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Fasting is a widespread practice across cultures and religions, involving abstaining from food and sometimes specific types of food for defined periods. Fasting has evolved into a ritualistic, spiritual, and health-promoting practice. Fasting encourages the body to create ketones. These molecules act as a substitute energy source for different body parts, with the brain particularly proficient at utilizing them for energy. This cascade of metabolic adaptations involves several key organ systems: the pancreas for hormonal regulation, the liver for glycogen storage and fat metabolism, adipose tissue as an energy reserve, and skeletal muscle as a potential protein source—all working in concert to maintain energy homeostasis during periods of fasting. Fasting can positively impact various markers of health, including cardiovascular disease, cancer, and neurodegenerative diseases. While highlighting potential benefits, the review also acknowledges potential negative effects as well. This review summarizes the literature on fasting and its effects on the body.