I heard that caffeine is good for losing weight so I am going to drink coffee.
I'm drinking coffee since I heard that caffeine is effective for losing weight.
「ダイエットする」は英語で to lose weight (減量する)と言います。
1)I heard that caffeine is good for losing weight so I am going to drink coffee.
good for 〜 = 「〜に良い」「〜に効果的」
2)I'm drinking coffee since I heard that caffeine is effective for losing weight.
effective = 「効果的」
caffeine is effective for losing weight = カフェインは減量するのに効果的