The Atkins diet involves limited consumption of carbohydrates to switch the body's metabolism from metabolizing glucose as energy over to converting stored body fat to energy. This process, called ketosis, begins when insulin levels are low; in normal humans, insulin is lowest when blood glucose levels are low (mostly before eating). Ketosis lipolysis occurs when some of the lipid stored in fat cells are transferred to the blood and are thereby used for energy. On the other hand, caloric carbohydrates (for example, glucose or starch, the latter made of chains of glucose) impact the body by increasing blood sugar after consumption. (In the treatment of diabetes, blood sugar levels are used to determine a patient's daily insulin requirements.)^[1] Fiber, because of its low digestibility, provides little or no food energy and does not significantly impact glucose and insulin levels.

In his book Dr Atkins' New Diet Revolution, Atkins made the controversial argument that the low-carbohydrate diet produces a metabolic advantage because "burning fat takes more calories so you expend more calories".^[2] He cited one study where he estimated this advantage to be 950 calories (4.0 MJ) per day. A review study published in Lancet^[3] concluded that there was no such metabolic advantage and dieters were simply eating fewer calories because of boredom. Professor Astrup stated, "The monotony and simplicity of the diet could inhibit appetite and food intake".

The Atkins Diet restricts "net carbs" (digestible carbohydrates that impact blood sugar). One effect is a tendency to decrease the onset of hunger, perhaps because of longer duration of digestion (fats and proteins take longer to digest than carbohydrates). Atkins states in his 2002 book New Diet Revolution that hunger is the number one reason why low-fat diets fail and that the Atkins diet is easier because you are allowed to eat as much as you want.^[2]

Net carbohydrates can be calculated from a food source by subtracting fiber and sugar alcohols (which are shown to have a smaller effect on blood sugar levels)^{[citation needed]} from total carbohydrates. Sugar alcohols contain about two calories per gram, and the American Diabetes Association recommends that diabetics count each gram as half a gram of carbohydrate.^[4] Fructose (for example, as found in many industrial sweeteners) has four calories per gram but has a very low glycemic index^[5] and does not cause insulin production, probably because β cells have low levels of GLUT5.^[6][7] Leptin, an appetite regulating hormone, is however not triggered following consumption of fructose. This creates an unsatisfying feeling after consumption, promoting binge behavior that culminates in an increased blood triglyceride level arising from fructose conversion by the liver.^[8]

Preferred foods in all categories are whole, unprocessed foods with a low glycemic index, although restrictions for low glycemic carbohydrates (black rice, vegetables, etc.) are the same as those for high glycemic carbohydrates (sugar, white bread). Atkins Nutritionals, the company formed to market foods which work with the Atkins Diet, recommends that no more than 20% of calories eaten while on the diet come from saturated fat.^[9]

Atkins' book, Atkins Diabetes Revolution, states that, for people whose blood sugar is abnormally high or who have Type 2 diabetes mellitus, the Atkins diet decreases or eliminates the need for drugs to treat these conditions. The Atkins Blood Sugar Control Program (ABSCP) is an individualized approach to weight control and permanent management of the risk factors for diabetes and cardiovascular disease.^[10] Nevertheless, the causes of Type 2 diabetes remain obscure, and the Atkins Diet is not accepted in conventional therapy for diabetes.