24-Hour Fasting: Do The Risks Outweigh The Benefits Of This Restrictive Eating Pattern?

Fasting has been used for religious purposes for thousands of years. More recently, it's become a popular way to boost weight loss, improve certain health conditions, and enhance overall well-being.

There are a number of fasting methods to choose from, but some experts believe that longer-term fasts, such as 24-hour fasts, may offer the most benefits. 

A 24-hour fast is exactly what it sounds like: a fasting method that involves completely abstaining from calorie intake for a 24-hour period. 

Also known as whole-day fasting, 24-hour fasting is considered a type of periodic fasting (PF), which refers to fasting methods that don't involve fasting every day or every other day.

Unlike other types of fasting, which are usually practiced every day or most days of the week, like 18:6 fasting, people using the 24-hour fasting method may only fast once a week or a couple of times per month.

When using the 24-hour fasting method, you'll only drink water or other zero-calorie beverages like herbal tea during the fasting window. Although there are variations to fasting where people take in a small number of calories—commonly known as "dirty fasting"—experts suggest that completely abstaining from calorie intake is the only way to ensure you're reaping the full benefits of fasting. 

"Ideally, zero calories should be consumed during fasting periods," Grant Tinsley, Ph.D., a professor at Texas Tech University and intermittent fasting researcher, tells mindbodygreen.

"While a splash of milk in your coffee may not 'break' your fast as much as eating a full meal, the only way to ensure that you are not breaking your fast at all is to not consume any calories," he says. 

Research shows that fasting benefits health in a variety of ways, but longer-term fasts may impact health differently than shorter-term fasting methods.

One of the most evidence-based benefits of 24-hour fasting is its effect on weight loss.

After fasting for around 24 hours1, your body uses up most of its glycogen stores. Glycogen is the storage form of glucose or sugar, which is your body's main source of energy. After using up stored glycogen, your body starts burning fat for fuel. 

A 2015 review2 led by Tinsley that included seven whole-day fasting studies found that people who followed a low-calorie diet and engaged in 24-hour fasts up to two times per week for 12 to 24 weeks experienced reductions in body weight of up to 9% compared to control groups.

"Longer fasts may be more effective at switching the body into a fat-burning state, which will boost weight loss," board-certified OB/GYN and functional medicine specialist Kyrin Dunston, M.D., FACOG, tells mindbodygreen.

Fasting has been linked to significant improvements in blood lipid levels like triglycerides.

Interestingly, longer fasts of 24 hours seem to be more effective for lowering blood lipid levels than shorter fasts because there's an increase in triglyceride breakdown and fat oxidation that occurs after fasting for 18 to 24 hours.

"If you're struggling with your lipids and particularly your triglyceride levels, then fasting may be the tool that can assist you in reducing that number into optimal ranges," says Dunston.

Metabolic syndrome is a term for a cluster of symptoms—including elevated blood sugar and blood pressure levels—that increase your risk for health conditions like heart disease.

Insulin is a hormone that shuttles sugar from your blood into your cells, where it can be used for energy. When someone is resistant to insulin5, cells don't respond well to insulin, and the pancreas starts producing more insulin to help cells absorb sugar. Insulin resistance increases your risk of health conditions6 like liver disease and Type 2 diabetes.

In addition to lowering blood lipid levels, 24-hour fasting has been shown to reduce something called trimethylamine N-oxide (TMAO).

TMAO is a compound created by gut bacteria from the breakdown of substances found in certain foods like red meat, eggs, and dairy. Having high levels of TMAO may increase the risk of heart disease.

Experts note that there's still a lot to learn7 about the effects of fasting on cellular aging. However, as fasting expert Jason Fung, M.D., explained on the mindbodygreen podcast, longer fasts (in the 24-hour range8) seem to enhance autophagy—the process of clearing away old or damaged cells. Enhanced autophagy may help reduce one's risk9 of developing a number of chronic illnesses, supporting longevity.

Yes, engaging in 24-hour fasting can be an effective way to boost weight loss.

A 2015 review2 that included seven whole-day fasting studies found that people who followed a low-calorie diet and engaged in 24-hour fasts up to two times per week for 12 to 24 weeks experienced a reduction in body weight of up to 9% compared to control groups.

However, it's unclear whether 24-hour fasting, or any fasting method for that matter, is more effective than more traditional dieting methods like continuous calorie restriction. 

For a quick refresher: In order to lose weight, you need to create a calorie deficit where you're burning more calories than you take in. Engaging in a 24-hour fast once or twice a week or even a few times per month will likely result in a calorie deficit, which can promote weight loss. But, if you're compensating for that calorie reduction by eating more on non-fasting days, then you're less likely to see results.

Twenty-four-hour fasting involves going for a long time without eating. Because it's a more extreme method of fasting, it's not safe or appropriate10 for some people. Pregnant and breastfeeding women, teens, children, those with eating disorders or a history of disordered eating, people taking certain medications, and people who are underweight shouldn't try 24-hour fasting.

People with medical conditions that impact their ability to regulate blood sugar levels, like those with Type 1 diabetes, shouldn't try 24-hour fasting unless they clear it with their health care provider first. Not taking in any calories for a prolonged period of time can lead to low blood sugar levels or hypoglycemia, which can lead to dangerous side effects like fainting. For people dependent on insulin, fasting interferes with medication requirements, which may also lead to adverse side effects.   

Sometimes, physicians may recommend fasting11 as a natural treatment method for people with certain medical conditions like some cancers. However, you shouldn't try intermittent fasting if you have a medical condition unless it's recommended and monitored by a health care provider for a specific reason.

Also, it's important to understand that fasting isn't necessary for improving any aspect of health. Some people find that fasting is a helpful tool to reach their goals, but it's not the right choice for everyone. Using methods like fasting to try to attain unrealistic weight loss isn't healthy and could cause you to develop an overly restrictive relationship with food12.  

Some studies2 have shown that 24-hour fasting regimens that involve one to two 24-hour fasts per week for up to six months are safe and not associated with any significant side effects. 

If you're a healthy person with no underlying conditions, it's likely that engaging in occasional 24-hour fasts won't negatively impact health. However, keep in mind that not only are prolonged fasting methods unnecessary to reach and maintain optimal health, they're not appropriate or safe for everyone. 

Also, most studies investigating the effects of longer-term fasts like 24-hour fasts show high dropout rates, meaning these protocols are hard for people to stick to. For example, in the 2021 study4 that found that 24-hour fasting helped reduce markers of metabolic syndrome, the researchers planned for a 15% dropout rate at the beginning of the study. At the end of the study, the researchers reported a higher-than-expected dropout rate of 24% in the participants randomized to the 24-hour fasting protocol. 

If you have no desire to go without calories for 24 hours, we hear you! Twenty-four-hour fasts are among the most extreme fasting methods and definitely aren't for everyone. Fortunately, if you'd like to reap the benefits of fasting without going 24 hours or more without eating, there are plenty of less-restrictive methods to choose from.

And of course, if you're on the fence about fasting, there are other ways to improve your health that don't involve skipping meals. 

Twenty-four-hour fasting involves completely abstaining from calories for 24 hours, usually once or twice a week. Studies show that this method could be helpful for encouraging weight loss, reducing insulin resistance, and improving blood lipid levels.

However, 24-hour fasting is a more extreme fasting method that may be hard to stick to, especially long term. Plus, it's not appropriate for some people, like those with disordered eating tendencies and people with health conditions like Type 1 diabetes.