Green tea consumption is associated with a number of health effects including weight loss and protection from cancer and cardiovascular disease. This evidence is supported by results from animal experiments and human clinical trials. The reason for the health effects is not fully understood, but it is thought that the presence of a group of phytochemicals called catechins, present in the tea plant (Camellia sinensis) in high concentrations, might be responsible for these effects. Catechins belong to a group of phytochemicals called the flavonoids and this group has been extensively researched for their health effects. Many experiments have demonstrated the bioavailability of green tea catechins. For example in one study1, researchers administered different amounts of green tea catechin solids (1.5, 3 or 4.5 grams) to human volunteers and then ,measured the catechins in their blood. Plasma levels of the main catechin increased for 1.4 to 2.4 hours after ingestion, with maximum concentrations being experienced after the 3 gram dose.
Therefore green tea catechins can be absorbed from the gut to the blood, and this may then allow them to have biological effects on tissues. However, is it a requirements that the catechins are absorbed for beneficial effects to occur? Green tea is associated particularly with a reduction in gastrointestinal cancer. This may suggest that the presence of catechins in the gut is sufficient for beneficial effects to occur. Evidence suggests that it is the association between gastrointestinal cancer and catechins that accounts for the total protective effect of catechins on cancer, and if this is the case then contact between the carcinogenic cells and green tea in its unmetabolized form may be a requirement of protection from cancer. In addition, the green tea catechins may also protect from the damaging effects of free radicals in the gut prior to absorption, for example, from lipid peroxides, and this may explain some of the cardiovascular benefits of green tea. Studies also indicate that red wine may be protective of health through gastrointestinal effects.