This method of drinking only water and not eating for a day is very unscientific and extremely unhealthy, and should not be adopted, let alone the so-called “optimal duration”. Doing so for a long time will cause many serious harms to the body： 1. **Metabolic disorders**: If the body does not get enough energy supply, the basal metabolic rate will drop significantly. Even if you return to a normal diet in the future, it will be easier to accumulate fat. 2. **Malnutrition**: Unable to obtain various essential nutrients, such as protein, carbohydrates, fats, vitamins, minerals, etc., resulting in impaired function of various organs of the body and decreased immunity. 3. **Muscle loss**: The body breaks down muscles to provide energy, affecting the normal structure and function of the body. 4. **Cause digestive system problems**: May cause gastrointestinal disorders, constipation, abnormal gastric acid secretion, etc. 5. **Increased burden on the cardiovascular system**: Imbalance of electrolytes in the blood, etc., increases the burden on the heart, and there is a risk of cardiovascular diseases such as arrhythmias. 6. **Affect brain function**: It causes dizziness, fatigue, inattention, memory loss, etc., which seriously affects daily life, work and study. Correct weight loss should be achieved through reasonable diet control (to ensure balanced nutrition) and appropriate exercise, step by step and sustainably, so as to achieve the goal of weight loss under the premise of health.