1. I'm ( going ) on a diet.

我在节食减肥。

2. I'm cutting down on sweet food.

我正在减少甜食的摄入量。

3. I just drink sugar-free beverage.

我只喝无糖饮料。

4. You've got to eat less and exercise more.

你得少吃多运动。

5. She's on a diet.That explains why she eats like a bird.

她在节食，这就是她吃得这么少的原因。