be on a diet

To participate in a specific nutritional plan, typically consisting of eating smaller amounts of food, eating (or avoiding) particular foods, or both, in an attempt to lose weight or achieve some other health benefit. A: "Would you like some of my lemon meringue pie?" B: "No, thank you. I'm on a diet. I'm aiming to lose 15 pounds by the summer." My doctor says I need to be on a diet of nothing but fruits, vegetables, and nuts if I'm going to get my cholesterol down. She's been on a strict diet ever since the doctor determined which foods were irritating her digestive tract.