Because there is no energy and protein intake, muscle tissue and adipose tissue will be consumed (glycogen will be consumed, but the proportion of glycogen in the total weight is very small, so it will be ignored), and the food residue in the digestive tract will be emptied, so you will soon be much lighter. However, there are also side effects. One is that the body consumes a lot of fat, which will lead to the symptoms of acidosis, the most obvious manifestation of which is bad breath. Second, it may lead to hunger stress reaction in the body, leading to overeating after dieting and rapid weight rebound. If you diet for a long time, you may induce anorexia; Third, there may be malnutrition, especially vitamin deficiency symptoms.