Very low calorie diets, or VLCDs, have calorie intakes of less than 800 calories per day, with a few going up to 1,000. They are typically administered when someone who is obese needs to lose a lot of weight quickly for medical reasons, such as undergoing surgery or fending off immediate health concerns. A doctor must supervise a VLCD; unsupervised fad VLCDs and poorly supervised medical VLCDs can have negative consequences. Even well-monitored short-term VLCDs, however, can put a patient at risk for certain conditions. VLCDs may have calorie levels as low as 400 calories per day, and a 500-calorie-per-day diet is subject to all the same effects as other VLCDs.
The National Institute of Diabetes and Digestive and Kidney Diseases states that between 10 and 25 percent of people who undergo a VLCD develop gallstones. Some of these patients--approximately one-third, according to the NIDDK--show symptoms that become severe enough to warrant surgery. Why this happens isn’t fully known. The Cleveland Clinic and NIDDK suggest that rapid weight loss might make the gallbladder contract less and expel less bile, leading to gallstone formation.
The UCLA Center for Human Nutrition warns that any diet with a daily calorie intake of less than 1,000 calories sends the body into starvation mode. The appetite suppression seen in starving people occurs about four or five days after beginning the diet. A 2006 study in the journal Obesity compared subjects on low-calorie and very low-calorie diets, and found that a decrease in food cravings was associated with caloric intake and not weight loss: Those on the VLCD had greater appetite suppression. Metabolism heads into starvation mode as well; Irving B. Weiner and colleagues write in the “Handbook of Psychology: Health Psychology” that those on VLCDs tend toward a “rapid regaining of weight” after the diet ends. In fact, a 2009 study published in the Journal of Proteome Research found the short-term, drastic weight loss resulting from a 500-calorie-per-day diet caused fat cells in subjects to start “screaming to take up glucose and fatty acids and store fat again.”
Part of the reason a 500-calorie-per-day diet needs competent medical supervision is that the extreme reduction in intake severely impacts nutrition. Although you can take vitamins, and the nutritional liquids and bars you have on the diet are fortified, the NIDDK reports that you may still have issues such as diarrhea and fatigue. If the diet isn’t properly supervised, or you go on a fad diet at home that restricts calories, you open yourself up to severe problems such as bone loss and anemia.