Green tea (Camellia sinensis) for the prevention of cancer

 

Abstract

Background

Tea is one of the most commonly consumed beverages worldwide. Teas from the plant Camellia sinensis can be grouped into green, black and oolong tea. Cross-culturally tea drinking habits vary. Camellia sinensis contains the active ingredient polyphenol, which has a subgroup known as catechins. Catechins are powerful antioxidants. It has been suggested that green tea polyphenol may inhibit cell proliferation and observational studies have suggested that green tea may have cancer-preventative effects.

Objectives

To critically assess any associations between green tea consumption and the risk of cancer incidence and mortality.

Search methods

We searched eligible studies up to January 2009 in the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, EMBASE, Amed, CancerLit, Psych INFO and Phytobase and reference lists of previous reviews and included studies.

Selection criteria

We included all prospective, controlled interventional studies and observational studies, which either assessed the associations between green tea consumption and risk of cancer incidence or that reported on cancer mortality.

Data collection and analysis

At least two review authors independently applied the study criteria, extracted data and assessed methodological quality of studies. Due to the nature of included studies, which were mainly epidemiological, results were summarised descriptively according to cancer diagnosis.

Main results

Fifty-one studies with more than 1.6 million participants were included. Twenty-seven of them were case-control studies, 23 cohort studies and one randomised controlled trial (RCT).

Twenty-seven studies tried to establish an association between green tea consumption and cancer of the digestive tract, mainly of the upper gastrointestinal tract, five with breast cancer, five with prostate cancer, three with lung cancer, two with ovarian cancer, two with urinary bladder cancer one with oral cancer, three further studies included patients with various cancer diagnoses.

The methodological quality was measured with the Newcastle-Ottawa scale (NOS). The 9 nested case-control studies within prospective cohorts were of high methodological quality, 13 of medium, and 1 of low. One retrospective case-control study was of high methodological quality and 21 of medium and 5 of low.

Results from studies assessing associations between green tea and risk of digestive tract cancer incidence were highly contradictory. There was limited evidence that green tea could reduce the incidence of liver cancer. The evidence for esophageal, gastric, colon, rectum, and pancreatic cancer was conflicting. In prostate cancer, observational studies with higher methodological quality and the only included RCT suggested a decreased risk in men consuming higher quantities green tea or green tea extracts. However, there was limited to moderate evidence that the consumption of green tea reduced the risk of lung cancer, especially in men, and urinary bladder cancer or that it could even increase the risk of the latter. There was moderate to strong evidence that green tea consumption does not decrease the risk of dying from gastric cancer. There was limited moderate to strong evidence for lung, pancreatic and colorectal cancer.

Authors' conclusions

There is insufficient and conflicting evidence to give any firm recommendations regarding green tea consumption for cancer prevention. The results of this review, including its trends of associations, need to be interpreted with caution and their generalisability is questionable, as the majority of included studies were carried out in Asia (n = 47) where the tea drinking culture is pronounced. Desirable green tea intake is 3 to 5 cups per day (up to 1200 ml/day), providing a minimum of 250 mg/day catechins. If not exceeding the daily recommended allowance, those who enjoy a cup of green tea should continue its consumption. Drinking green tea appears to be safe at moderate, regular and habitual use.

 

Plain language summary

Green tea for the prevention of cancer

Fifty-one studies with more than 1.6 million participants, mainly of observational nature were included in this systematic review. Studies looked for an association between green tea consumption and cancer of the digestive tract, gynecological cancer including breast cancer, urological cancer including prostate cancer, lung cancer and cancer of the oral cavity. The majority of included studies were of medium to high methodological quality. The evidence that the consumption of green tea might reduce the risk of cancer was conflicting. This means, that drinking green tea remains unproven in cancer prevention, but appears to be safe at moderate, regular and habitual use.

 

Abstract

Zeleni čaj (Camellia sinensis) za prevenciju karcinoma

Background

Tea is one of the most commonly consumed beverages worldwide. Teas from the plant Camellia sinensis can be grouped into green, black and oolong tea. Cross-culturally tea drinking habits vary. Camellia sinensis contains the active ingredient polyphenol, which has a subgroup known as catechins. Catechins are powerful antioxidants. It has been suggested that green tea polyphenol may inhibit cell proliferation and observational studies have suggested that green tea may have cancer-preventative effects.

Objectives

To critically assess any associations between green tea consumption and the risk of cancer incidence and mortality.

Search methods

We searched eligible studies up to January 2009 in the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, EMBASE, Amed, CancerLit, Psych INFO and Phytobase and reference lists of previous reviews and included studies.

Selection criteria

We included all prospective, controlled interventional studies and observational studies, which either assessed the associations between green tea consumption and risk of cancer incidence or that reported on cancer mortality.

Data collection and analysis

At least two review authors independently applied the study criteria, extracted data and assessed methodological quality of studies. Due to the nature of included studies, which were mainly epidemiological, results were summarised descriptively according to cancer diagnosis.

Main results

Fifty-one studies with more than 1.6 million participants were included. Twenty-seven of them were case-control studies, 23 cohort studies and one randomised controlled trial (RCT).

Twenty-seven studies tried to establish an association between green tea consumption and cancer of the digestive tract, mainly of the upper gastrointestinal tract, five with breast cancer, five with prostate cancer, three with lung cancer, two with ovarian cancer, two with urinary bladder cancer one with oral cancer, three further studies included patients with various cancer diagnoses.

The methodological quality was measured with the Newcastle-Ottawa scale (NOS). The 9 nested case-control studies within prospective cohorts were of high methodological quality, 13 of medium, and 1 of low. One retrospective case-control study was of high methodological quality and 21 of medium and 5 of low.

Results from studies assessing associations between green tea and risk of digestive tract cancer incidence were highly contradictory. There was limited evidence that green tea could reduce the incidence of liver cancer. The evidence for esophageal, gastric, colon, rectum, and pancreatic cancer was conflicting. In prostate cancer, observational studies with higher methodological quality and the only included RCT suggested a decreased risk in men consuming higher quantities green tea or green tea extracts. However, there was limited to moderate evidence that the consumption of green tea reduced the risk of lung cancer, especially in men, and urinary bladder cancer or that it could even increase the risk of the latter. There was moderate to strong evidence that green tea consumption does not decrease the risk of dying from gastric cancer. There was limited moderate to strong evidence for lung, pancreatic and colorectal cancer.

Authors' conclusions

There is insufficient and conflicting evidence to give any firm recommendations regarding green tea consumption for cancer prevention. The results of this review, including its trends of associations, need to be interpreted with caution and their generalisability is questionable, as the majority of included studies were carried out in Asia (n = 47) where the tea drinking culture is pronounced. Desirable green tea intake is 3 to 5 cups per day (up to 1200 ml/day), providing a minimum of 250 mg/day catechins. If not exceeding the daily recommended allowance, those who enjoy a cup of green tea should continue its consumption. Drinking green tea appears to be safe at moderate, regular and habitual use.

 

Plain language summary

Zeleni čaj (Camellia sinensis) za prevenciju karcinoma

Zeleni čaj za prevenciju karcinoma

U sustavni pregled o učinku zelenog čaja na prevenciju karcinoma uključena je 51 studija s ukupno 1,6 milijuna ispitanika. Radilo se uglavnom o tzv. opažajnim studijama – u kojima istraživač ne odlučuje tko će biti izložen nekom utjecaju ili nekoj terapiji, nego samo promatra podatke o postojećem stanju. U analiziranim studijama je ispitana povezanost između pijenja zelenog čaja i karcinoma probavnog trakta, ginekoloških karcinoma uključujući i karcinom dojke, karcinoma mokraćnog sustava uključujući karcinom prostate, karcinoma pluća i karcinoma usne šupljine. Glavnina uključenih studija bile su srednje ili visoke metodološke kvalitete. Dokazi o tome može li konzumiranje zelenog čaja smanjiti rizik od karcinoma bili su proturječni. To znači da i dalje nema dokaza da je pijenje zelenog čaja korisno za prevenciju karcinoma. S druge strane uzimanje zelenog čaja nije bilo povezano s nuspojavama pa ga je sigurno konzumirati umjereno i redovito.

Translation notes

Translated by: Croatian Branch of the Italian Cochrane Centre
Translation Sponsored by: Ministry of Education, Science and Sports

 

摘要

背景

綠茶(Camellia sinensis)用於防癌

茶是全球最受消費者歡迎的飲料之一。可製茶的為Camellia sinensis這一屬植物，可以製成綠茶、紅茶與烏龍茶。各種不同文化的飲茶習慣也不同。Camellia sinensis含有活性多分成份，其中的一部份稱為兒茶素，兒茶素是強力抗氧化劑。一般認為綠茶多酚可以抑制細胞增生，而觀察型研究認為綠茶可能有防癌效果。

目標

嚴謹評估飲用綠茶和發生癌症之風險與死亡率之間的任何關聯。

搜尋策略

我們在Cochrane Central Register of Controlled Trials (CENTRAL)、MEDLINE、EMBASE、Amed、CancerLit、Psych INFO和Phytobase等資料庫，搜尋至2009年1月止符合條件的相關研究，以及之前的回顧和納入研究的參考目錄。

選擇標準

我們納入所有的前溯研究、控制型介入研究與觀察型研究，每篇研究都評估飲用綠茶和癌症發生率風險的關聯、或者有報告癌症死亡率。

資料收集與分析

至少有2位回顧作者獨立的根據研究規範，進行資料摘錄與評估研究方法之品質。基於納入之研究的本質大多數為流行病學研究，結果是依照癌症診斷用描述方式呈現。

主要結論

納入51篇研究、超過160萬名研究對象。其中27篇為病例對照研究，23篇為世代研究與隨機控制試驗(randomised controlled trial (RCT))。有27篇研究試著建立飲用綠茶和消化道癌症之間的關聯，多數為上胃腸道、5篇探討與乳癌的關聯、5篇探討與前列腺癌的關聯、3篇探討與肺癌的關聯、2篇探討與卵巢癌的關聯、2篇探討與膀胱癌的關聯、1篇探討與口腔癌的關聯、3篇研究納入各種癌症診斷的病患。根據NewcastleOttawa量表測量研究方法之品質，有前溯世代的9篇巢式病例對照研究(nested casecontrol studies)，研究方法品質高，其他13篇屬中等品質、1篇低品質。1篇回溯性病例對照研究屬於高品質研究方法，其他21篇屬中等品質、5篇低品質。評估綠茶和消化道癌症發生風險之關聯的結果為高度矛盾。綠茶可以降低肝癌發生率的證據有限。食道癌、胃癌、結腸癌、直腸癌、胰臟癌的相關證據互相衝突。至於前列腺癌，高品質研究方法之觀察型研究與唯一納入的隨機控制試驗認為，飲用較多綠茶或綠茶萃取物的男性罹癌的風險降低。不過，飲用綠茶降低肺癌風險的證據屬有限到中等，特別是男性，而膀胱癌風險在後來甚至會升高。有中度到強烈證據認為飲用綠茶不會減少死於胃癌的風險。關於肺癌、前列腺癌與大腸直腸癌的證據屬有限中等到強烈。

作者結論

建議飲用綠茶以防癌的證據不足且互相矛盾。本回顧之結果，包括所提的與癌症的關聯傾向，都需謹慎解讀，且能否一概而論還有疑慮之處，因為納入的多數研究是在亞洲完成(n = 47篇)，也是飲茶文化風行的區域。適合的綠茶飲用量為每天3−5杯(一天最多1200 ml)，可提供至少每天250 mg的兒茶素。如果未超過每天建議許可量，那些喜歡每天喝個幾杯茶的人可以繼續飲用。適度、規律、習慣性的飲用綠茶看來是安全的。

翻譯人

此翻譯計畫由臺灣國家衛生研究院(National Health Research Institutes, Taiwan)統籌。

總結

納入此系統性回顧的有51篇研究、超過160萬名研究對象，本質大多數為觀察性研究。這些研究探討飲用綠茶和消化道癌症、婦科癌症包括乳癌、泌尿道癌症包括前列腺癌、肺癌與口腔癌之間的關聯。納入的研究之研究方法品質多數屬於中等到高等。飲用綠茶而降低癌症風險的證據互相矛盾，這表示，飲用綠茶之於防癌尚未獲證實。但是，適度、規律、習慣性的飲用綠茶看來是安全的。