This type of dietary pattern would be difficult for someone who eats every few hours (e.g., snacks between meals, grazes). It would also not be appropriate for those with conditions that require food at regular intervals due to metabolic changes caused by their medications, such as with diabetes. Prolonged periods of food deprivation or semi-starvation places one at risk for overeating when food is reintroduced, and may foster unhealthy behaviors such as an increased fixation on food. [7,8]
Essentially, the bodies starvation response kicks in (like a crash diet ), in order to conserve energy. In the first 24 hours of the fast your body will utilize glycogen (easily-accessible storage form of glucose, stored in small quantities in the liver and muscles) to provide energy for your various body processes to keep functioning. Once your carbohydrate reserves have been exhausted, your body utilizes stored fats. After several days of fasting, your body begins to break down protein, which is converted into glucose. Much of our muscle mass is protein and thus muscle tissue is often lost.
Furthermore, fasting for fast weight loss carries additional health risks. Whilst fasting for a day or two is rarely a problem if you are healthy, “it can be quite dangerous if you are not already eating a healthy diet, or if you’ve got liver or kidney problems, any kind of compromised immune system functioning, or are on medication – even Tylenol,” says Dr. Fuhrman.