During the first day of a fast, the body burns sugar stores known as glycogen. After this occurs, the body begins to burn fat for fuel, while the brain continues to require blood sugar. Production of Insulin goes down whilst Glucagon and Growth Hormone (especially between the 18-24 hour mark) go up.
During the second day, if muscle is not needed (lack of appropriate exercise) some muscle tissue may be broken down into amino acids, which are converted by the liver into glucose to feed the brain.
During the second to third day, the body goes into what is called ketosis. In ketosis, the liver converts stored fat into chemicals called ketones, which can be used by the brain, heart and muscles for energy. It is generally during this period when people lose their hunger pains and experience increased energy levels and a heightened sense of awareness, which often includes clarity of the mind and spirit.
People may lose up to two pounds a day during this stage. As fat is increasingly burned as fuel, fat-stored toxins are released into the bloodstream to be metabolized by the liver and kidneys.””
Fasting has been proven study after study to improve longevity and reduce ailments like high blood pressure, cholesterol, excess body fat, diabetes and so on. Fasting if longer than 24 hours should be supervised by a qualified (knowledgeable) health professional/doctor.