Insulin is a very important hormone in the body. It is the main hormone that regulates blood sugar levels and energy storage.
One of the functions of insulin is to tell fat cells to produce and store fat, and to hold on to the fat that they already carry. It also tells other cells in the body to pick up glucose (blood sugar) from the bloodstream, and burn that instead of fat.
So, insulin stimulates lipogenesis (production of fat) and inhibits lipolysis (the burning of fat). It is actually well established that low-carb diets lead to drastic and almost immediate reductions in insulin levels.
According to many experts on low-carb diets, including Gary Taubes and the late Dr. Atkins, lower insulin levels are the main reason for the effectiveness of low-carb diets.
They have claimed that, when carbs are restricted and insulin levels go down, the fat isn’t “locked” away in the fat cells anymore and becomes accessible for the body to use as energy, leading to reduced need for eating.
The bottom line is that blood levels of the hormone insulin go way down when carb intake is reduced. High insulin levels contribute to fat storage, and low insulin levels facilitate fat burning.