Green tea is one of the most popular beverages consumed worldwide. It is made from the leaves of the Camellia sinensis plant and contains a number of bioactive compounds that may provide health benefits. One of the purported benefits of green tea is protection against cancer. This article will examine the evidence on whether green tea has anti-carcinogenic effects and can help prevent or treat cancer.
Green Tea Composition
Green tea contains a number of bioactive compounds, including:
– Catechins: Green tea is particularly rich in a subgroup of antioxidants called catechins. The main catechins in green tea are epigallocatechin gallate (EGCG), epigallocatechin (EGC), epicatechin gallate (ECG), and epicatechin (EC). EGCG is the most abundant and potent catechin in green tea.
– Caffeine: Green tea also contains caffeine, although in much lower amounts than coffee. An average cup of green tea contains around 30-50mg of caffeine.
– Amino acids: Green tea contains small amounts of amino acids such as theanine, which may work synergistically with caffeine to provide health benefits.
– Vitamins and minerals: Green tea contains B vitamins, vitamin C, selenium, manganese, and zinc.
– Flavonoids: Green tea contains flavonoids like kaempferol, myricetin, and quercetin which have antioxidant and anti-inflammatory activities.
The combination of these compounds gives green tea its unique medicinal properties. The catechins, and specifically EGCG, are believed to be the main anti-carcinogenic compounds in green tea.
Anti-carcinogenic Mechanisms of Green Tea Catechins
Green tea catechins may help protect against cancer through several mechanisms:
– Antioxidant activity: Green tea catechins are powerful antioxidants that can neutralize free radicals and prevent oxidative damage to cells. Oxidative damage is linked to cancer development.
– Anti-proliferation: The catechins inhibit the growth and spread of cancer cells. EGCG prevents uncontrolled cell division by interacting with a number of cell signaling pathways.
– Pro-apoptotic effects: Catechins induce programmed cell death (apoptosis) in cancer cells. They also inhibit anti-apoptotic pathways in cancer cells. This pushes damaged cells towards cell death.
– Anti-angiogenesis: Angiogenesis allows tumors to grow and spread by developing new blood vessels. The catechins prevent tumors from creating their own blood supply by inhibiting angiogenic factors.
– Anti-metastatic activity: Catechins prevent cancer cell invasion, migration, and adhesion to other tissues. This helps contain the cancer cells and prevents metastasis.
Through these diverse mechanisms, green tea catechins can modulate pathways involved in cancer development and progression at multiple levels. The polyphenols have also demonstrated synergy with cancer drugs and radiotherapy to enhance therapeutic efficacy.
Epidemiological Evidence on Green Tea and Cancer
A number of large cohort studies and meta-analyses have examined the association between green tea consumption and cancer risk:
Prostate Cancer
– A meta-analysis of 13 studies found that green tea intake was associated with a reduced risk of prostate cancer. Men who consumed over 5 cups per day had a 51% lower risk of developing prostate cancer compared to non-tea drinkers.
– In the Ohsaki Cohort Study in Japan, green tea consumption was associated with a 48% lower risk of advanced prostate cancer.
Breast Cancer
– A meta-analysis of 15 case-control and cohort studies found that women who consumed the most green tea per day (over 3-5 cups) had an approximately 20% lower risk of developing breast cancer compared to non-drinkers.
– The Shanghai Women’s Health Study observed that regular green tea intake over a 10-year period reduced breast cancer risk by 22%.
Colorectal Cancer
– A meta-analysis of 8 studies found that green tea drinkers had a 29% lower risk of colorectal cancer.
– In the Japan Public Health Center-based Prospective Study, green tea consumption was linked to a 27% risk reduction for colorectal cancer.
Lung Cancer
– The Ohsaki Cohort Study found a 39% decreased risk of lung cancer in people who drank 5+ cups of green tea/day compared to non-drinkers.
– A meta-analysis reported an approximately 20% reduction in lung cancer risk for the highest green tea consumers, however results were not statistically significant.
While promising, most of these studies were observational in nature. They cannot prove cause and effect. However, the consistency of the findings across different populations provides compelling evidence that green tea may help prevent various types of cancer. The results appear strongest for prostate and colorectal cancers.
Clinical Trials on Green Tea and Cancer
A number of clinical trials have directly evaluated the effects of green tea catechins on cancer biomarkers, tumor progression, and treatment side effects:
Prostate Cancer
– In men with prostate cancer, taking 600mg EGCG supplements daily for 1 month before prostatectomy led to reductions in prostate tumor markers PSA, VEGF, and HGF compared to placebo.
– In men with precancerous prostate lesions, a 1-year intervention with 600mg green tea catechins reduced the progression of prostate lesions compared to placebo. Only 3% of the green tea group progressed to prostate cancer versus 30% of the placebo group.
Breast Cancer
– In women with breast cancer, taking EGCG supplements for 2-6 weeks before surgery increased tumor caspase-3 levels, indicating enhanced cancer cell death.
– Women with early-stage breast cancer were given 800mg EGCG daily for 6 months. The EGCG group had greater reductions in Ki-67, a marker of cancer cell proliferation compared to placebo.
Colorectal Cancer
– In colorectal cancer patients, 2 grams of green tea extract daily for 4 weeks reduced tumor cell proliferation by over 50% compared to baseline.
– In people with colon polyps, a 6-month intervention with 500mg green tea extract reduced polyp number and size compared to placebo.
Leukemia
– Chronic lymphocytic leukemia patients took 2000mg green tea extract daily. After 1 month, the green tea group had an average 30% reduction in absolute lymphocyte count.
– Individuals with low-grade B-cell malignancies were given 2000mg green tea extract for 6 months. This significantly improved lymphocyte counts and certain biomarkers compared to baseline.
While most trials have been small and short-term, the results demonstrate anti-cancer effects of green tea compounds in actual cancer patients across different tumor sites. Larger and longer-term trials are still needed.
Green Tea and Cancer Treatment
In addition to cancer prevention, green tea may also benefit cancer patients undergoing treatment by reducing treatment side effects:
Radiation
– Breast cancer patients undergoing radiotherapy were given EGCG capsules during treatment. The EGCG group had significantly lower markers of oxidative stress compared to placebo.
– Another trial found that head and neck cancer patients who took EGCG supplements during radiotherapy experienced less salivary gland damage and dry mouth symptoms compared to placebo.
Chemotherapy
– In children undergoing chemotherapy for acute lymphoblastic leukemia, high-dose EGCG supplementation reduced liver toxicity compared to placebo.
– Women with breast cancer receiving doxorubicin and cyclophosphamide chemotherapy were given EGCG capsules. This resulted in lower cardiotoxicity, hepatotoxicity, and oxidative stress versus placebo.
– Multiple studies show that EGCG can protect normal cells from chemotherapy agents without interfering with their anti-tumor activity.
Green tea catechins appear to mitigate some of the adverse events of cancer treatment by virtue of their antioxidant and detoxifying properties. More research is needed to determine optimal doses for radioprotection and chemoprotection.
Optimal Intake for Cancer Prevention
Most of the cancer-preventive effects of green tea in human studies occurred with intakes of at least 3-5 cups per day. The typical green tea catechin content is:
– Brewed green tea: approximately 60-100 mg catechins per 8oz cup
– Green tea extract supplements: Standardized to 50% EGCG, 200-500mg per capsule
To reach beneficial levels of 500-1000mg of catechins daily, 3-5 cups of brewed tea or 2-4 capsules of standardized extract can be used. Higher doses may be optimal for adjunct cancer therapy under medical guidance.
More human data is needed to establish the ideal cancer-protective doses. However, consuming 3-5 cups of green tea daily as part of a healthy lifestyle appears quite safe and beneficial based on existing evidence.
Is Drinking Green Tea Enough?
While green tea intake appears protective against cancer, it should not be relied on as the sole strategy for cancer prevention. Green tea is likely most effective when consumed alongside:
– A diet high in fruits, vegetables, whole grains, legumes, fish, and healthy fats
– Regular physical activity
– Avoidance of smoking and excessive alcohol
– Weight management
– Early screening/testing based on age and risk factors
Green tea polyphenols enhance the beneficial effects of a comprehensive healthy lifestyle. But green tea should be viewed as an adjuvant rather than alternative to conventional cancer prevention strategies.
Possible Risks of High Intake
Green tea catechins are generally safe at moderate consumption levels. However, there are some potential concerns with excessive intake:
– Liver toxicity: High doses of EGCG supplements over 800-1000mg long-term may raise liver enzymes and cause toxicity in some individuals.
– Iron absorption inhibition: EGCG can bind to non-heme iron from plant sources and reduce absorption. Those with low iron status or on iron-restricted diets should avoid very high EGCG doses.
– Caffeine overdose: While green tea caffeine is generally low, very high intakes coupled with other caffeinated beverages may cause caffeine-related side effects like headache, insomnia, irritability, etc.
– Drug interactions: EGCG may interact with certain medications broken down by the liver. Check with a doctor before taking green tea supplements.
Moderation is key. For cancer prevention, the bulk of evidence supports reasonable intake of 3-5 cups green tea daily or 200-500mg standardized catechin supplements under medical guidance. More is not necessarily better when it comes to green tea.
Bottom Line
In summary, there is considerable epidemiological and clinical evidence that green tea intake may help prevent various types of cancer, including prostate, breast, colorectal, and lung cancers. The anti-carcinogenic catechins in green tea operate through a variety of mechanisms to block tumor growth and progression at multiple stages. Although larger clinical trials are still needed, green tea is emerging as a promising and affordable dietary strategy to reduce cancer risk as part of an overall healthy lifestyle. Consuming 3-5 cups of green tea per day or taking catechin supplements under medical supervision appears safe and potentially beneficial for long-term cancer protection.