If you’re looking for scientific evidence, here’s a very-frequently-cited 1976 article (before Atkins, but describing an Atkins-like "ketogenic" diet): Composition of weight lost (percentage) during the ketogenic diet was water 61.2, fat 35.0, protein 3.8. During the mixed diet, composition of loss was water 37.1, fat 59.5, protein 3.4. ... During starvation, mean rate of weight loss was 750.7+50.9 g/day, the composition (percentage) being water 60.9, fat 32.4, protein 6.7.In other words, on either the Atkins-like diet or the starvation diet 60% of the weight loss was just water weight. The "mixed" diet (basically, a lower-calorie diet, radically so in the case of this study) subjects did lose more fat weight than water weight, over the course of several weeks, but again, in the earliest part it was primarily water loss.The results of this study are well accepted and have been repeated in various forms, further refining our understanding. The upshot: eating less, but not going overboard on any particular kind of food, is the best way to produce real fat weight loss.You just can't burn fat in the first week of a diet. It takes time for your body to shift to other metabolic pathways, even if you shock it with a diet designed to force your body to burn fat. Instead, the body shifts into a starvation mode, burning up all of its stores of sugar, which is then excreted as water and CO2.