I would mark you up, but wanted to add that carbohydrates, or perhaps some specific high carb foods, are the cause of a whole slew of physical problems that a lot of people have. When I did low carb, not only did I lose weight fast, but allergies went away, I had consistently high energy throughout the day ... and at the end of the day my energy level dropped like a rock and I slept solidly through the night whereas, before that, I consistently had trouble sleeping. Even skin problems cleared up - dandruff, eczema went away completely.
I also want to add to what people are saying - you don't absorb every calorie you eat. That's been mentioned in a few posts above, but not enough, I think, although you cannot use that fact as an excuse to overeat. But beyond that, a lot of low-carb naysayers don't get that you can lose more calories than you actually burn when you're in ketosis. The simple (calories in) - (calories out) is just far too simple, and it's just accurate.