No, thanks. I am on a diet.
    不，謝謝。我在節(jié)食。
    on a diet意思為吃規(guī)定的食物，即節(jié)食。
    例句：
    You're skinny enough without going on a diet!
    你不必節(jié)食就已經(jīng)夠瘦的了!
    I'm supposed to go on a diet.
    我應該節(jié)食。
    He is on a diet to reduce some weight.
    他正在節(jié)食以減輕體重。