The most well-known chemical constituent of tea is caffeine, which is stimulating in nature. Caffeine in tea is approximately 4 percent in dry weight. Tannin is the other principal constituent of tea.
Caffeine, theobromine, and theophylline are polyphenols belonging to the phenolic group of compounds. The major flavonols in tea are- catechin, epicatechin, epicatechin gallate, gallocatechin, epigallocatechin, and epigallocatechin gallate. Epigallocatechin gallate is the most active of these catechins. Another compound – L-theanine, is an amino acid found in tea. Theanine in green tea plays a role in reducing stress. Oxidized catechins (theaflavins in black tea) are said to reduce cholesterol levels in the blood.
Tea decreases lipid and carbohydrate absorption, increases lipid metabolism, inhibits de novo metabolic production of fat, and increases carbohydrate utilization. (Grove et al., 2010) Green tea intake is associated with increased weight loss due to diet-induced heat generation attributed to the catechin epigallocatechin gallate. (Shixian et al., 2006)
High caffeine intake was associated with weight loss through heat and fat oxidation and suppressed leptin in women. (Leptin is a molecule that triggers a signal of satiation to the brain). The green tea-caffeine mixture improved weight management in habitually low caffeine consumers, partly through heat generation and fat oxidation. (Westertep-Plantenga et al., 2005)