It's healthier to be slim./苗条更健康。

A: I try to stay slim.

A: 我努力保持苗条。

B: So do I. But it's hard. You have to exercise and watch your diet.

B: 我也是。但是很难。你得做运动，并且注意饮食。

A: I know.  I'm careful about my diet.

A: 我知道。我对饮食很小心。

B: So am I. It's healthier to be slim.

B: 我也是。苗条更健康。

A: Yes, it's healthier and you look better.

A: 是的，苗条更健康，而且你看起来更好看。

B: And you feel better, too.

B: 你也感觉更舒服。1. Many people nowadays talk about how they watch their diet, diet: I'm careful about my diet. / I'm careful about my diet.
现在很多人都谈论他们是怎样注意饮食的，diet /饮食: I'm careful about my diet. /我对饮食很注意。Notes 注释

2. Most people believe looks and health are related. You can say: It's healthier to be slim.  / It's healthier to be slim; You look better /You look better;You feel better, too. /You feel better, too.
大多数人相信外表和健康有关系。你可以说：It's healthier to be slim.  /苗条更健康；You look better /你看起来更好看；You feel better, too. /你也感觉更舒服