Carbs and fat are your body's two sources of fuel. A low-fat diet is high in carbs, so when you eat more carbs than your body needs it stores them as fat. Atkins is an effective diet that transforms your metabolism from one that stores fat into one that burns fat. High-carbohydrate diets raise blood sugar, which in turn signals the body to secrete more insulin—you wind up with more fat on your body and a "sugar burning" metabolism. The Atkins diet restricts foods that are known to raise blood sugar and insulin. Limiting carbohydrates forces your body to use fat for energy rather than sugar.
The classic Atkins approach, which has been renamed Atkins 20®, is the traditional four-Phase program starting with Induction at 20 grams of Net Carbs per day. In addition, Atkins has decided to offer another option, with more flexibility and more food choices, called Atkins 40®.
On Atkins 40®, you begin with 40 grams of Net Carbs a day and move up progressively from there as you get closer to your goal weight, while still burning fat for fuel. In a nutshell (and yes, you can eat nuts from day one on Atkins 40!), Atkins 40 helps you make a variety of food choices from a selection of vegetables, nuts/seeds, a variety of fruits, whole grains, legumes, as well as dairy, including whole Greek yogurt and cheeses. Atkins 40 also focuses on portion control, and emphasizes nutrient-dense carbohydrates. Added sugars are still a no-no.