agreed on the calories thing. I've been on diets reliant on ketosis (atkins, dukan, low-carb, etc), and the weight loss is pretty steady and good but it's nowhere near as high as with a VLCD.
Having said that, the losses seen in the first week can also be achieved on a normal ketosis diet... i lost 10lbs in the first week of a VLCD last week, but on the Dukan diet where you can eat as much as you like as long as it's only protein... i also lost 10lb in the first week... and i ate a LOT. But the weightloss didn't continue at the same rate, it slowed to 1 to 2 lb a week (vs 3 to 5 lb on VLCD). The dramatic first week loss is mostly down to water lost during the process of your body using up it's glycogen stores.
To burn 1lb of fat you have to 'lose' 3500 calories.
If you need 2500cal a day (or more like 2000 if you are close to goal weight) and you are eating just 600 a day then...
In 1 week you need 17500 calories
You actually eat 4200 calories
Calorie deficit is 13300 calories, which needs to come from elsewhere (fat burning) and at 3500cal for 1lb of fat that is 3.8lbs worth of fat melting in one week. Cool eh
. This also explains why it is harder to lose weight towards goal, because firstly, your amount of needed calories decreases when on any kind of diet the longer you are on it (around 10% to 15%), and on top of that, a smaller body needs much less calories per day, so the calorie deficit is much less in a small person.