VLCD stands for Very Low Calorie Diet and usually contains 800 Calories or less. They are generally in the form of a packaged shake or soup and are low in carbohydrate. The value of being low in carbs is that your body uses up its stores of glycogen (sugar) and must burn fat for energy. The sign that this is happening is the production of ketones.
You can test for the presence of ketones using urine test strips called ketostix from the chemist. A VLCD is generally used where rapid weight loss is required. It is also helpful to remove you from your usual eating habits for a few weeks so that you can wipe the slate clean and recreate your eating habits.
We also find a VLCD is handy to use for 2:5 intermittent fasting approaches where women consume just 500 Calories for 2 non consecutive days per week, and men 600. This is generally made up of 1 shake and 1 main meal per fasting day with a portion control approach of 1300 – 1600 Cals on the other days of the week.