As far as I know, the Energy drink is just a normal treat.
It is the favorite one of Androtchi/Robotchi.
To make your Tama lose weight you need to buy the Diet Soda - that looks different, it doesn't have an 'E' on it - there's a pic that looks more like old fashioned weighing scales. The Diet Soda takes your Tamas weight down to 30lbs instantly!