How might Green tea cause weight loss?

Green tea contains several elements that may promote weight loss. Crucially, it has catechins which are plant derived polyphenols that have potent anti-oxidant action. They act to damp the effect of free radicals in the body.

Of the catechins in green tea, epigallocatechin gallate, also known as EGCG, has been of especial interest. Its catechins act to inhibit the action of catechol o-methyltransferase (COMT).

Since COMT acts to degrade transmitters like norepinephrine, preventing its action may play a role in increasing energy use by the body.

Green tea also contains caffeine. Caffeine is a popular, mild stimulant that could to some degree promote weight loss. As a stimulant, it activates the body. More specifically, it may act on phosphodiesterase, to regulate cAMP levels.

Together, these combined actions of substances in green tea can in theory increase energy expenditure and cause fat oxidation.
What do the studies say?

Unfortunately, the science supporting the ability of green tea to cause weight loss is limited.

One analysis combined the results from 15 studies including a total of 1,243 patients. The studies were to see if green tea extract combined with caffeine could result in weight loss. They ran from 2 months to 6 months long. The results were disappointing.

In the studies, consumption of green tea led to a decrease in body mass index (BMI) of anywhere from lowering it by 0.6 to an increase of 0.2. Overall, in terms of BMI, green tea had little effect.

Additionally, while the studies did technically show a weight loss effect of green tea, it was a very minor effect.

In some studies, people gained a little weight, but in most, people lost a little. Ultimately, drinking green tea was associated with an overall weight loss of just .14kg, or less than half a pound.
Green tea and blood pressure

But while the data supporting green tea as a way to lose weight is somewhat limited, there is a good deal of evidence that green tea can improve heart health and possibly lower blood pressure.

In a study of 1507 Taiwanese subject, those who drank several cups of green tea a day had a drop in risk of developing hypertension of up to 65%. On the other hand, other studies have not shown such a benefit for lowering hypertension but have, in general, supported green tea as a way to improve heart health.

Source:

http://healthlifeandstuff.com/2011/20/green-tea-and-weight-loss/