Over the past few decades, numerous studies have been conducted by researchers to explore the nutritional value and pharmacological activity of tea. In 1991, Chen provided a comprehensive review of the main developments in this field. Since then, a considerable amount of research has focused on the biochemical and cellular mechanisms underlying the effects of tea on diseases such as cardiovascular and brain diseases, cancer prevention, and radiation prevention.
Tea is available in different forms due to variations in processing, which gives rise to three different types of tea products – green tea (unfermented), black tea (fermented), and semi-fermented tea like Oolong tea and Paochung tea. Green tea extract (GTE) and black tea extract (BTE) are extensively used by most scientists.
Green tea is known to contain polyphenols, which make up around 30% of the dry weight of tea leaves. These polyphenols comprise flavanols, flavandiols, flavonoids, and phenolic acid, with most of them being flavanols or catechins. The major catechins found in green tea are (-)-epigallocatechin gallate (EGCG), (-)-epigallocatechin (EGC), epicatechin-3-gallate (ECG), (-)-epicatechin (EC), (+)-gallocatechin (GC), and (+)-catechin (C).
During the fermentation process, a series of complex chemical reactions occur, with the most critical one being the oxidation of polyphenols. This leads to the formation of oxidized-polymerized compounds like theaflavins (TF), thearubigins (TR), and various aroma compounds that give black tea its distinctive flavor. However, the oxidation of polyphenols leads to a decrease in the number of catechins, and thus, BTE is found to be less effective than GTE in most experiments.
Catechins found in tea have various biochemical properties such as antioxidative, antiaggregation, and free radical scavenging activities, among others. These properties are attributed to the cancer preventive effects of tea. However, the biochemical mechanisms of tea in different diseases may vary due to several factors such as the concentration of catechins, their bioavailability, and the presence of other active components.
In conclusion, tea is a complex beverage that contains several active compounds that have potential health benefits. The biochemical and cellular bases of tea and their relationship with different diseases have been extensively studied by scientists, with the catechins being the most studied component. However, further research is needed to understand the full spectrum of tea’s benefits and to develop effective strategies for using tea in disease prevention and treatment.