Just my ignorant opinion, but I think calories are the worst thing to consider. Atkins book discusses the Metabolic Advantage, which is that by limiting the carbs, the amount of fat eaten doesn't matter. The excess fat is broken down into ketone bodies, pyruvic, citric and lactic acid and excreted into urine and stool. In mice being fed low carbs, the percentage of unused fat calories being excreted in urine and stool rose from 10 to 36%.

I don't remember where I read it, but I remember this: Fat burns in a carbohydrate flame. That suggests to me that a very small amount of carb is needed to light the fat-burning furnace. Finding the right amount of carbs is the goal.

Right now my treadmill says I'm burning 1000 calories in a one-hour workout. I'm running 40 miles a week on 10-15 grams of carbs a day and unlimited protein and fat. No bonking so far.