Here's What Happens to Your Body on a 5-Day Water Fast
Fasting is the quickest way to reinvigorate the body
Well, kids, I was sick last week. I figured it was an exciting new flavor of Covid, but it turns out is was bacterial. Twelve hours after my first dose of antibiotic I felt fine.
I blame myself.
Looking at my logs, it’s been three months since my last prolonged fast, which, at my age, is just enough time for things to start breaking down.
I knew I was due to take a break from eating a few weeks ago. I’ve learn to recognize the signs:
Lack of energy, especially mental energy.
Bad mood 30% of the time.
Left knee, right ankle, and wrist pain (arthritis from various sports injuries.)
These symptoms sneak up on me whenever I go more than a quarter without a prolonged fast. I also start straying from my low-carb, high-fat diet a few months after fasting.
I also noticed it’s been awhile since I badgered the world with my fasting evangelization apostolate. So, I went through the archives to see what aspects of fasting I haven’t written about.
Unfortunately for you, I found one: fasting timelines.
When I quit smoking, which I did regularly for about 15 years, one of the most motivating tools was a timeline of what happens to your body after your last cigarette. Stuff like:
1 hour: Yell at your wife.
3 hours: Slam a door.
6 hours: Hyperventilate trying to suck in second-hand smoke on the patio of a suburban strip mall bar.
Since I’ve been studying fasting for a few years, I thought it would be helpful to offer such a timetable to people who wonder, “why would I want to go five days without eating?”
So, let’s dig in. Assume you began your fast with end of your Sunday dinner.
Nothing. Everybody goes an hour without eating every day. It’s normal.
This is when most people start thinking about a snack. But consider this: your body goes at least 8 to 10 hours without food every night while you’re asleep. If you were smart, you began your fast after dinner, so you should be in bed now, anyway. Even if 6 hours of fasting did something wonderful (or terrible) to your body, you’d have no clue.
Okay, now we’re getting somewhere. 12 hours after your last calorie, even the most sticky, slow-digesting food is out of your small intestine and no longer providing nutrition to your body. If you finished dinner at 6 p.m., it’s now 6 a.m. Your blood sugar level is stable and, if you’re not diabetic, below 100.
If you’ve never fasted before, you suddenly have a whole new respect for the survivors of the Donner Party. If you are an experienced faster, you might pop a dose of BHB to get a jolt of ketones. If you’re measuring your blood glucose, there’s a good chance it’s spiking over 100 as your liver dumps glycogen into the blood for energy. (The liver is supposed to cycle its glycogen stock every day, but with with a normal eating schedule of meals every six hours and snacks in between, it never gets a chance. So, when you go 16 hours without food, the liver, out of practice, dumps too much sugar into your veins. This issue will go away shortly.)
More importantly, about 16 hours in, your body accelerates the natural process of recycling the oldest, weakest cells. It’s call “autophagy,” and it’s the most important benefit of fasting. Autophagy is believed to be the reason fasting slows or reverses many forms or cancer, as your body decommissions the deranged cells and uses their healthy parts in the creation of new ones.
20 to 24 hours:
Your liver is running out of highly available liquid glycogen, so your blood glucose level is falling. You’re starving and angry and maybe a little sluggish. But your body is also starting to turn fat into ketone bodies for energy. Congratulations: you are now, most likely, in light to moderate ketosis.
Autophagy accelerates. Your body is burning fat. Your brain will begin to function a little better shortly. The energy drop you experienced four hours ago is gone. (But a weird energy phenomenon is about to kick in. More about that later.)
It is now 6 p.m., and you’ve gone 24 hours without a single calorie. You’re still alive. Congratulations. Though humans went 24 hours without food for millenia, you’re probably in the top 5 percent of the population when it comes to food discipline. And bedtime is only a few hours away: don’t quit now.
You’re technically on the third day of your fast. Even though you had food in your system for 12 hours after that last meal, fasting begins when you stop eating, which was your last bite of food on Sunday.
Your liver is still trickling out some stored glycogen (unless you’re a keto practitioner). That’s because, as I mentioned earlier, our modern eating habits provide the liver with a constant influx of sugar. Over the years, your liver starts compressing that syrup into hard crystals which melt slowly when you stop eating. Don’t worry, though: regular fasting and low-carb diets will eventually deplete those stores of glycogen and all the nasty stuff that got packed in their with it.
Autophagy increases by about 330% at this point, according to several studies. This means dangerous and weak cells are disappearing from your body. Their healthy parts being recycled into new, healthy cells.
But there’s bad news: if this is your first long fast—or first in a long time—this day will probably be the worst. You’ll feel cold, like you have a fever. You might be a little melancholy. Sad memories might hit you out of nowhere. You might have brain fog and energy so low it would make Jeb Bush jealous.
What’s happening? Your body is adjusting to running on ketones instead of sugar (or alcohol) for energy. Our bodies naturally run on ketones, glucose, or alcohol, but it burns them in reverse order. Alcohol requires no chemical change to burn, so your body will always burn that first. Sugar requires little chemical conversion effort, so that burns next. When there’s no sugar or alcohol available, your body starts consuming its fat stores.
Most Americans have never forced their bodies to burn fat as its primary source of energy. Just as the liver was clumsy in kicking out glycogen, your whole body is ham-fisted with ketones the first time it tries to use them in large numbers.
Be patient. Your body will learn. And once your body gets in the groove of burning fat, it will operate better than it ever did on sugar. Much better.
So, 36 hours in is a key moment. You cannot stop now. Keep pushing. Drink water. Take a long walk. Work out. Do something to accelerate ketone adaptation to shorten the difficult period.
You’ve gone two days without a calorie, and the worst of it is over. Autophagy jumps up again, another 50% leap in bad-cell destruction. You’re about four hours from bedtime, but it’s been a tough day. Consider hitting the rack early.
At this point, your brain is undergoing rapid repairs. Your hippocampus, in particular, sees significant neuroplasty—generation of new brain cells. Memories that have been buried under the fog of damaged cells are reconnected to the prefrontal cortex by a bridge of healthy, young cells.
Now, we’re getting into serious numbers. If you started your fast after dinner Sunday, Wednesday morning is the first day in your new body. You might catch an occasional whiff of fingernail polish remover. That’s acetone. Overnight, your body got really good at breaking down fat into ketones. Too good, in fact. The combination of massive amounts of ketones and being asleep means your body has to get rid of excess ketones. Ketones turn in to acetone which you exhale. Yes, you smell (mildly) like nail polish remover. This will go away as you become more active.
Autophagy is nearing its max, but other anti-aging and damage-repairing functions now join the party.
Autophagy rapidly breaks down deranged cells. With no food coming in, cells are not growing and dividing. Something miraculous is about to happen to prevent your body from wasting away. Two things, actually,
First, sometime after the 60 hour mark, your body starts creating new cells from stem cells. Then, to accelerate this process, you also produce growth hormone as if you were a growing child.
What’s miraculous about this process—and one of the greatest health benefits of fasting—is that these new cells, created from stem cells, have perfect DNA.
Stem cells are cells with a perfect DNA pattern that have not yet been assigned a job. They are not liver cells or brain cells or skin cells, but they can become those things if needed.
The number of stems cells available for transformation decline with age, but prolonged fasting seems to temporarily reverse or slow their gradual loss. In other words, regular prolonged fasting of 72 hours or more keeps your body armed with perfect DNA to replace cells with damaged DNA.
Cell division produces cells that are copies of copies of copies of copies. Errors in one generation of cells are passed along to the next generation. Which introduces new errors before splitting into the next generation. In other words, a normal adult goes through life full of bad cells.
When you kill old, bad cells through rapid autophagy while denying your body the food needed for cell division, your body has no choice but to go back to square one and create new cells from the original blueprint.
This stem-cell recovery process is similar to the process that allows kids to grow larger. Cell division isn’t fast enough to make a 14-year-old boy go from 5’6” to 6’2” in a year—it takes massive amounts of brand new cells from stem cells to feed that growth rate. These growth spurts also require a ready supply of growth hormone.
That’s what happens after 60 hours of fasting. You replace 50-something-year old-cells with cells identical to when you were four. This may explain why some people experience a big of age regression after a fast.
A physician who did not further identify herself recently wrote about her first 11-day water fast. It’s a great post worth reading. But one point stands out: her skin.
Flawless skin: I couldn’t stop touching my face. No face ritual. I was in isolation and didn’t need to use any products. I noticed soft cheeks and flawless skin somewhere around day 5. My face was so smooth like a baby’s butt. I felt like I had regenerated skin. I also noticed a glow on my forehead. I was lighter in complexion. I compulsively touched my face all day for the reason that it was outrageously soft.
Once you make it to 60 hours, your body is literally rebuilding itself from the original plan.
Autophagy continues, but the pace of cell-breakdown is at its max. It can’t go any faster.
New cell genesis is approaching its fastest pace, as well.
Fat is falling off your body. Though the liver is still squirting out bits of dissolved glycogen cubes, the vast majority of your energy is old fat.
Here, I should make a note about “water weight.”
Skeptics will tell you, “yeah, so you lost seven pounds. Big deal. Most of it was water weight.” If they’d stop there, they’d be 100% correct. But they never do.
“You’re just gonna put all back on as soon as you start eating again. Ha ha.”
Yes, fasting causes your body to lose “water weight” early on. Here’s the thing: no matter how much water you consume while fasting (and you only need 8 8-ounce glasses of water a day even while fasting), you would still lose that water weight.
Another term for water wight is “inflammation.” Remember that I said glucose requires more chemical reaction to turn into fuel than alcohol does? Well, that chemical reaction requires a lot of water. About 8 times as much water (in weight) as sugar. When you’re running on carbs, every cell of your body is holding onto water to help with that glucose chemistry experiment.
When you stop eating, your body burns the sugar that’s available. It uses that stored water for the chemical reaction.
When the sugar burns up, the water goes with it. Since you’re not introducing new pallets of sugar during a fast (or on keto), you’re not retaining the excess water, either.
That water weight is not in some reservoir in your gut—it’s water stored in every cell of your body, including blood cells.
Without that excess water (aka, systemic inflammation), every cell, every function, of your body works better. The swelling in my knee, ankle, and wrists goes away because the water goes away.
Again, no matter how much you drink, you will not retain the water while fasting because your cells don’t need it to convert glucose.
If, after your fast ends, you begin a low-carb diet, that water weight will not return. Even if you go back to the All American, 200-grams of sugar per day diet, it will take about two weeks for your chronic, systemic, body-killing, pain-inducing inflammation to return to normal. (Don’t let it.)
When I end a 5-day water fast, I continue to lose weight for about 3 to 5 days. Then, I hold for about week. And I typically lose about 8 to 12 pounds on a fast.
I have never regained weight after fasting until I start eating carbs like a regular American. In 2021, I went from 187 to 175 during a fast in May, then I dropped to 172 and stayed within 2 lbs of that until December—when Christmas meant eating sugar like I’m trying to hide it from the Feds.
I’m telling you: a 5-day fast followed by a diet of less than 20 grams of carbohydrates per day, and that “water weight” will never return.
All that fat burning, cell munching, and stem-cell genesis continues at warp speed, but something else happens now. Something that’s difficult to explain but absolutely amazing. Something I crave when I’m not fasting.
From 96 hours to . . . as long as you fast (and some people go 21 days once a year), every bodily sense goes through the roof.
Your vision gets sharper—way sharper
Your mind operates at genius level
Your body is more coordinated
Your hearing becomes acute
Your ability to manage your mood and energy seems perfect
But most amazing is your sense of smell. You can smell things you didn’t know had a smell
Once, on day 4 of a fast, I was walking through a parking lot that’s surrounded by woods. I smelled something. I can’t describe the smell—it was just there. And it demanded my attention.
I stopped and looked. I knew the direction the smell was coming from, almost by instinct.
Far off in the woods—maybe 300 yards—was a deer. I knew immediately the deer was the thing I smelled. I could smell and see it 300 yards away in the woods. It was only about 100 feet inside the tree line, so it wasn’t like I had X-ray vision, but I saw it only because I smelled it first.
It smelled neither alluring nor foul. It just had a distinct scent that I could recognize from 300 yards.
But smell might not be the sense most affected by your fast. The doctor I mentioned earlier, the one with the soft skin, noticed her vision:
Better vision: When I woke up, I not only felt instantly alert, but my vision was super clear. Though I wear glasses to correct a structural defect, I could feel my eyes at the back of my head. No joke! The light in my room felt brighter and clearer. Everything looked picture perfect. It almost felt psychic. I saw what I needed to see, not just what I wanted to see.
That’s how tuned and sharp your senses get after 96 hours.
The Last Day
It’s Friday. You haven’t eaten since Sunday. You’re not even thinking about eating.
You’ve now learned that, as my friend Jim says, food is an option. Food is rarely a necessity. Many people do a 21-day water-only fast once a year.
You have learned something about yourself—something you’d have scoffed at the mention of before: you don’t need food.
You don’t need the next meal. You don’t need the one after that. You know this because you just skipped 15 consecutive meals and any snacks you might have had in between.
You had no soda, no cream in your coffee, no candy, no chips, not even sweetener. For the last weeks, your treats were black coffee and herbal teas. (Chamomile and peppermint are my favorites during a fast. They’re kind of bland any other time.)
Your entire digestive system has been resting and recovering—getting shiny new cells from stems—for almost a week.
You are clean, and your senses are so sharp you could survive being thrown into the wild and left for dead. (Bring a knife to skin and dress that deer, though.)
You’ve learned something interesting about your body on ketones: it becomes very efficient at adapting to the energy necessary—and no more.
Remember, I said I’d elaborate on the energy thing? It’s time.
When you’re fasting or deep into ketosis, your body provides exactly the right amount of energy. No more, no less.
This means that if you sit for a few minutes, you will feel tired. You might even be afraid that you won’t be able get up when you need to.
But once you get up to do something, your energy level rises to the occasion. If the occasion is a 3-mile run, you have plenty of energy to do it. If it means lifting weights, you’ve got it. If it’s working four hours straight on a retaining wall installation, you can knock it out without feeling fatigued.
But you never have extra energy. You’re never nervous or fidgety, or anxious. Your mood is right down the middle, but you can laugh uproariously at something funny.
I believe fasting is the ultimate cure for depression, lethargy, melancholy, whatever. It’s the cure for low energy, low spirits, and low IQ. It clears up brain fog and improves memory.
Combined with a good keto diet when not fasting, it keeps your body at a high level of autophagy and stem-cell genesis, reducing the chances of many forms of cancer.
As Donald Trump might say, what the hell do you have to lose?
Some “experts” say you need to ease back into food very carefully.
Hogwash. Five days is not that long without food. (Seven days or longer, a different story.) Eat a celebratory meal. Since it’s Friday, no meat, but eat something you really want to eat. Nate Eliason, who wrote the greatest blog post about 5-day fasting ever, says this:
I think people are too careful with fasting. Numerous sources suggested waiting two or more days before reintroducing meat. If you were in the wild, and you hadn’t eaten in four days, and you managed to kill a deer, you wouldn’t go forage for berries before eating it to “work yourself up to it.” You’d eat the damn deer.
After your 5-day fast, you are stronger in mind and body. If you combined the fast with intense prayer, your soul is in better shape, too.
You are now better prepared to handle emergencies—like no time for lunch. In a real emergency when cut off from what we call civilization, people’s first thoughts include “what I’m going to eat.” Even though shelter is more important than food, modern man tends to prioritize food first.
Not you. You know you can go a work week without a single calorie. You will be focus on clean water and shelter. (Granted, you can get a lot of water from food, so don’t overlook that source.)
Because you’ve experienced fasting, you can continue to function while others are freaking out. You’ll be in charge quickly. You and the other fasters. You know what to do. The loss of luxuries like warm showers and hot meals, while inconvenient, won’t throw you off your game.
See, that wasn’t so bad now, was it?
If you supplement (vitamins etc.) do you increase, decrease or maintain the regimen during the fast?
Great read. Couple of questions:
I’ve done 3-4 day fasts in the past, but would have a couple of cans of beef stock. 3-4g of protein @ 15 cal ea. Do you see a problem with that? Seemed to go thru same stages as you described.
Also, were/are you still doing Starting Strength training during the fasts? If so, do you step down the weight during the fast as it relies on a protein excess.