Actually, Green Tea supplementation has been shown to increase fat burning by 17% and insulin sensitivity by 13%. This study was published in March of this year.
However, they were using Green Tea supplements in the range of 890mg. Which means you’d need to take Green Tea pills like Mega-T that has a 775mg blend to see these results. I take the Green Tea pills, drink Green Tea at work, and Iced Tea (a black tea) at home, and I can tell the difference. It's not as dramatic as an ECA stack, but it does help.
A thread here about it.
Another study suggesting Green Tea increases thermogenesis beyond the effects of just caffeine.
And another study.
http://www.nature.com/oby/journal/v15/n ... 7176a.html
Another study that proposes how Green Tea might work. As of now, it is not entirely understood.
Several potential mechanisms responsible for the altered energy and fat metabolism have been proposed. Dulloo et al. (1999) have suggested a synergistic interaction between catechins and caffeine due to sympathetically released noradrenaline (NA). Since catechins are known to be capable of inhibiting catechol O-methyltransferase (COMT) (the enzyme that degrades NA) (Borchardt & Huber, 1975), and caffeine to inhibit trancellular phosphodiesterases (enzymes that break down NA-induced cAMP), it is proposed that the green tea extract, via its catechins polyphenols and caffeine, is effective in stimulating thermogenesis by relieving inhibition at different control points along the NA-cAMP axis. Taken together, both caffeine and tea catechins would be expected to increase and/or prolong the stimulatory effects of norepinephrine on energy and lipid metabolism.