It's not that they don't lose weight: they do, but when they go off the diet, the kilos creep back.

It's not that they don't lose weight: they do, but when they go off the diet, the kilos creep back. 
go off: ngừng làm, ngừng hoạt động
Các từ còn lại:
go on: tiếp tục
go back: đi ngược về
go away: đi khỏi