It's virtually impossible to quickly lose bodyfat by not eating enough food or not eating at all for long periods of time (i.e., intermittent fasting). Notice, I mentioned losing bodyfat, not body weight. You may lose body weight, but this will most likely be due to a reduction in muscle mass known as catabolism. Your body essentially will feed off of its muscle tissue before it resorts to fat stores. In other words, your body weight will decrease but at the expense of lost muscle mass. Essentially you'll become a skinny fat person with less body weight but more bodyfat. This is what usually happens when people follow weight-reduction diet programs. The human body does not need muscle to survive. In fact, it's not natural to have an inordinate amount of muscle on the body since there's no biological advantage to having lots of muscle from an evolutionary perspective. Remember, the body only needs fat to survive. This is why having less than 3 and 8 percent bodyfat for males and females, respectively, may affect bodily functions (i.e., organ functions) and become life-threatening.
BOTTOM LINE: To lose bodyfat while preserving muscle mass, eat plenty of calories and then gradually reduce caloric consumption by no more than 500 calories per day. Be sure to perform resistance training (i.e., weight training) in conjunction with some form of cardiovascular exercise (e.g., running, swimming, hiking, rowing, biking, etc.).
Brian Danley, CFT
"Motivation is what gets you started. Habit is what keeps you going."