Learn what happens during a 24-hour fast, including possible benefits, risks, exercise tips and the best way to break your fast safely.

What Happens During a 24-Hour Fast? Benefits, Risks and the Science

Fasting has become a major wellness trend, with devotees promising everything from rapid weight loss to better brain function and even a longer life. But what actually happens during a 24-hour fast—and which of those benefits are backed by science?

I was introduced to fasting at the Lanserhof health clinic in Austria a few years ago. Fasting is part of “the cure” their proprietary method to promote health and longevity. Most guests begin a fast on the first day of the one-week program. While I struggled with hunger pangs initially, by the end of the week, I was a believer. I felt lighter, more focused and definitely more energetic.

what happens during a 24-hour fast — Drinking lots of herbal tea during my 24-hour fast

The 24 Hour Fast

The first day at the clinic kicked off the 24 hour fast, after that, we were working on having small meals 3x a day with no snacking. We let our food digest between meals and stopped eating around 6pm and didn’t commence again until 7 or 8 the next morning. But with these notes, I want to focus on the initial full day fast.

A 24-hour fast is one of the more approachable forms of extended fasting. It might mean finishing dinner at 7 p.m. and waiting until 7 p.m. the following day to eat again. During that window, you consume water and other noncaloric beverages but no food.

Going an entire day without eating can produce measurable metabolic changes. However, some of the more dramatic claims you’ll see online—especially those involving “detoxing,” extreme fat loss or cellular rejuvenation—are often overstated.

Here’s a realistic look at what happens during a 24-hour fast, its potential benefits and the important risks to consider.

This article is for informational purposes only and is not a substitute for personalized medical advice. Talk with your healthcare provider before fasting if you have a medical condition or take prescription medication.

What Happens to Your Body During a 24-Hour Fast?

The exact timeline varies based on your last meal, activity level, metabolism, diet and previous experience with fasting. These stages should be viewed as estimates rather than a metabolic stopwatch.

0–6 Hours: Your Body Digests Its Last Meal

During the first several hours, your body is still processing your most recent meal.

Glucose from carbohydrates provides much of your immediate energy, while insulin helps move glucose and other nutrients into your cells. Any excess energy may be stored as glycogen in the liver and muscles or as body fat.

At this point, very little has changed metabolically. You are in the normal post-meal state.

6–12 Hours: The Transition Begins

As digestion winds down, insulin levels begin to fall. Your liver starts breaking down stored glycogen to keep your blood sugar within a healthy range.

Fat burning gradually increases, although your body is still drawing heavily on its stored carbohydrates.

You may begin to feel hungry during this period, especially if it overlaps with a meal you normally eat. Hunger often arrives in waves and may fade once your usual mealtime passes. If start after a early dinner (which is what I normally do), these will be evening hours (with tea) and sleep. Easy peasy.

12–24 Hours: The Metabolic Shift Becomes More Noticeable

As the fast continues, liver glycogen becomes increasingly depleted. Your body relies more heavily on fatty acids for energy and begins producing ketones. I find that light exercise (think a chill yoga class, easy hike or a walk) helps at this point of the fast.

This transition is sometimes called the “metabolic switch.” According to the National Institute on Aging, the timing of this switch can vary considerably from person to person.

Some people will produce measurable ketones before reaching 24 hours, while others may take longer. Your previous meals, exercise habits and familiarity with fasting can all affect the timing. In my experience hours 16-18 are the hardest. Hours 22-24th seem to breeze by.

What Are the Possible Benefits of a 24-Hour Fast?

Researchers are still working to separate the proven benefits of fasting from the social-media hype. Most studies also examine repeated intermittent-fasting patterns—not the effects of completing one isolated 24-hour fast. For me, I try to do at least 2x a month.

Increased Fat Burning and Mild Ketosis

As insulin drops and glycogen becomes less available, your body begins relying more heavily on stored fat. Ketone production may also rise.

That does not mean a single fast burns a dramatic amount of body fat. It simply means the proportion of energy coming from fat increases during the fasting period.

Improved Metabolic Flexibility

Metabolic flexibility is your body’s ability to switch between carbohydrates and fat for fuel. Fasting gives your body practice making that transition.

This is one reason occasional fasting appeals to people interested in metabolic health rather than rapid weight loss. That’s where I fall. Full 24-hour asting for weight loss seems unsustainable.

Will a 24-Hour Fast Help You Lose Weight?

You may see the number on the scale drop after a 24-hour fast, but most of that immediate change is not body fat.

Short-term weight loss can include:

Water
Glycogen stored with water
Food moving out of the digestive system
A relatively small amount of body fat

Meaningful fat loss occurs when fasting helps create a sustainable calorie deficit over time.

Importantly, the most current evidence does not show that intermittent fasting is clearly superior to traditional approaches to weight loss. A 2026 Cochrane review analyzed 22 randomized trials involving nearly 2,000 adults and found no clinically meaningful weight-loss advantage over standard dietary advice.

That doesn’t make fasting useless. Some people find it simpler to limit when they eat than to count calories every day. The best approach is generally the one that is safe, nutritionally sound and sustainable for your lifestyle.

Does Fasting Improve Mental Clarity?

Many people report greater focus, steadier energy and improved mental clarity once the first waves of hunger pass.

Ketones can provide the brain with an alternative source of energy, but individual responses vary. Other people experience headaches, irritability, fatigue or difficulty concentrating.

The idea that everyone will feel mentally sharper during a 24-hour fast is not supported by strong evidence. Pay attention to how your own body responds. For me, I do see a boost in creativity and focus in the last portion of the fast. And I schedule writing sessions to make the most of this energy increase.

Can You Exercise During a 24-Hour Fast?

Healthy adults can often tolerate light or moderate activity during a short fast. Good options may include:

Walking
Easy running
Gentle cycling
Yoga
Light strength training

Long runs, heavy lifting and high-intensity interval workouts may feel harder because you have less readily available glycogen. A fasting day is probably not the ideal time to chase a personal record.

If you are an active midlife woman, protecting muscle mass and supporting exercise recovery are especially important. Avoid allowing fasting to crowd out the protein, carbohydrates and total energy your body needs consistently. Schedule demanding workouts on days when you can fuel and recover properly.

Stop exercising and end the fast if you feel faint, confused, unusually weak or unwell.

What Can You Drink During a 24-Hour Fast?

Water should be your priority. Many people also include:

Plain sparkling water
Unsweetened tea
Black coffee

Avoid turning the day into a test of how little fluid you can consume. Dehydration can contribute to headaches, dizziness and fatigue.

Coffee on an empty stomach can also cause shakiness, heartburn or digestive discomfort in some people.

How to Break a 24-Hour Fast

There is no need for an elaborate fast-breaking ritual after 24 hours. Most healthy adults can simply eat a balanced, moderately sized meal.

A good post-fast meal might include:

Lean protein such as chicken, fish, eggs, tofu or Greek yogurt
Vegetables
Healthy fats
Complex carbohydrates such as brown rice, quinoa, beans or potatoes

Eat slowly and give your body time to register fullness. An enormous, heavy meal may cause bloating or digestive discomfort and can quickly erase the calorie deficit created during the fast.

Who Should Not Try a 24-Hour Fast?

A 24-hour fast is not appropriate for everyone. Avoid fasting—or speak with a qualified healthcare provider first—if you:

Are pregnant or breastfeeding
Are a child or teenager
Have a current or previous eating disorder
Are underweight
Take insulin or certain diabetes medications
Have a medical condition affected by food or fluid intake
Take medication that must be consumed with food
Frequently experience fainting or low blood pressure

People with diabetes face particular risks from fasting, including low blood sugar, high blood sugar and dehydration. The National Institute of Diabetes and Digestive and Kidney Diseases recommends planning any fast with a healthcare professional who can evaluate medication and monitoring needs.

The Bottom Line on 24-Hour Fasting

A 24-hour fast can increase fat burning, encourage mild ketone production and give your body practice switching between fuel sources. Repeated intermittent-fasting schedules may improve certain markers of metabolic health for some people.

But fasting is not a detox, a guaranteed path to weight loss or a proven shortcut to longevity. The evidence for dramatic autophagy, anti-aging and mental-performance claims in humans remains limited.

For a healthy adult who enjoys fasting and has medical clearance, an occasional 24-hour fast may be a reasonable wellness practice. It should complement—not replace—a nutritious diet, regular exercise, adequate protein, good sleep and stress management.

Ultimately, the question isn’t whether you can go 24 hours without eating. It’s whether fasting helps you build a healthier routine you can actually sustain.