What happens if you don’t eat for a day? Timeline and effects

Fasting is a longstanding part of many religious traditions, including the Jewish and Muslim observances of Yom Kippur and Ramadan. A form of fasting known as intermittent fasting has also gained popularity as a weight-loss tool.

Many studies have examined the benefits and risks of giving up food for a day, including how it affects weight loss.

In this article, we look at what happens to the body during fasting, as well as what a person can do to make fasting safer.

What happens during fasting?

Whether a person is fasting or not, the body still needs energy. Its primary energy source is a sugar called glucose, which usually comes from carbohydrates, including grains, dairy products, fruits, certain vegetables, beans, and even sweets.

The liver and muscles store the glucose and release it into the bloodstream whenever the body needs it.

However, during fasting, this process changes. After about 8 hours of fasting, the liver will use the last of its glucose reserves. At this point, the body enters into a state called gluconeogenesis, marking the body’s transition into fasting mode.

Studies have shown that gluconeogenesis increases the number of calories the body burns. With no carbohydrates coming in, the body creates its own glucose using mainly fat.

Eventually, the body runs out of these energy sources as well. Fasting mode then becomes the more serious starvation mode.

At this point, a person’s metabolism slows down, and their body begins burning muscle tissue for energy.

Although it is a well-known term in dieting culture, true starvation mode only occurs after several consecutive days or even weeks without food.

So, for those breaking their fast after 24 hours, it is generally safe to go without eating for a day unless other health conditions are present.

Drinking enough water is essential for maintaining everyday health, whether a person is eating or not.

Many health authorities recommend drinking eight 8-ounce glasses (about 2 liters) of water every day.

Unless a religious observance forbids it, a person can take advantage of the fast by drinking plenty of water to help curb hunger pangs.

When fasting for 24 hours, some people consume other beverages such as tea, black coffee, or zero-calorie sweetened drinks.


Although it is generally safe, going a day without eating can be risky for some people, including:

  • people with diabetes
  • people with a history of eating disorders
  • people using medications that they must take with food
  • children and adolescents
  • those who are pregnant or breastfeeding

What is the safest way to break a fast?

According to Chelsey Amer, a registered dietitian nutritionist, there are several ways a person can break their fast safely:

  • Drink water: This is especially important if circumstances prevented it during the fast.
  • Eat a small meal: Eating a large meal immediately after a fast can strain the digestive system.
  • Chew food thoroughly: Chew each bite at least 30 times.
  • Eat cooked foods: Go for foods that are easier to digest, such as cooked vegetables instead of raw.
  • Avoid experimenting: Trying new foods after a fast can make digestion harder and may make a person feel ill.


Going a day without eating is generally safe and can be beneficial in several ways, including as a weight-loss tool.

Fasting does not help weight loss any more than other conventional approaches and can be harder to stick with over the long term.

If a person is fasting for health reasons, it is essential that they do it safely and for no longer than is necessary. Long-term fasting starves the body of essential nutrients and can cause many complications.

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