Very low Energy Diets otherwise called VLEDs are diets where you eat <800 calories/day or <3350 kJ/day.

VLEDs involve replacing all meals with a specific meal replacement formula during the intensive early phase. These high protein-low carbohydrate diets induce fat burning and mild ketosis, which results in suppression of hunger and promotion of fullness. Treatment duration with a VLED is generally 8–12 weeks, however, safe year-long use under strict medical supervision has been reported in some studies. Generally you go through an intensive phase for a few weeks then a transition and maintenance phase for a few months where you replace the low calorie shakes/meals that you bought with healthy food you and have cooked. For example you might only have meal replacement shakes and bars for 3-4 weeks, then add one normal healthy meal and the rest meal replacements, and then 2 meals and one meal replacement etc.  This is great because for those who need a clean slate to start with it allows you to phase one out and phase one in slowly, giving you time to adjust to making healthier habits. You can also use them safely and effectively on an as needed basis, intermittently, for example, to help keep the weight off.

When used under the medical supervision of a GP and dietician, VLEDs are able to induce rapid weight loss and have been shown to achieve an average weight loss of 18–20% with better-sustained weight reduction. In addition to weight loss effects, the rapid weight loss offered by VLEDs has been shown to improve sugar control in patients with type 2 diabetes, improve blood pressure and reduce total cholesterol. The DIRECT trial shows that weight can be lost and kept off and meal replacements really do work.

VLEDs may not be suitable for use for all obese patients and it is important to consider the costs associated with purchasing suitable nutritionally complete meal replacements. VLEDs are contraindicated for use in pregnant or lactating women, infants, children, adolescents (under 18 years), elderly (over 65 years), patients with a history of psychological disorders, alcohol misuse or drug abuse, in the presence of a condition called porphyria, recent heart attack or unstable heart disease.

It is important to have a health professional monitor you because some people feel a bit crappy on it, partly from the mild ketosis it produces and a range of side effects like low blood pressure, bowel changes and period changes which occur when you are rapidly losing weight.

It is best to find some good brands of low calorie shakes, bars, soups, and buy good ones, not cheap ones.  They are not overly expensive, you might be looking at about $40 for 21 shakes, $6 per proper meal. There are some good info sheets you can google, and I have put some links for this below.