Ketosis is a metabolic reaction that occurs in the body, when following the OPTIFAST® VLCD™ Program Intensive phase. This means the body shifts from using primarily carbohydrates to using fats as an energy source resulting in a reduction of body fat stores. Ketones are by-products that are created when the body has to switch to fats as an energy source. It means stores of glycogen or carbohydrates have been depleted. One of the consequences of ketosis is a decrease in appetite, which helps make following the OPTIFAST® VLCD™ Program Intensive phase much easier. As the Intensive phase causes only a mild ketosis; it is a very safe approach to weight loss.