on a diet 控制饮食

OK. It's official. I'm on a diet for one month starting today!

好，我决定了。我从今天开始控制饮食一个月！

cut back 少吃

Are you sure? We're all supposed to go out for pizza tonight.

你确定吗？我们今天晚上都准备出去吃披萨的呀。