24 Hour Fast Part 2: How Tap into Ancestral Hunger for Optimal Benefits

Dive into the world of intermittent fasting with Emily Wei's expert dietitian insights on the 24-hour fast. Learn about calorie restriction, metabolism myths, and muscle preservation for a sustainable approach.

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By Staff Writer
Jovan Mijailovic
Edited by Jovan Mijailovic

Updated July 12, 2024.

A woman sitting on a yoga mat looking at her phone - Top Hacks for the 24 Hour Fast

Believe me when I say that the first time I heard of purposefully abstaining from food for an entire day, I was more than apprehensive. I can recall one other time that I’ve done this in my life: after a very regretful trip to the buffet.

After actually fasting for 24 hours, I was pleasantly surprised by both the results of the fast and by myself. This is part 2 of our blog series on intermittent fasting (IF), where we present the basics of the process, hacks to help you do it successfully, and the benefits and drawbacks of this method.

» Go beyond the benefits of a 24-hour fast with a blood biomarker analysis

24 Hour fasting: Calorie restriction = Weight loss

Brad Pilon, author of Eat Stop Eat, says,

Prolonged calorie restriction is the only proven nutritional method of weight loss. [1] While much confusion surrounds diet trends and recommendations, this is the closest to an irrefutable truth.

To lose weight, you need a negative energy balance. [2] In other words, you must spend more calories than you take in. So you can eat less, exercise more—or better yet, combine the two to achieve weight loss.

» See what you should know before starting a fast

Why I chose the 24-hour fast

For many of us, restricting calories is hard, especially in the long term. I picked the 24-hour fast because I find it easier to control myself entirely for one day a week than to give up foods I love for good. Remember, you should still be eating within reason on the other six days of the week.

» Think going without food for a day is easy? Try fasting for 4 days

For example, if you need 2,000 calories daily, eat 2,000 calories six days a week and fast for one day. You can't eat 4,000 calories six days a week and fast for one day and still lose weight.

What the 24-hour fast looks like

Once a week, do a 24-hour fast. Once the period passes, eat as if you didn't start it in the first place.

While Eat Stop Eat suggests fasting one to two times per week, I’m not sure how sustainable two days of fasting are for the average person. When I tried fasting for two whole days, I wasn't successful.

A blue poster with a info on intermittent fasting on it - Tapping Into Ancestral Hunger, Part 2: Top Hacks for the 24 Hour Fast

But wait, I have some concerns about fasting for 24 hours!

Won’t my metabolism decrease if I don’t eat for 24 hours? 

Your metabolism is dependent on your body weight and, in particular, depends on your lean body mass. Contrary to popular belief, several studies have shown that fasting less than 72 hours will not affect your basal metabolic rate. [3,4] 

You won’t slow down your metabolism by fasting for a day.

» Learn how to avoid age-related weight gain

Won’t I lose my hard-earned muscle?

When you fast for 24 hours, your body first uses the sugar stored in your liver for energy and then relies on fat stores. You’ll actually be burning fat instead of muscle.

But there's a small catch:

To maintain muscle, you must do resistance exercises such as weightlifting. These exercises do not have to be done on your fast day.

The rules: How to do a 24-hour fast

1. No foods are off the table

You still have to eat within normal means during your six feeding days. Eat sensibly and as healthy as possible, but don’t feel restricted.

» Check out new foods to add to your diet

2. No calories during the 24-hour fast

Calorie-free drinks are allowed and encouraged. That means there’s no need to forego coffee—the five calories in your creamer are acceptable—tea or even sugar-free gum.

» Coffee has many other benefits. Discover how it can help you when biohacking your body

3. Break the fast with balance and reason

Break the fast with a regular-sized meal. Though there are benefits to IF beyond calorie restriction alone, you won’t lose much weight if you shovel down a day’s worth of calories each time you break the fast.

You spent the day helping to reset your appetite thermostat. So, take advantage of that fact, and remember to reframe your hunger perspective.

Break your fast the healthy way!

Eat a balanced meal with protein, healthy carbs, and fats. Unless you want to feel pretty awful, I wouldn’t recommend breaking the fast with a stack of pancakes.

4. More does not mean better

If you can fast for one whole day a week, that would be great. Doing this twice a week will give you more power. But limit your IF to a maximum of two days a week.

5. Keep pumping iron

Consistency is key to maintaining the muscle you've built. Continue weight training. This will send signals to your body to hold onto that hard-earned muscle mass.

» Learn more about protein and muscle recovery

Iron shopping list with a fork and knife - Tapping Into Ancestral Hunger, Part 2: Top Hacks for the 24 Hour Fast

Pros of the 24-hour fast

  • It is straightforward: No complicated meal plans or prep are involved. You stop eating for one day, once a week.
  • It’s non-restrictive: For the rest of the week, you will live your everyday life.
  • It’s maintainable and sustainable: I can see myself fasting for one day out of the week for the rest of my life. I believe that many people would be able to practice the 24-hour fast for a very long time.
  • Perceivable cognitive benefits: During the later hours of my fast—think 16-24 hours—I felt like I was seeing the world in a new light. I felt sharp and focused on whatever task was at hand.
  • Excellent appetite regulation: After 24 hours of fasting, my appetite regulation was superb. While I thought that the opposite would happen, fasting for a day actually helped curb my appetite. I was no hungrier after one day than I sometimes am three hours after a meal.
  • I regained self-control: I constantly talk about how hungry I get when I don’t eat, but I realize that food doesn’t control me. I decided not to eat for 24 hours and was just fine.
  • You can build up your tolerance: While fasting for 24 hours might initially be difficult, it gets easier with practice.

You can (and should still) work out and fast for 24 hours!

When you fast, your body first uses the sugar stored in your liver for energy and then relies on fat stores. Muscle glycogen remains available, so you can still exercise in the fasted state. There’s nothing wrong with taking a workout rest day on your fast day.

» Find out how food helps you recover after a workout

Cons of the 24-hour fast

  • Twenty-four hours feel like a lifetime: Many people worry about getting low blood sugar if they don’t eat every few hours. This is mainly psychological. Research suggests that it's rare unless you have diabetes. [5] During a 24-hour fast, you'll most likely maintain normal blood glucose levels.
  • Potential headaches: The first time I fasted for a day, my head hurt a bit in the later hours of the fast. But I didn’t have any problems the second time. A combination of slowly increasing your fasting periods and staying hydrated can keep any pain at bay.
  • Beware of post-fast binge-eating: After the first two times I fasted, I binged and broke my fasts with meals of around 1200 calories. Be wary, and don’t go crazy. Eat a regular meal as if you had not fasted at all.
  • Your breath stinks: Despite staying extremely hydrated, this is just inevitable after a day of fasting.
  • Think twice about a 24-hour fast if you're an athlete: If you're an elite athlete training multiple times a day or are participating in some type of prolonged endurance sport, the 24-hour fast isn't for you. [6]

» Explore why insulin is your key to metabolism and healthspan

Changes in weight after fasting for 24 hours

Be patient with the scale. The day after the fast, you will see the number drop significantly—anywhere from 1.5 to 3 pounds. [6] While much of this is water weight, you still lose fat. [7] After a few days, you'll gain weight as you hydrate yourself. But this doesn't mean you haven't shed any pounds.

What happens is you're decreasing fat at a slow, sustainable rate. While the amount lost depends on body size, short-term fasting has been shown to help individuals lose around 2% after 3 weeks of fasting. [7] Also, you can use IF to lose weight in the long term.

» Promote your endurance with zone 2 heart rate training

Golden rules of intermittent fasting success:

  • If you decide to try IF, stay in tune with your body! Fasting may require more adjustment for some than others; make sure you give your body time to adapt. It may not be for everyone, and nothing is wrong with that.
  • Stay busy. Don’t torture yourself by sitting on the couch and thinking about food all day. Instead, take some time to clear your head and get your body moving. Go for a walk or catch up with friends.
  • Hydrate, hydrate, and hydrate. Water, tea, coffee… keep the liquids coming.
  • Re-think your hunger.

My takeaways from the 24-hour fast

Financially speaking, I have no skin in the IF game. However, as both a self-declared spokesperson for wellness and a serious skeptic about the latest dietary trends, I would be ecstatic if you tried the 24-hour fast. I found IF not only to agree with my lifestyle but also to give me a sense of empowerment.

As it hasn’t been three months since my last blood test, I can’t yet report on any of my biomarker changes. Suppose my mood and appetite regulation improvements indicate changes in my InsideTracker results.

In that case, however, I expect to see improvements in my blood glucose levels and a drop in my Inner Age.

Effects of doing regular 24-hour fasts

I can say that I have regained control over my eating habits, have obtained a new perspective on the benefits of ‘hunger,’ and have even rediscovered what it feels like to appreciate food. After four rounds of the 24-hour fast, my clothes fit better. While I don’t like feeling restricted, I am plenty willing to fast once a week for the results I desire.

Check out part 3 of the blog series, where I’ll share my experience with the Breakfast Skip, an alternative IF method. In the meantime, what do you have to lose besides a couple of pounds? 

» You don't have to rely on a 24-hour fast to lose weight. Improve your diet with a blood biomarker analysis


  1. Pilon, B. (2012). Eat Stop Eat (5th ed.). Strength Works, Inc.
  2. Deighton, K., Batterham, R., & Stensel, D. (2014). Appetite and gut peptide responses to exercise and calorie restriction: The effect of modest 2 energy deficitsAppetite, 81, 52-59.
  3. Verboeket-Van De Venne, W., Westerterp, K., & Kester, A. (1992). Effect of the pattern of food intake on human energy metabolismBritish Journal of Nutrition, 70, 103-115.
  4. Keim, N., & Horn, W. (2012). Restrained Eating Behavior and the Metabolic Response to Dietary Energy Restriction in WomenObesity, 12(1), 141-149.
  5. Eckert-Norton, M., & Kirk, S. Non-diabetic HypoglycemiaThe Journal of Clinical Endocrinology & Metabolism.
  6. Pilon, B. (2012). Eat Stop Eat (5th ed.). Strength Works, Inc.
  7. Heilbronn, L, et al. (2004). Alternate-day fasting in nonobese subjects: Effects on body weight, body composition, and energy metabolismThe American Journal of Clinical Nutrition, 81, 69-73.

Disclaimer: InsideTracker doesn't diagnose or treat medical conditions. Consult your physician if you have any health concerns.