Atkins is a departure from the previously prevailing metabolic theories. Atkins said that there are important unrecognized factors in Western eating habits leading to obesity. Primarily, he believed that the main cause of obesity is eating refined carbohydrates, particularly sugar, flour, and high-fructose corn syrups. Atkins involves restriction of carbohydrates to more frequently switch the body's metabolism from burning glucose as fuel to burning stored body fat. 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). Caloric carbohydrates (e.g., glucose or starch (itself made of chains of glucose)) produce most of the blood sugar after meals and can be calculated to even determine the insulin needs of diabetics. Because of its low digestibility, fiber provides little or no food energy and does not significantly impact glucose and insulin levels. Ketosis involves lipolysis in which some of the lipid stores in fat cells are transferred to the blood.