اولا ً : نگاه کن به حرف اضافه ای که با diet اورده یعنی on . خود on معنی روی چیزی و می ده و معنی مکان از بر می آد
کلا went on a diet ظرف مکانه . بهمین خاطر where اورده
ثانیا : تو انگلیسی آمریکایی خیلی مرسومه که به جای which از where استفاده کنی .
مثالی دیگر :
Kim went on a diet where she ate several bananas and drank a lot of water everyday
recently I went on a "diet" where I just gave my body the foods
They went on a diet where you purchased the meals
when I was 13 and we went on a diet where you ate only certain foods each day
So if you once went on a diet where you had to restrict your food intake to the point where you constantly felt hungry
he went on a diet where he ate nothing but potatoes for 60 days
If I went on a diet where I felt restricted by what I could eat I'd give up and end up regaining the weight