No, thanks. I am on a diet.
不，谢谢。我在节食。

on a diet意思为吃规定的食物，即节食。

例句：

You're skinny enough without going on a diet!
你不必节食就已经够瘦的了!
I'm supposed to go on a diet.
我应该节食。
He is on a diet to reduce some weight.
他正在节食以减轻体重。