Reviewed: Dufresne CJ, Farnworth ER. A review of the latest research findings on the health promotion properties of tea. Nutritional Biochemistry 2001;12:404-21.
In this extensive article, the authors review and interpret the large body of research on tea (Camellia sinensis (L.) Kuntze, Theaceae), focusing on its health-promoting properties.
In recent years, scientists have performed much research on the health benefits of substances found in foods of plant origin, including tea. Attention has focused on polyphenols, especially the catechin flavonoids, which are potent antioxidants that may protect cells from damage caused by reactive oxygen species (ROS). Tea is the best dietary source of this class of very active flavonoids.
Tea contains many different catechins, as well as flavonols, methylxanthines, organic acids, and other chemical constituents. The composition of tea, particularly unfermented green tea, is now well known, but “knowledge about the digestion, absorption and metabolism of tea by humans is at its infancy,” the authors say.
The authors discuss the anti-cancer effects of tea in detail, writing, “It is evident that tea polyphenols exhibit many protective activities and different metabolic pathways are involved. They act as antioxidants, they selectively inhibit specific enzyme activities, they target and repair DNA aberrations.” Some tea components appear to act synergistically, producing even stronger protective effects when administered together than the effects of their individual activities added together.
In vivo animal studies indicate that tea inhibits many types of cancer, including skin, lung, and digestive tract cancers. Human studies of tea consumption and cancer risk are difficult to conduct and interpret because human diets are complex and are constantly changing. In epidemiological studies, “the confounding factors are generally more variable than the effect tested, and the results are not conclusive,” the authors say. However, some cohort studies suggested that green tea may protect against colon, stomach, pancreatic, bladder, and esophageal cancers.
The authors also describe the mechanisms by which tea components appear to inhibit cardiovascular disease (CVD) development. Several substances in tea interfere with the process of lipoprotein oxidation, thereby limiting a key step in the development of atherosclerosis. Further, tea catechins and rutin have anti-inflammatory effects that may inhibit atherosclerosis, now recognized as an inflammatory process. Caffeine and other components of green tea may promote fat oxidation, thereby inhibiting obesity, a major risk factor for CVD. And its diuretic effects increase elimination of procarcinogens and mutagens, especially through regular use.
Overall, animal studies support the theory that tea protects against CVD, but human studies have produced inconclusive results. One possible reason why human studies have not yielded consistent results is the genetic heterogeneity of lipid metabolism in human populations. Research has shown that the effects of tea consumption differed among the three human genotypes of a gene affecting apolipoprotein E. Subjects with one genotype showed beneficial effects of tea consumption, while subjects with another genotype had no significant effects. “In the near future, it will be possible to identify sub-populations at risk for CVD that will benefit most from the addition of tea to the diet,” the authors say.
Tea components clearly influence many cellular functions. Green tea contains a higher concentration of, and resulting protective effect from polyphenols than fermented black or partially fermented oolong teas. Tea may protect against various diseases and conditions in addition to cancer and CVD. The authors provide a brief discussion of tea’s potential beneficial effects on renal function, diabetes, skin and eye tissue, arthritis, and dental caries. Evidence supporting the antiviral, antibacterial, and antihistamine effects of tea is also discussed. Finally, studies have shown that tea may have psychological and neurological effects, such as increases in alertness and information processing; these effects were not due only to caffeine, but were linked to theanine in tea.
In summary, evidence of tea’s beneficial properties continues to accumulate. Tea is now considered an important part of a healthful diet. New products, such as tea extract capsules sold as dietary supplements (as well as the old familiar iced tea as an alternative to soft drinks), are available to consumers.
—Christina Chase, M.S., R.D.