Back tea is produced by a lengthy fermentation process on the leaves of the Camellia sinensis plant. This process results in the destruction of much of the polyphenol content of the leaves because the enzyme polyphenol oxidase metabolises many of the polyphenols within the leaves to theaflavins and thearubigins. In contrast, green tea is made from the steaming of the tea leaves, which inhibits polyphenol oxidase and conserves many of the originals polyphenols. Oolong tea is prepared with partial fermentation and so the levels of polyphenols is somewhere between that of green tea and black tea. This partial fermentation results in the formation of unique chemicals such as theasinensins, which are dimmers and oligomers of the original polyphenols. All of the teas contain methylxanthines, which includes about 5% caffeine by weight as well as the theophylline and theobromine.
Green tea has become of interest to researchers because the polyphenols present have been shown to confer weight loss effects. Green tea polyphenols belong to the flavon-3-ol (catechin) class of flavonoids and include epigallocatechin gallate (EGCG), epicatechin (EC), epigallocatechin (EGC), and epicatechin gallate (ECG). A typical cup of green tea contains roughly 240 to 320 mg of flavon-3-ols with roughly 50 % of this amount being made up of EGCG. Although the weight loss effects of green tea are not fully understood, a combination of animal, human, in vitro and epidemiological studies are starting to build a clearer picture of the mechanisms of action (figure 1). Much of the research has focused on the effects of EGCG because it is the most plentiful flavon-3-ol in green tea, although black and oolong tea have been less well studied.
Figure 1. The multiple mechanisms of action of green tea polyphenols and their physiological consequences.
Green tea catechins work to reduce bodyweight via a number of different mechanisms. Green tea can inhibit the absorption of both glucose and lipids in the small intestine because catechins inhibit the SGLT1 transporter and lipase enzyme respectively. In addition, catechins are able to act on the liver where they decrease the synthesis of lipids (de novo lipogenesis) as well as increase their oxidation. They may also be able to increase the storage of glycogen in the liver, reducing blood sugar levels. In skeletal muscle, catechins increase insulin sensitivity by up regulating glucose transporters, thus increasing the catabolism of carbohydrates. In contrast, in adipose tissue, catechins may decrease the uptake of glucose by inhibition of glucose transporters. In both skeletal muscle and adipose tissue, fatty acid oxidation may be increased in the presence of catechins from tea.
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