I’ve had amazing results over the last 2 years or so using a combination of:
- Heavy weightlifting just 3 times per week
- And intermittent fasting
I’ve built strength and muscle, along with shedding at least 15 or more pounds of body fat.
But right now, as I write this, I’m about 4 months into a bulk.
Using intermittent fasting on a bulk is a great way to stay lean and moderate your calories, but no matter how you slice it, there’s always a little bit of fat gain when building muscle.
And I’m a little fluffier right now than I’d like to be!
I wanted to know if doing a 24 hour fast would help me lean out and burn some fat while still continuing on my path to gaining strength and muscle — (in other words, a 24 hour fast while bulking!).
So I decided to try a 24 hour fast and write down my results before and after (plus pictures).
But first, I needed to know a lot more about long, extended fasts and how they affect your body.
(By the way, did you know you can use a simple breath sensor like this one to detect if your body is in ketosis, aka fat-burning mode, during a fast?)
What actually is fasting? (24-hour fast definition)
There’s a lot of confusion out there about what’s actually considered “fasting.”
So let’s clear that up right now.
Fasting is any prolonged period of time over around 8 hours without food.
So, technically, just having dinner and going to sleep (usually) constitutes a fast!
Around this 8 hour mark, the last food and calories you ate have been digested and processed.
(Many of those calories, and especially carbohydrates, are turned into glucose and stored in the liver and muscles to use as energy.)
Somewhere around 8-12 hours without any food or calories, your body will run out of these glucose reserves.
At this point, your body begins to tap into other sources, primarily body fat, for new energy.
During a 24 hour fast, then, that means your body’s running on mostly burned body fat for somewhere between 12-16 hours before you eat again and provide more material for glucose production.
In much longer fasts (beyond 72 hours without food), your body may enter starvation mode and begin slowing down its metabolic rate (how fast it uses energy) and tapping into muscle tissue for energy.
(That’s not a good thing!)
Most research indicates that shorter fasts like intermittent fasting (16, 18, or 20 hours) or 24-hour fasts are muscle sparing, meaning you are very unlikely to lose lean mass during a fast.
Your body far prefers to burn glucose, first, then body fat for energy.
What are the benefits of a 24 hour fast? (Or why I decided to do it)
First, let’s run through the science-backed benefits of fasting (whether that’s a shorter 16-hour fast or a longer 24-hour one.)
According to Healthline, fasting can:
- Reduce inflammation
- Reduce insulin resistance (keeps blood sugar levels steady)
- Improve blood pressure, cholesterol & other health markers
- Boost metabolism & how many calories you burn
- Provide a surge of growth hormone
- Improve appetite regulation
- Lead to a loss of water retention & water weight
There are some cool side bonuses of fasting, like the fact that it forces you to hydrate thoroughly (zero-calorie liquids are all you can have!).
It also helps you distinguish between actual hunger (lack of nutrients) and boredom or psychological hunger, which could lead to a healthier relationship with food.
Then there are the anecdotal benefits of fasting like:
- It can help improve your skin and clear acne
- It’ll improve your energy and focus
- Some people swear fasting fights disease
For my purpose, the benefit of fasting that I’m most interested in is fat loss and fat burning.
Again, I’m going into this from the perspective of someone who wants to continue gaining strength and muscle while staying lean.
I want to test out whether it’s possible to burn off body fat during a fast and keep it off when you resume eating in a calorie surplus the next day.
(If it clears my skin and gives me more energy, that’s just a bonus! Losing a little water weight sounds pretty good, too.)
You can tell your body is converting fat into energy while fasting by using a handheld sensor like this one that detects acetones in your breath.
What are the risks of a 24 hour fast?
There are definitely drawbacks of doing a long fast, like, oh, I dunno, the fact that it sounds hard!
After all, eating is fun. Not eating for a whole day sounds kind of not fun.
But in general, most sources that I’ve read agree that 24-hour fasts are completely safe.
The exception would be if you have an existing medical condition related to blood sugar, blood pressure, or cholesterol, all of which can be altered by dieting.
You should consult with your doctor before doing a 24 hour fast if that’s the case, and especially if you live with a condition like diabetes.
For most of us, though, our bodies are completely capable of burning through glucose and then a little bit of body fat for one measly day.
As long as you stay well-hydrated and eat nutritious foods before and after the fast, there’s very little concern to be had.
Will your body go into “starvation mode” during a 24 hour fast?
It’s highly unlikely.
Starvation mode is (mostly) a myth.
The idea is that, during fasting or dieting, your metabolism slows down so much in an effort to survive (and not starve to death) that it becomes impossible to lose more fat.
This can (maybe, sort of) happen, but it’s not at all caused by eating a few hundred calories less per day or doing a short fast.
Starvation mode, in reality, is what happens when you’re actually starving to death and have been deprived of nutrients to a severe degree.
That’s when your body starts eating away at its own muscle tissue in order to survive.
This would most likely take your body several days (or more) without any food at all, and it wouldn’t happen until you chewed through a substantial amount of body fat first.
This, ACTUAL starvation mode, is really dangerous for obvious reasons, including the fact that muscle breakdown of this level can release toxic proteins into your system.
There’s plenty of evidence out there of people who do regular two or three-day fasts without any negative health issues, so you’re definitely not going to starve doing a short, one-day fast.
Related: How many calories do you burn during a 24 hour fast?
The number of calories burned during a 24 hour fast depends on your age, gender, weight, and activity levels. The fast doesn’t really make your body burn any more or less calories.
In general, moderately active adult women will burn about 1600-2400 calories per day and men between 2000-3000, according to Everyday Health.
Preparing for the 24-hour fast and the “rules”
It’s time for me to get ready to do a full one-day fast, and there are a couple of things to think through.
Namely, what am I allowed to have during my 24-hour fast?
Based on some research and my own thoughts, I’ll be able to drink:
- Black coffee
- Naturally flavored zero-calorie sparkling water
- Water (duh)
- Green tea
I’m going to try like hell to avoid artificially sweetened drinks like diet soda. They can be a nice treat during a fast but sometimes they can make your hunger cravings worse when you’re done.
The other thing I need to prepare is my post-fast meal.
I’m going to start fasting after dinner on Day 1 and resume eating at dinner on Day 2.
Since I’m trying to hit my normal bulking calories (or at least come close), I have to choose Day 2 dinner really carefully. It’ll be hard to get enough calories in that day in such a condensed amount of time.
Of course, I could cram in lots of junk calories easily, but to properly hit my macros I really need a dinner that will be:
- Calorie dense
- Protein heavy
- Have some whole grains & veggies
I’m thinking of cooking up some big old chicken breasts for a massive bomb of protein, and topping it off with some pasta (heavy on the sauce and oil for caloric density) and veggies for dinner.
After a large dinner, I’ll try to eat some sweet but protein-rich foods like protein-fortified oatmeal, milk, and/or Greek yogurt.
The last rule is that I’m not allowed to just sit on my ass (or sleep!) all day. I have to get up and at least go for a walk or be active in some way, though I won’t be fasting during a normal workout day.
What happened before, during & after my first 24 hour fast
Here’s a timeline!
1 pm Wednesday – I work out. It’s leg day! I have a good workout and leave the gym feeling pretty good. I weigh in at 138.4 pounds (at 5′ 5″) fasted before my work out.
8 pm Wednesday – Dinner. Spaghetti with chicken meatballs. I eat up knowing my eating-window is about to close for a long time!
9 pm Wednesday – I finish off a post-dinner snack of protein oatmeal and a big glass of milk. I have to force this down a little, and I’m stuffed. Normally I would eat this post-dinner snack a half hour to an hour later, right before bed.
(Note: I really wanted to stop eating earlier in the night than this, but alas, life happened and the schedule got pushed back some. Now I have to make it really deep in the day on Thursday without eating.)
I round out the day somewhere around 2250-2500 calories for bulking.
7 am Thursday – I wake up. I’m about 10 hours into the fast at this point, which is nothing. Though I stopped eating slightly earlier than I normally would at night, I feel pretty much normal.
730 am Thursday – Black coffee! This is how I always start my day with intermittent fasting.
1030 am Thursday – I’m feeling my first hunger pangs. This is pretty normal during a fast, and I find that hunger comes and goes in waves throughout the day before you eat. However, this is a little earlier than normal for me to be hungry.
I’m keenly aware of the fact that I don’t get to eat for a long time, and it’s a weird feeling. It’s easy to underestimate how much you look forward to eating delicious food just because it’s fun.
At this point I’m 13.5 hours into the fast, give or take.
11 am Thursday – I keep myself busy with work and drinking regular water. So far this is a pretty normal day, as I don’t usually eat anything until around 1 pm or so.
I also check my weight again as a gauge. I’m still exactly 138.4 pounds.
1 pm Thursday – I drink a sparkling water (naturally flavored, zero calories… an off-brand La Croix basically). This is pretty much normally what I would do at this time during my usual 16/8-ish fasting.
130 pm Thursday – I’m not particularly hungry, but I am definitely feeling like I want to eat. I usually eat at this time, give or take. My brain is definitely telling my body that it’s time to eat and that eating will bring me happiness.
Have to just ride the wave and focus on something productive.
We’re a little over 16 hours into the fast at this stage.
230 pm Thursday – I am peeing like crazy, which makes sense. First, I’m drinking a lot of water, and second, peeing a lot is usually a sign of ketosis (when your body begins burning fat for energy and releasing its glycogen stores).
I can feel another wave of hunger.
3 pm Thursday – The hunger (or food cravings) have subsided a lot. But I’m still peeing like a madman.
During this phase of the afternoon, I took my dogs out for a walk at some point to get moving and distract myself.
330 pm Thursday – I decide to treat myself with another black coffee to get me through the afternoon.
At this point the actual hunger is not that intense at all, but the desire for just some kind of flavor or something to do with my mouth is getting stronger.
5 pm Thursday – I’ve just been chugging water and peeing, chugging water and peeing. This has got to be the most hydrated I’ve been in a long time.
The hunger is coming back on. At this point this is the deepest into a fast I have ever been at 20 hours.
I’m going to be ready to seriously crush dinner in a few hours.
6 pm Thursday – I grab another sparkling water. Again, it’s not the hunger getting me (though that’s starting to ramp up), it’s just the desire for something other than just plain water.
I’ve now been fasting for 21 hours.
830 – 9 pm Thursday – The rest of the evening is pretty uneventful, except I am SUPER looking forward to a post-fast feast.
The hunger is there but it’s not overwhelming by any means. I keep busy with my usual end of day stuff and drink lots of water. Surprisingly, my mood and energy levels are really good.
Right around 9 pm it’s finally time to eat!
I ditch my original idea of pasta in favor of something quick: I decide to eat an entire frozen spinach pizza and an extra large chicken breast on the side for more protein.
(I have a toddler and life doesn’t always go exactly according to plan!)
I take it slow at first, starting with just three slices and the chicken breast.
It’s such a bizarre feeling; I can feel the hunger creeping on even as I’m eating.
It’s like there’s this huge void in my stomach that’s dying to be filled!
I end up eating almost the entire pizza (my wife has one slice), the chicken breast, and a glass of wine.
(Side note: When she comes home she mentions I look extremely lean and that my skin looks clear and well-hydrated.)
That’s around 1200 calories total and about 60 grams of protein or so.
10 – 11 pm Thursday: After letting things settle for a bit, I need to take in some more calories.
Before bed I finish off with:
- A protein bar (230 calories and 20g of protein)
- A small glass of milk (100 calories and 7g of protein)
- A Greek yogurt dessert bar (100 calories and 10g protein)
At this point, I’m pretty stuffed and it’s time to get to bed. I’d love to take in a few more calories and some more protein, but I think it’s time to call it a day.
The total tally for my meal(s) after the 24 hour fast is roughly 1650 calories and 100g of protein.
(Conveniently enough, the bulking workout program I’m currently on actually calls for one low-calorie day per week to stay lean.)
Still, I worry that this is a little low and might affect how I feel in the gym tomorrow.
Friday morning: I wake up feeling pretty normal, though I slept like a rock and it takes me a while to shake off the cobwebs.
I also feel a little bit more depleted than normal, which makes sense after a deep fast and a low-calorie day.
I feel lean but not overly so. I did stuff a lot of food into my face in a really short amount of time (1600+ calories in 2 hours, yeesh!)
I weigh in at 138.0 pounds, that’s just under a half pound down from the previous day.
I suspect I might weigh less than that later in the day as my body “processes” all the food I gave it late the previous night..
Friday afternoon: Time to work out. I’m fasted, as usual, and haven’t eaten since 11 pm Thursday.
(I do hit some Kino Octane [Amazon link], my favorite pre-workout for intermittent fasting.)
To be honest, it’s not my greatest lift ever. It feels like my top-end strength is there but the endurance just isn’t.
I definitely feel a little bit more drained than usual. I think this has more to do with the low calorie intake on Thursday vs the actual fast itself.
At least, that’s the theory.
I get in a decent workout when all is said and done. I’ve had far better but I’ve also had worse.
Before and after my 24-hour fast photos
Alright, alright. I know you want to see the results (if there are any to see).
Here’s a quick shot of my face before and after doing a 24-hour fast.
(Excuse my bedhead! I took these first thing in the morning the day before and the day after.)
I think there’s noticably less puffiness in my face in the After shot, and maybe even a more consistent skin tone. Am I imagining it? What do you think?
And here’s the body and midsection. Again, I think the difference is VERY small, maybe imperceptible. (After all, the scale shows about .4 pounds difference, which is practically nothing), but I do think there’s more leanness in the After photo, especially in my lower stomach area.
(That’s where my body loves to hold fat.)
It’s not much, but it’s definitely an incremental improvement in terms of leanness.
It could be fat, it could be water retention, it could be both, but I definitely think I came out of the 24 hour fast a little bit leaner.
(Here’s the full shot before and after. I have to say I nailed the posing here. I’m standing in the exact same spot and posing the exact same way before and after. Anyway, I could be imagining it but I definitely think I look like, 10% puffier in the Before shot.)
Will I do it again and would I recommend a 24 hour fast?
Getting through a full 24 hours without eating was not nearly as hard as I thought it would be.
(It helps that I’ve been bulking and eating plenty of calories… It would probably be harder to do a 24 hour fast on a cut when you’ve been eating in a calorie deficit for a while.)
I really thought I’d be famished by the end and tempted to break the fast and start refeeding early.
But I found that a 24 hour fast is really not that different from other forms of intermittent fasting.
Hunger comes and goes in waves. You may be starving one minute, but if you hydrate and focus on something else it will fade.
I learned that hunger is SO much more psychologically driven than anything else.
Eating is fun. It triggers the pleasure centers of your brain. It can be social. It breaks up the day and can be a reward for a job well done.
But does your body really need to eat every time you feel hungry? Probably not. Most of us will never even come close to the point of needing food to prevent starvation.
I can definitely see myself doing this again to flush my system and shed a little, tiny bit of fat at once, but I can’t see myself doing it super frequently.
Eating is truly one of life’s great pleasures, and while I enjoy intermittent fasting, I don’t want to feel like I’m depriving myself on a regular basis.
But I’d recommend anyone who’s interested in fasting give an extended, 24-hour fast a try because:
- It’ll (surprisingly) help you improve your relationship with food, hunger, and your own body’s signals
- You’ll hydrate like crazy and “flush your system”
- You will burn a little bit of fat and come out looking a tad leaner
- You’ll feel great about your ability to exercise discipline and self-control
- It will force you to eat fewer calories (if that’s your goal)
Related: Should you do a 24 hour fast while bulking?
It’s hard to say based on a sample size of one time.
For me, I just found it too difficult to get all the calories and protein I needed in a severely restricted eating window.
It’s hard enough to eat in a surplus when you’re eating all day, let alone cramming that many calories into like a 3-hour window.
I think the long fast and the low calories definitely led to a less-than-great workout the next day.
BUT… I do think it helped me lean out a bit, so in some respects, it might have been a decent trade-off.
What I might try, instead, is incorporating some longer fasts (18-20 hours or so) occasionally into my week, especially on rest days.
That gives me a little more time to eat enough and should give me some of the additional benefits of a longer fast.
If you want to try a 24-hour fast on a bulk, just make sure you structure your day with enough time to get all of the calories you need. And plan your post-fast meals ahead of time to get the right macros!
Related: Can you do a 24 hour fast every day? How about once a week?
You can do a 24-hour fast every day, and a lot of people do!
That’d be pretty similar to the style of eating known as OMAD, or One Meal a Day.
A lot of dieticians and health experts don’t advise sticking with an OMAD diet over a long period of time. Conservationists, however, tout the benefits of eating only once a day (especially plant-based) for resource consumption and environmentalism.
You could easily do a 24-hour fast once a week. And, in fact, this might be a great idea to stay lean while you’re bulking or just occasionally “flush” the system out.
Try it once to see if you can tolerate it, and if you have any health concerns, DEFINITELY talk to a doctor first.
Trying a 24-hour fast was definitely a cool experience, and I felt proud of myself for making it through at the end.
I do think there were some solid benefits, like waking up a little leaner the next day, and getting super-hydrated from all the water.
During my 24 hour fast, my weight held steady, but the next day after eating, I had lost about half a pound
24-hour fasts or OMAD probably aren’t 100% ideal for bulking, but for weight loss they can be a great strategy to kickstart fat burning.
Let me know in the comments if you’ve ever tried it, or if you’re considering giving it a shot!
(If you’re interested in fasting, especially as part of the Ketogenic diet, consider getting a Keyto breath sensor to better track your progress and get helpful biofeedback.)
Hope this was helpful!