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.
(..) for four months. I mean, it's pretty good, right? Let's stop screwing around and just get it over with. Who cares? Okay, obviously your diet of power bars and boxed wine has scrambled your brain. We got as great thing going here, you know? Why mess with it? Yeah, you're probably right. (..)2010 TvShow: Scrubs Title: Our Thanks Season: 9 Episode 13