There are several theories as to why fasting provides physiological benefits, they’ve been studied extensively, and have had experiments designed to test cells that are under a mild stress and how they respond to the stress, and cope with stress and possibly resist disease.
Though the word “stress” is often used in a negative sense, challenging the body and mind has benefits. Consider intense exercises, which extensively stress various muscles groups and the cardiovascular system. As long as you give your body time to recover, it will grow stronger. There is considerable similarity between how cells respond to the stress of exercise and how cells respond to fasting.
What is fast?
To improve health, the goal should be to lose weight by reducing the total caloric intake rather than focusing on when those calories are consumed. A fast starts with restricted eating for some days and then further to limit what you eat the other days. Periods of voluntary abstinence from food and drink have been practiced since earliest of ancient times by people around the globe. Fasting is a practice that dates back centuries and plays a fundamental role in many cultures and religions. In general, most types of fasts are performed over 24–72 hours.