Now, to answer your question, I got this right off another area on this site.
"What is the Lowest Amount of Calories You Can Eat in a Day?
When wondering how many calories you should eat to lose weight, you should concentrate on healthy diets. Most dieticians will tell you that the lowest amount of calories you should eat in a day is 1,200, unless you are extremely overweight and under a physician’s care. They will then probably say that even 1,200 is a bit low, and 1,400 is the advisable amount to keep you from feeling hungry and stay healthy."
In my opinion, 600 calories a day will destroy your metabolism and has more in common with anorexia/disordered eating than with a healthy diet that will allow you to lose weight safely.
Here's a CNN article about VLCDs. http://articles.cnn.com/2009-12-15/h...s?_s=PM:HEALTH
"Diets of less than 800 calories can lead to numerous complications, according to Jampolis, including heart arrhythmias, which could lead to death. Extreme dieters are also at risk of dehydration, electrolyte imbalance, low blood pressure and high uric acid, which could lead to gout or kidney stones, she says. Also, losing weight quickly could lead to gallstones and thinning hair because dieters are getting the minimum amount of nutrition, which can affect hair and bone density.
While different programs restrict patients to different calorie counts, Hernried believes some diets provide too few calories to their patients. "No matter how good the nutrition is in the product, the brain will start to break down under 600 calories."