From the official Atkins Web site:
To begin, Atkins is not a diet, and that's not just marketing hype. It's called the Atkins Nutritional Approach (we'll be calling it the ANA for short), not the Atkins Diet. That's because there's more than weight loss at stake here: what really matters is your overall health and well-being. In fact, many people who don't need to lose weight choose to follow the ANA because of all the health benefits it provides.
When you give your body too many carbohydrates to metabolize, it burns them for energy. But if you carefully control your carb intake, your body burns fat instead. And that, in a nutshell, is the secret -- if there is one -- of the ANA.