Of course the magic calorie number is individual for each person based on their metabolism, level of activity, genetics, type of food eaten, etc. But there is a rough level of calories that exists to maintain any given weight for any given person if you consider these factors.

If I weigh 200 lbs, in order for me to get down to 125 lbs, I must find the most efficient way to boost my metabolism and create an "energy deficit" to take me down there. I could eat the same way I'm eating and increase exercise (but chances are if I am overweight at 200 lbs, in addition to the exercise it would be beneficial to change how and what and why I eat as well, because as soon as I stop exercising, the weight will come back on, moreover, even with a 500 per day calorie deficit, ultimately the maintenance calorie level will come down with a lower body weight, and unless you also eat less, you will plateau--then stop exercising, and you are back to square one.

What low-carbing does is first of all give our metabolism about a 200-300 calorie per day boost on what would otherwise be an average calculation of maintenance calories. In addition, low-carbing cuts down on hunger, so it is easy to manage smaller (reasonable) portions and still feel comfortable, then over time get accustomed to eating less. I think that in addition to eating the wrong foods, those of us who have a serious weight problem have also been eating portions that were too large--for boredom, comfort, or simple hunger (often induced by the quality/type of the food itself stimulating insulin which in turn stimulates hunger).

However, we will all reach a point where intake and expenditure level out and we will hit a plateau. At that time we have to decide how we want to create the deficit again if we need to lose weight (and how fast we want to do it which should take into account how much weight we want to lose as well).

For long-term committment, particularly as we approach the last 5 or 10 pounds to our goal, making realistic small changes will have the most lasting effect over time. For many, it's a lot easier to cut out 100 calories a day of food intake, than to commit to a daily full hour of calisthenics. The best approach is probably a combination of less intake and more activity, moreover one that we can realistically expect to keep doing for the rest of our lives.

Dr. Atkins proposes that as we approach our goal, the method of adding healthy carbs back in in grams while simultaneously reducing total dietary fat will help us to achieve our long-term goals (pre-maintenance and maintenance). In essence, this is reducing calorie-dense food and replacing it with less calorie-dense food, but very gradually, so as not to trigger an insulin (and hunger) effect.

Those of us who will add carbs back in but not cut back proportionately on fat, will indeed start gaining weight again, perhaps at an accellerated rate because the combination of high fat and high carbs will trigger weight gain and dangerous body chemistry in the blood lipids. Of course if you do that, simply you are also adding extra calories which are not taken away elsewhere thus creating an overabundance of calories as well.

I think this is why a lot of folks who initially succeed on low-carb fail to maintain it, and why Dr. Atkins specifies 4 stages to his lifetime eating plan.