Normal gym 'regimes' entail educating the client about long term balanced, healthy eating. The curves eating system is borderline ketogenic, which results in a lot of water loss..I've often done this myself(due to experimentation with bodybuilding techniques for muscle definition)..cut carbs out and lost ten pounds in two weeks..However most of this was water and when I reintroduced carbs(which you will have to at some point) I regained 90% of the weight over 48 hours..Since curves do not have an accurate body composition analysis they cannot say how much is fat loss or water loss..
Because the curves system does not provide a sufficient stimulus to retain lean muscle mass during such restricted eating, the rest of the weight loss is lean tissue..which is never a good idea...
Heres the background theory!...
Carbohydrates are stored in our bodies as glycogen, and with each gram of glycogen, three grams of water are stored. When we stop eating foods with carbohydrates, we start using stored glycogen to meet our energy needs. As we use glycogen, the water that was stored with it is lost through our urine. Our bodies normally have a reserve of approximately 800 grams(depending on body size), two pounds, of glycogen. So if we were to use all our stored glycogen, we would lose 2,400 grams, or six pounds of water.
On low carbohydrate diets, most of this water weight loss occurs in the first week of the diet, during which time the majority of the stored glycogen is consumed.After this lean tissue is lost at a rapid rate unless very demanding training is performed on each bodypart.
and to recap on the curves exercise system :
The machines are so inferior to anything else..the results being that muscles are not worked throughout a full range of possible movement . . . resistance is limited to an amount that can be moved in the weakest position . . . little or nothing is done in the way of improving flexibility, since there is no resistance in the fully extended position . . . and no resistance is provided in the fully contracted position..