Whether you fast for religious or spiritual reasons or try intermittent fasting for health, do you know what happens to your body during a 24-hour fast?
Read on to learn the physiology behind a day without food.
Many people will soon observe Yom Kippur, the Jewish Day of Atonement, which is often marked by a 24-hour fast. Fasting is an ancient practice found in nearly every religion: Muslims during Ramadan, Jews on certain holidays like Yom Kippur, Catholics on specific observances, and others. Beyond spiritual reasons, fasting has become popular for fitness and weight-management, and a growing body of research has explored its metabolic effects.
What is intermittent fasting?
Intermittent fasting refers to cycling between periods of eating and fasting. Common schedules include 16:8 (16 hours fasting, 8 hours eating), 20:4, or OMAD (one meal a day). The basic idea is that during a fast your body shifts which energy sources it uses. As a registered dietitian, I caution that these approaches are not appropriate for everyone—especially women and people with a history of disordered eating.
If fasting triggers loss of control for you, I do not recommend it. Consider guidance to build a balanced relationship with food.
Below is a clear, practical explanation of what happens in the body during a 24-hour fast so you can understand the physiological changes and how to break a fast sensibly.
What happens to your body during a 24-hour fast?
You will not shrivel up—your body is designed to adapt. Metabolic shifts occur over the course of the fast as your body transitions between fuel sources.
Here’s a straightforward timeline:
About eight hours after your last meal, you enter a fasting state once digestion and absorption are mostly complete. At this point your body relies on carbohydrate (glucose) stored as glycogen for energy.
During an overnight fast you deplete much of your liver glycogen. Research indicates that after a full 24-hour fast, muscle glycogen can also be reduced significantly—studies show reductions around 50% without exercise.
As glucose stores fall, the body increasingly mobilizes fat stores for fuel. Fat breakdown and fatty acid oxidation begin prior to total depletion of glycogen, which is one reason intermittent fasting is associated with increased fat-burning pathways.
In longer or more severe fasting, the body may begin catabolizing some muscle protein for energy or gluconeogenesis. Hormonal changes—principally reductions in insulin and shifts in glucagon, epinephrine, and other signals—drive these transitions between fuel sources.
How does the body select fuel?
Hormones and enzyme activity regulate which substrates are used. Lower insulin and higher glucagon and catecholamines during fasting promote glycogen breakdown, fat mobilization, and later, limited protein breakdown. These coordinated responses preserve blood glucose for tissues that need it while allowing other tissues to adapt to fatty acids and ketones.
How should you break a 24-hour fast?
Even in families that traditionally break the fast with rich spreads, it’s wise to reintroduce food thoughtfully. After a prolonged fast, digestion and appetite signals may be altered, so a gentle approach minimizes discomfort and supports steady blood sugar.
Tips for breaking a fast
- Hydrate first—drink water, especially if you also abstained from fluids.
- Start with a small, nutrient-dense meal featuring fruits, cooked vegetables, or light protein.
- Chew thoroughly—slower eating and thorough chewing aid digestion and help you recognize fullness.
- Choose easy-to-digest options initially, such as cooked vegetables rather than large amounts of raw produce.
- Stick with foods you tolerate well. Avoid immediately overeating to “make up” for the fast.
If you prefer a traditional break-fast, a moderated choice could be half a bagel with avocado, smoked salmon, tomato, cucumber, and a side of fruit—balanced and not overly heavy. Wait an hour or two to reassess hunger before eating more.
Tell me: have you fasted for religious or health reasons? What do you prefer to eat when you break your fast?
Note: I do not recommend starting an intermittent fasting regimen without medical supervision and input from a registered dietitian if you have health concerns. Fasting can have different effects depending on individual health status, medications, and personal history.
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