The catechins found in Japanese green tea have been studied to be capable of inhibiting the actions of α-amylase and sucrase. Inhibiting these two digestive enzymes suppresses the decomposition of starch and thus prevents the production of glucose.
By reducing the amount of fat produced from glucose, this is believed to indirectly prevent the drinker from gaining excess weight and reduce the risk of obesity.
In an experiment conducted on obese men, a research report found that participants who drank approximately 482mg of catechin daily had significantly less abdominal fat as compared to those who consumed only 180mg of catechin (Masa Hase, Ichiro Tokiko et al., 2001).
This benefit of catechin is believed to be compounded by the added interaction with caffeine, which is also present in tea.