Fasting for a day or 24 hours can have various effects on the body, such as affecting the way the body uses energy.
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. However, it is always a good idea to discuss with a healthcare professional if someone is considering fasting.
This article explains what happens to the body during fasting and what a person can do to make fasting safer.
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
When glucose in the blood sufficiently meets the body’s energy demands, the liver and muscles will store some of the extra glucose in the form of glycogen and release it into the bloodstream whenever the body needs it.
However, during fasting, this process changes. The liver will use the last of its glycogen reserves around
With no carbohydrates coming in, the body creates its own glucose using mainly fat. Eventually, the body runs out of these energy sources, too. 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 occurs only after several consecutive days or 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.
According to a 2022 article, fasting is a common weight loss method. However, its effectiveness may vary.
Popular diet plans include 12-hour or 16-hour fasting periods, as well as the 24-hour fast. Some diets require people to drink only water during the fast, while others allow zero-calorie beverages.
Fasting is not necessarily better than any other weight-loss method, including reducing daily calorie intake by a small amount.
A 2022 review of 43 studies found that intermittent fasting led to more significant reductions in body weight, waist circumference, and fat mass than a non-intervention diet — a person’s usual diet with no changes.
However, intermittent fasting had similar effectiveness to caloric restriction diets.
The limits of fasting may have less to do with its physical effects than how it fits into a given lifestyle.
For example, an older
The researchers concluded that fasting might be more challenging to maintain over time.
Another possible concern is post-fast binging, in which a person eats large amounts after fasting. However, a 2020 review highlights research in which people with obesity who did alternate day fasting did not develop binge-eating habits.
As well as aiding weight loss, not eating for a day can have other potential health effects.
Research in a 2022 review suggests that intermittent fasting may help to reduce fasting glucose and insulin levels in people with type 2 diabetes.
Intermittent fasting may also help to reduce the risk of cardiovascular disease by reducing factors such as total body weight and fats in the blood.
Research in the review also suggests that body weight reductions due to intermittent fasting may improve systemic inflammation, which links to certain cancers and other health conditions.
Drinking enough water is essential for maintaining everyday health, whether a person is fasting or not.
Many health authorities recommend drinking six to eight 8-ounce glasses (about 2 liters) of water every day.
Unless a religious observance forbids it, people 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 challenging 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 nursing
According to the National Institutes of Health, people who regularly fast for longer than 16 hours may have a higher risk of gallstones.
People may benefit from speaking with a healthcare professional before fasting for a day to ensure they are staying safe.
There are many ways to break a fast safely, including:
- Drink water: This is especially important if circumstances prevent 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 thoroughly to get the full benefit.
- 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.
If a person fasts for 24 hours, the body runs out of its stored energy source, known as glycogen. The body then uses fat and protein as an energy source. Effects on the body may include weight loss, preventing certain diseases, and improving cardiovascular health.
People can drink plenty of water to help curb hunger unless a religious observance forbids it. Other beverages people may drink during a fast include plain, unsweetened tea and coffee.
A 24-hour fast includes not eating any food for 24 hours. People may still intake unsweetened liquids, however.
Going a day without eating is generally safe and can be beneficial in several ways, including as a weight-loss tool.
Fasting, however, does not help weight loss any more than other conventional approaches and may be harder to stick with over the long term.
If someone is fasting for health reasons, it is essential that they do it safely and for no longer than is necessary. People should always speak with a healthcare professional first if considering fasting.