There are a few principles I need you to understand in order to put Atkins into action. Let's startwith a couple of definitions. First, a calorie is simply a unit of energy-precisely the amount ofheat needed to raise I gram of water 1 degree Celsius at sea level.
    Second, metabolism is the sum of the physical and chemical processes
by which food istransformed into energy. Now let's move on to some concepts and misconceptions.

Don't Excess Calories Cause Weight Gain?

It is true that gaining weight results from taking in more calories than you expend. But excesscalories certainly cause you to pile on the pounds-and this is a gigantic "but"-only when you areeating a lot of carbohydrate along with fat.
    So it's time to abandon the assumption that the only way to lose weight is to strictly controlyour intake of calories. Many people think that only one thing matters: how many calories youtake in and use up. It's not that simple.
When you follow a controlled carbohydrate approach, you get what I call a "metabolicadvantage.
When you control carbohydrate consumption sufficiently, your body will switch fromburning glucose derived from carbohydrate to burning primarily fat for energy. This means youcould eat, say, 2,000 calories and still begin losing pounds and inches.
    In contrast, if you were consuming 2,000 calories on a low-fat diet, you might not lose weight,and you might actually gain weight. The metabolic advantage is that burning fat takes moreenergy so you expend more calories. And if you eat fewer calories-as many Atkins people dobecause their appetite is usually diminished-you'll likely lose weight even faster. So it's not thatcalories don't count, it's just that you will burn more of them, with less hunger, when your bodyis operating on a fat-based metabolism.
    When I published the first edition of this book ten years ago, that claim was quitecontroversial. Today it is galloping into acceptance among scientists who study the humanmetabolism. Glance through the references listed at the back of this book and you will see thenumerous studies and articles published in the past six years. And, we will deal with themetabolic advantage in greater detail in Chapter 7.