Ketosis is a metabolic reaction that occurs in your body when following the OPTIFAST® VLCD™ Program Intensive phase. This means your 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 your body has to switch to fats for energy. It means your 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 OPTIFAST® VLCD™ Program causes only a mild ketosis, it is a very safe approach to weight loss.


Before your body transitions into ketosis, you will most likely feel hungry and you may experience some side-effects such as fatigue, lack of concentration, nausea & headaches. These side-effects should only last about 3 days and most symptoms usually pass by in 4 - 6 days. This is what we refer to as the 3 Day Challenge.