What happens to the body after 25 hours of fasting?
A concise article about the physiological processes in the body during a 25-hour Yom Kippur fast. The body transitions from using glucose from the pre-fast meal to glycogen stores, and then to fat and the production of ketone bodies. Sensations vary and include hunger, thirst, fatigue, headache, and difficulty concentrating, depending on the fasting person's drinking and eating habits.
An article by Channel 14 explains what happens to the body during the Yom Kippur fast, which lasts about 25 hours. In the first hours after the pre-fast meal, the body uses glucose from food as an available energy source and begins to utilize glycogen stores in the liver and muscles. As the fast progresses, the body increases its use of fat as an energy source, and the liver produces ketone bodies from fatty acids that the brain can use. Sensations during the fast vary from person to person: lack of fluids can cause thirst, dry mouth, headache, and dizziness. A decrease in caffeine intake can cause headaches in regular coffee drinkers. The lack of food and drink manifests as fatigue, weakness, difficulty concentrating, hunger, and thirst, depending on preparation for the fast and the body's condition. The article emphasizes that the body knows how to adapt itself to the significant change from routine, gradually transitioning from using energy from food to using internal reserves.