Consumption of green tea slightly increases metabolism, which in turn burns calories. As far as I know, drinking 5 cups of green tea in a day burns as much calories as a 5 mintute jog (which isn’t a lot actually). I have been drinking 1-1.5 litres of green/black/herbal tea daily for the past one year, and I’ve definitely seen changes in weight. Also, if you drink it on an empty stomach, it prevents you from getting hungry. I’d recommend consulting your doctor as well prior to consuming it in larger quantities. From what I know, it also slightly reduces estrogen levels in women, increases androgen levels in men (though I’ve experienced no change), and if consumed occasionally (but not daily), it reduces the red blood cell count.
I don’t know how lemon helps, but for best results, use loose-leaf teas only.