Prior to entering ketosis, many diets call for reduced calories. This is a mistake as the body will find it easier to turn to protein rather than fat, leading to a potential loss of strength. Reducing calories outside of ketosis leads to hunger, increasing the calories consumed. It is this roller coaster that causes diets to fail.
When carbs are reduced to less than 50 grams per day (one donut), in about 1-4 days the body’s metabolism shifts to ketosis. During ketosis, the body manufactures sugar (glucose) from fat. Weight loss starts by reducing carbs and increasing fats until the body enters ketosis. At that point, when the body is manufacturing glucose from fat calories can be safely reduced without the insulin/hunger rebound or loss of muscle mass.