He's 'on a diet'. [=he is eating less food or only particular kinds of food in order to lose weight]
Environmental changes are depriving the birds of the insects that form their 'staple diet'.
We've been given a steady 'diet of' political scandals in recent months.
(..) Okay, deal . What's your problem? You're my problem . But I think you're stressed. That's why you eat so much. It's hard to get fat on a diet. I'm not fat. It's all this fur. It makes me look puffy. All right, yeah. Fat hair. But when you're ready to talk, I'm here. Just drop it on the land. (..)2002 Ice Age