water and what happens to your metabolism due to your diet. CICO ignores all of these things. I can go on a 500 calorie grapefruit diet and I will lose weight quickly, but much of it will be muscle, my metabolism will slow down, my hormones, insulin, etc... will be all out of whack, and as soon as I get off that diet I will gain the weight back much faster in the future due to a now slower metabolism. I work in weight loss. I literally study and talk about this stuff all day everyday.

You can lose weight on any calorie deficient diet, but the effectiveness of losing the weight, whether your losing fat vs muscle, keeping your metabolism from slowing down, etc... whether you can keep the weight off long-term, etc..., all depends on what you eat.