While tea consumption has often been associated with better health, its effects on the risk of oral cancer are unclear. Based on this, researchers at China's Huazhong University of Science and Technology and Jingchu University of Technology conducted a meta-analysis to clarify the relationship between tea intake and oral cancer risk, as well as to quantify any dose-response link between the two.
They performed a search within databases such as Pubmed, Embase, and Web of Science to identify potential studies that had evaluated the association between oral cancer risk and tea intake, extracting pooled odds ratios with 95% confidence intervals and combining them to assess the strength of their associations.
The T about tea
They also conducted dose-response analysis in order to quantitate the link between tea consumption and oral cancer risk, and included a total of 14 articles involving 5,920 patients and 10,553 controls in their final analysis.
Eight of the studies were performed in Asia, two in America, three in Europe and one in Africa. Four of these studies explored the link between tea intake and the risk of oral cancer in women, while three investigated the same association in men.
The dose-response analysis' linearity model showed that with one added cup of tea a day, the risk of developing oral cancer decreased by 6.2%, while according to sub-group analysis, there was an "inverse association" between tea consumption and oral cancer risk.
The exception to this was sub-group analysis of the effects of black tea consumption on oral cancer risk in American subjects, which suggested that "ethnics and gene polymorphism may have a significant affection on the effect of tea consumption in oral cancer prevention".
Furthermore, stratified analysis based on the type of tea consumed found that black tea intake was not linked to oral cancer risk, possibly because most black tea polyphenols are dimerised by extended oxidation with enzymatic reactions, thereby diminishing the tea’s anti-cancer effects during the preparation process.
Catechins versus cancer
The association between tea consumption and lower oral cancer risk was attributed largely to catechins, which are considered major polyphenolic compounds in tea.
More specifically, the catechin epigallocatechin-3-gallate (EGCG), was reported as the "most prevalent" and thought to be the main molecule to have prevented carcinogenesis.
The researchers explained that EGCG was observed to have altered the activity of certain enzymes and genes in different cancer cells — in vivo and in vitro.
EGCG was also said to have helped suppress cancer growth by decreasing the expression of Class I histone deacetylases (a class of enzymes that allow highly alkaline proteins to wrap the DNA more tightly), and to have induced apoptosis of cancer cells by regulating the expression of several microRNAs.
Still, the exact mechanism by which tea consumption was able to prevent the formation of oral cancer remained unclear.
Brewing for precision
The researchers noted that this study was the first dose-response meta-analysis to assess linear and non-leaner relationships between tea consumption and oral cancer risk at different exposure levels.
In addition to the effect of the dosage of tea on oral cancer risk, they also analysed the link between the duration and concentration of tea intake and the risk of oral cancer.
However, they acknowledged that their meta-analysis lacked associated cohort studies and relied on case-control studies instead, raising the possibility of publication and selection bias.
The pooled data used as the final analysis of their meta-analysis (in the absence of individual data) also meant they were unable to obtain more exact analysis and precise results.
In conclusion, they wrote: "The results of our meta-analysis indicated an inverse association between the tea intake and the risk of oral cancer. Dose-response analysis suggested that dietary high-dose, long-term and high concentration of tea intake may be associated with the reduced risk of oral cancer.
"Finally, our study suggested more large-scale pooling and high-quality prospective studies are necessary for detecting the precise relationship between tea intake and oral cancer risk in the future."
"Tea consumption is associated with decreased risk of oral cancer: A comprehensive and dose-response meta-analysis based on 14 case–control studies (MOOSE compliant)"
Authors: Hao Zhou, et al.