Extreme energy-restricted diets provide fewer than 800 calories per day, and starvation or fasting diets provide fewer than 200 calories per day. These diets are frequently involved as a part of religious or protest regimen or in a personal effort to lose weight. Under these circumstances it is seldom continued long enough to produce the serious neurologic, hormonal, and other side effects that accompany prolonged starvation.
More than 50% of the rapid weight reduction is fluid, which often leads to serious hypotension problems. Accumulation of uric acid can precipitate episodes of gout. Gallstones can occur. Sometimes what starts as extreme energy restriction to lose weight leads to an eating disorder – anorexia.
Diets providing 200 to 800 calories are classified as very low calorie diets. Little evidence suggests that intakes of fewer than 800 calories daily are of any advantage. Most these diets are hypocaloric but relatively rich in protein. They are designed to include a full complement of vitamins, minerals, electrolytes, and essential fatty acids, but not calories. They are given in a form that completely replaces usual food intake. Many very low calorie diets are commercially prepared formulas of 800 calories or less that replace all usual food eaten. Others, such as the well-known grapefruit diet (Hollywood Diet), rely on eating a lot of the same low-calorie food or foods. Very low calorie diets are not the same as over-the-counter meal replacements, which are substituted for one or two meals a day. They are usually given for a period of 12 to 16 weeks. Their major advantage is rapid weight loss. Because of potential side effects, prescription of these diets is reserved under proper medical supervision and for persons with a BMI above 30 for whom other diet programs with psychotherapy have been unsuccessful.
The very low calorie diet can lead to an increase of urinary ketones that interfere with the renal clearance of uric acid, resulting in increased serum uric acid levels, often manifested as gout. Higher serum cholesterol levels resulting from mobilization of adipose stores pose a risk of gallstones. Gallstones, which frequently develop in obese people (especially women), are even more common during rapid weight loss. The reason for this may be that rapid weight loss appears to decrease the gallbladder's ability to contract bile. Additional adverse reactions that are common include cold intolerance, fatigue, light-headedness, nervousness, euphoria, constipation or diarrhea, dry skin, thinning reddened hair, anemia and menstrual irregularities. Some of these are typical of thyroid deficiency.
A recent meta-analysis of very low calorie diets compared to low calorie diets showed that even though there were significantly greater weight losses with the very low calorie diets in the short term, there were no significant differences in the weight losses in the long term. Thus there does not seem to be reason to recommend very low calorie diets over more moderate calorie restriction except in very rare instances.
For those who have lost weight on a very low calorie diet, limiting dietary fat intake and maintaining physical activity are both important factors for the prevention of weight regain based on 3 year follow-up study.
To promote better weight loss outcomes, people should limit their fat intake to less than 30% of calories, and increase high activity levels.
To be healthy, we need a balance of foods from different food groups. It's quite difficult to get good nutrition in as few as 800 calories (even a dietitian would have trouble doing this), especially if one eats the same foods every day. Also, once you go off the diet, you will likely regain your weight unless you change your lifestyle and commit to healthy eating and regular physical activity.