In one month or say 6 weeks , I seem to have lost 4 kgs of weight. I was 84 kgs & now 80. Cite reasons. Is it because of green tea alone?

If you are drinking only green tea, or drinking only water, or a beverage and nothing else, you are depriving yourself of essential nutrients and is an unwise decision if you want to maintain you health. Nothing, absolutely nothing, is worth losing your wellbeing and health (if indeed you had that distinction in the first place).

You don't say male or female. Each has their own bodily requirements to maintain a healthy lifestyle. Diet and exercise properly regulated are paramount.

You are missing protein, essential to maintain muscle mass.

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