doughnut only diet and lost weight because he was at a caloric deficit. However what that doesn't take into account is what type of weight you're losing, (muscle, water or fat), how efficiently you're losing weight and what your diet is doing to your metabolism. A lot of people will do an internet fad diet, lose a lot of muscle as well as fat, slow down their metabolism and as soon as they stop doing the diet gain more weight back then when they started.