cancer Epidemiol Biomarkers Prev. 2007;16:1219-1223.

In a population-based, prospective cohort study (The Shanghai Women's Health Study), Japanese women were studied for the possible association between green tea consumption and colorectal cancer. The study recruited 74,942 women (aged 40 to 70 years) between 1996 and 2000 from 7 urban communities of Shanghai, with a participation rate of 92.7%. This study excluded subjects who reported a history of cancer (n = 1490), diabetes (n = 3302), or familial adenomatous polyposis (n = 86) at baseline, subjects with an extreme total energy intake (< 500 or > 3,500 kcal/day; n = 132), subjects lost to follow-up since enrollment (n = 10), or subjects who drank black or oolong tea regularly and exclusively (n = 381). After these exclusions, a total of 69,710 women remained for the study. The study lasted 6 years.

Biennial follow-ups determined the occurrence of cancer and other chronic diseases during home visits with the cohort members or next of kin for those who were deceased. The majority of cases of cancer (n = 246; 96.1%) were pathologically confirmed, with the remainder (n = 10; 3.9%) diagnosed with endoscopy, radiography, or ultrasound. Tea consumption was assessed at the baseline survey and reassessed 2 to 3 years later for more than 91.4% of the participants at the first follow-up survey. Each subject was asked whether she drank green tea regularly (at least 3 times per week for at least 6 months) and at what age she started. This was followed by questions on the type and amount of tea consumed during the past year, as well as the current level of consumption.

The multivariate relative risk of colorectal cancer (CRC) was 0.63 (95% confidence interval, 0.45-0.88) for women who reported drinking green tea regularly at baseline compared with non-regular tea drinkers. A significant dose-response relationship was found for both the amount of green tea consumed (P trend = 0.001) and the duration in years of lifetime tea consumption (P trend = 0.006). Compared with non-drinkers, each 1.67 g increase (approximately equal to the amount of tea in one tea bag) in daily green tea consumption was associated with a 10% reduction in CRC risk (RR, 0.90; CI: 0.80-1.00). Additional 5-year consumption of green tea was also associated with a 10% reduction in CRC risk (RR, 0.90; 95% CI: 0.83-0.97) after fully adjusting for potential confounding variables (eg, cigarette smoking, alcohol consumption, exercise). The reduction in risk was most evident for those women who reported to drink tea regularly at both baseline and during follow-up surveys (RR, 0.43; 95% CI: 0.24-0.77).

Commentary

The data on the chemopreventive benefits of green tea continue to grow with this large, prospective cohort study of women in Shanghai. The results demonstrate an inverse correlation between green tea consumption and the risk of CRC — most notably those women who regularly consumed green tea over a longer period of time.

With research showing protection against ovarian cancer1,2 and breast cancer,3,4 as well as a reduction in mortality due to cardiovascular disease in women,5 it's time for practitioners to begin advising their female patients to drop the coffee and start adding green tea to their daily regimen.

Conclusion

As noted in an earlier review, large population studies in women continue to suggest that risk of cardiovascular disease, ovarian cancer, breast cancer, and now colorectal cancer are all reduced with increased intake of green tea. As previously noted, it's important to remember that these studies were looking at women consuming green tea as a beverage and not in capsules. While studies have suggested possible weight loss benefits for concentrated green tea extracts,6 case studies suggesting a possible link between a few of these encapsulated extracts and hepatotoxicity suggest the need for more safety studies before recommending them as a substitute for a few cups of green tea each day.7

References

1. Larsson SC, Wolk A. Tea consumption and ovarian cancer risk in a population-based cohort. Arch Intern Med. 2005;165:2683-2686.

2. Zhang M, et al. Green tea consumption enhances survival of epithelial ovarian cancer. Int J Cancer. 2004;112:465-469.

3. Wu AH, et al. Green tea and risk of breast cancer in Asian Americans. Int J Cancer. 2003;106;574-579.

4. Nakachi K, et al. Influence of drinking green tea on breast cancer malignancy among Japanese patients. Jpn J Cancer Res. 1998;89:254-261.

5. Kuriyama S, et al. Green tea consumption and mortality due to cardiovascular disease, cancer, and all causes in Japan: The Ohsaki study. JAMA. 2006;296:1255-1265.

6. Nagao T, et al. A green tea extract high in catechins reduces body fat and cardiovascular risks in humans. Obesity (Silver Spring). 2007;15:1473-1483.

7. Bonkovsky HL. Hepatotoxicity associated with supplements containing Chinese green tea (Camellia sinensis). Ann Intern Med. 2006;144:68-71.