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.
In this article, we're taking a deep dive into 24-hour fasting and sharing all the details about its potential benefits, risks, and safety.
What is 24-hour fasting?
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.
There's not a lot of research investigating the effects of dietary supplements on fasting, but it's likely that vitamin, mineral, and antioxidant supplements are OK to take as long as they don't impact blood sugar levels. This means that gummy vitamins and calorie-containing vitamin powders are a no-go during fasting.
Research shows that fasting benefits health in a variety of ways, but longer-term fasts may impact health differently than shorter-term fasting methods.
Here are some of the ways that a 24-hour fast, in particular, may benefit your health:
It can support weight loss.
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.
However, similar to other studies3, the review noted that it's unclear whether 24-hour fasting leads to greater weight loss compared to continuous caloric restriction.
It can improve blood lipid levels.
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.
The 2015 study2 mentioned above found that whole-day fasting methods were associated with a 5-20% reduction in total cholesterol and a 17-50% reduction in triglyceride levels.
It supports metabolic health.
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.
A 2021 study4 that included 103 people who had at least one marker for metabolic syndrome found that those who performed water-only 24-hour fasts twice weekly for four weeks and then once weekly for 22 weeks experienced significant reductions in their Metabolic Syndrome Score (MSS). Specifically, they had mild to moderate improvements in diastolic blood pressure, heart-protective HDL cholesterol, blood sugar, and triglycerides. People in the control group who followed their normal diets experienced an increase in their MSS score.
It can improve insulin resistance scores.
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.
The 2021 study4 mentioned above found that participants randomized to the 24-hour fasting protocol experienced a 32.5% improvement in insulin resistance scores, while the control group experienced a 3.7% improvement.
It may improve heart health.
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.
A 2018 study in 24 people found that water-only fasting for 24 hours reduced blood levels of TMAO by an average of 10.8 ng. Researchers suggested that more research be completed to understand how prolonged fasting methods may impact TMAO levels and heart disease risk.
It may support cellular health and longevity.
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.
However, Tinsley adds, "If someone is already eating a nutritious diet, maintaining a healthy body composition, and exercising, I am not sure how much additional benefit for longevity would be gained from fasting."
Is 24-hour fasting effective for weight loss?
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.
By this logic, the most effective weight loss program is one that you can stick to. If you find that 24-hour fasting once or twice a week is effective for you, then it might be a helpful weight loss tool. Others may find that following less intense fasting regimens or simply reducing total calorie intake on a daily basis is better for their lifestyle and health needs.
Downsides and side effects
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.
Additionally, 24-hour fasting may lead to the following side effects13:
- Extreme hunger
If you're looking for a more gentle, less extreme method of fasting that is less likely to result in adverse side effects, consider trying out the 16:8 method, which involves fasting for 18 hours and then eating within an eight-hour time period. This method more closely resembles a normal eating pattern and is much less extreme than 24-hour fasting.
How long can you do it safely?
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.
This isn't surprising, as completely abstaining from food for an entire day is likely difficult and unpleasant for most people.
24-hour fasting vs. other types of fasting
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.
Here are a few popular alternatives to 24-hour fasting that still deliver some similar benefits:
- OMAD:OMAD stands for One Meal a Day. It's a more extreme type of time-restricted feeding (TRF) where the eating window is very small. When following OMAD, people only consume one meal per day and then fast for the rest of the day. It may help reduce body weight14 and improve metabolic health, but it's highly restrictive and not appropriate for everyone.
- 16:8: This is another type of TRF that involves eating within a defined window. Following the 16:8 method, you'll fast for 16 hours and then consume all of your calories within an eight-hour window each day. It's much easier to follow compared to 24-hour fasting and OMAD, so it's a good choice for fasting newbies. It's also linked to a number of health benefits, including weight loss15, reduced blood sugar16, and improved blood lipid levels17
- Modified Alternate-Day Fasting (ADF):Alternate-Day Fasting (ADF) is a fasting method that involves eating normally one day and then fasting the next. Unlike traditional ADF, modified ADF allows you to eat a small number of calories—typically under 500—during your fasting days. This method is somewhat easier to stick to than traditional ADF and may be effective for promoting weight loss18 and reducing inflammatory markers19, like high-sensitivity C-reactive protein (hs-CRP).
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.
For a simpler and more enjoyable way to improve your metabolic health, increase your longevity, reduce your disease risk, and reach and maintain a healthy body weight, consider trying out simpler dietary modifications like eating more plant foods like vegetables and beans and cutting back on ultra-processed foods and added sugar.
How often should you do a 24-hour fast?
Most studies have investigated the effects of protocols that involve 24-hour fasts once or twice a week for up to six months. However, that regimen may not be appropriate or safe for everyone. If you’re interested in trying out a longer-term fast, work with a qualified health care provider to make sure you're going about it in a safe way.
24-hour fast vs. 16-hour fast: Which one's better?
It's hard to say which one is better, but most experts recommend that people new to fasting start with less intensive methods, like a 16:8 fast. A longer 24-hour fast may offer more benefits in terms of fat oxidation, but it's harder to stick to and may lead to more side effects like extreme hunger and lightheadedness.
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.
If you're interested in trying out fasting, consider choosing a less extreme method like 16:8 before easing into a longer fast. Check out this introduction to different fasting types for more information on other, less restrictive methods and how to safely incorporate them into your well-being routine.
Jillian Kubala, MS, RD is a Registered Dietitian based in Westhampton, NY. She holds a master's degree in nutrition from Stony Brook University School of Medicine as well as an undergraduate degree in nutrition science.
In addition to her private practice where she uses a unique and personalized approach to help her clients achieve optimal wellness, she works as a freelance writer and editor and has written hundreds of articles on nutrition and wellness for top digital health publishers.
Jillian and her husband have a backyard farm where they grow their own food and keep chickens. She runs a small cut flower business specializing in organically grown dahlias.