Fasting Graphic

[DISCLAIMER: This information is not intended to be understood as scientific information. This is a very brief overview of what happens to your body when you fast. For full information, please consult your physician and your local Google.]

I’ve been fascinated for years about what happens to our bodies when we fast. God intended us to undergo the physical ramifications and changes that He built into us before time began when we fast, so I’ve been eager to understand what those are.

I’m not a biologist or doctor or scientician. I’m just a fascinated guy who has tried to educate myself as best I can through the years, so feel free to fact-check me and dispute anything I say here. I’d appreciate the correction.

Fasting takes on different forms at different times after we stop eating. Before we begin looking at what those changes are, let’s define what fasting technically is.

Whenever you finish a meal, your body begins processing it (actually, it starts the moment you begin eating). The body’s process for dealing with food is to digest it in the stomach and then move it along. Your body expects a regular cycle of food coming in because it breaks that food down into it’s components and uses the energy stored in it: the calories.

It’s all about the calories, then. When we completely stop, or greatly reduce, the calories coming in, our bodies are designed to react to that. The first reaction begins when fasting truly begins.

Days 1-4

Within only a few hours, our bodies begin the process of fasting. Once our stomachs are empty, hunger pangs quickly set in. Whereas we wouldn’t call going 4 hours without food fasting, medically this is the start. Our body’s first reaction to fasting is the hunger pangs we feel. We usually wake up in the mornings hungry because our bodies have emptied our stomachs while we slept and it alerts us that it requires further energy before a fasting state sets in. There’s a legitimate reason we call our morning meal “break-fast.”

If you choose to continue not eating after those initial hunger pangs set in, your body almost immediately kicks into a fasted state. It begins preparing for how to deal with lower or no energy coming in. God designed it this way and its a fascinating thing.

Over the next several days, depending on your body and metabolism and how many calories you’re actually taking in (between nothing and a few hundred if you’re on a “juice fast”), something remarkable happens. The energy source your body prefers is called glucose. It’s basically sugar that our bodies extract out of food. Brain function and the cellular exchange of nutrients depends on it. We can only get glucose from food because our bodies won’t store it. Our fat deposits aren’t made of glucose.

Our bodies have a response, though. It’s called “keytones.” I don’t understand why they called it that, but that’s what they are. Keytones are non-glucose based energy stores that are custom-built to last a very long time. They’re so efficient giving our bodies only the energy it needs that they’re targeted at providing it only to specific functions. Brain function isn’t one of them, at least not higher-level brain function.

Over the course of the first 1-4 days the amount of energy our body derives from keytones increases dramatically until it reaches a peak around day 3-4. That entire time we generally feel miserable. If you’re doing an extended fast, the first few days are always the hardest and this is why. Every cell in your body is altering its normal function to exist on keytones instead of glucose. If you’ve gone through this before, you know how painful a process it is.

Days 4-21

After you have completely shifted over to fueling yourself with keytones, your body is in full-on “starvation mode.” This is where every normal bodily process that is not absolutely necessary is shut down to conserve energy. The keytones trigger this reaction in your cells, and it’s truly a majestic thing to think that God designed us this way.

Things like producing heat and bowel movement are almost completely shut down. The body seals itself off, if you will, from losing energy unnecessarily. Extended fasts in wintertime prove to be very uncomfortable because of this. 98.6-degree heat in the summertime is a welcome thing if you’re on an extended fast. You’ll also find it a lot harder, but not impossible, to sweat. Sweat is just energy leaving your body and at this point it doesn’t like to do it.

Depending on how much keytone “reserve” your individual body possesses (love handles, belly paunch, thighs and seat-cushion comfort), your body can go an incredibly long time surviving without calories.

During this time your body is existing primarily on these fat stores. If you try strenuous activity, you may find that your muscles hurt very quickly. That’s because it takes energy to power those guys and your body shuts it off. I remember foolishly trying to do a pull-up on day 35 of a fast one time because I was feeling so spritely with all the weight I’d lost. After one pull-up I ached for hours.

You may find that you’ve lost muscle mass after a long fast and that’s primarily from atrophy. Your body will eat away at your muscles during a fast, but it prefers fat stores. Lack of use of muscles will cause your body to chip away at them slowly because they hold a higher-quality energy, but your body is masterfully designed to favor fat first.

Day 21 On

At some point your body will run out of fat stores. Depending on how well you’ve stored up, that can be anywhere from 21 days to 60 or more. Also, if you’re taking in some calories through juice, the effects of keytone breakdown are extended. Supplementing your body’s keytone usage with glucose from juice gives you the ability to fast longer, especially if you’re a skinny person, but eventually everyone will run out. That’s when your body turns to drastic measures.

Before your body has completely exhausted all fat stores it will begin breaking down anything else it can get its hands on. This is when it’s time to stop fasting. You’ll probably have a hard time finding any medical professional who’ll encourage you to do extended fasts because they’ve seen so many people bound up by eating disorders, but you’d probably be hospitalized and committed against your will by one if you continued to fast beyond this point.

From personal experience I can say that I’ve never reached this point. Unfortunately for me, my body has almost always had enough “reserves” to last me longer than 40 days. I have encountered fasts that are less pleasant toward the end because I have fewer fat stores, but I’ve never run out.

How to Choose The Right Fast

I’ve known skinny people who have fasted 40 days on nothing but water and made it through just fine and I’ve seen “healthy” people need to quit after a few weeks. Your required energy output is a big factor. You can’t physcially fast on water for any length of time and do physical labor. Your body will shut down on you, and it can be catastrophic.

You have to match your required level of work to the type of fast you plan on doing. A “water fast” (water only) just won’t work very long if you do manual labor. It won’t work well, either, if you have a stressful desk job. Your brain tires quickly without glucose. I’ve done a 21-day water fast before and I was physically limited to no more than 2 hours a day of physical activity. What I mean is that the longer the fast went, the more time I had to lay down and rest after just walking a mile or two.

If you don’t have the luxury to put your life on hold, then a juice fast might be the way to go. The supplement of glucose keeps your brain function going better and gives you more energy for physical activity. The amount of calories you take in will determine the amount of effort you can exert.

I’ve personally found that after a while it’s hard to continue drinking juice. The acidity starts to adversely affect my body after a while and I have to supplement with salty calories somehow. I’ve added broths to my fasts when I can’t do acidic juices anymore.

God Sees You

In the end, no matter what you do God sees your heart. Taking in 1,000 calories a day in juice doesn’t mean you’re not fasting. Fasting is more in the heart of what you’re doing than the physical limitations you put on yourself. I’m giving you this information to help you make some decisions when you choose to fast, not to say there’s one way that’s “more holy” than the other.

As I said in my previous post, fasting is about giving up physical sustenance, but its also in the heart behind it. I’ve known people to diet on 1,000 calories a day, but it wasn’t fasting. Fasting is denying our bodies something it needs for a spiritual purpose. You have to be doing it because you’re desperate for God, and God alone. When you do that, God sees it. No matter how weak our attempts may be, God sees them and they matter to Him.


I’ve been intentionally as scientific-less as possible here. There’s lots of good resources out there on exactly how the mechanics of all this work if you ask Google or your local library. There’s also physical disorders that fasting for any length make potentially dangerous, so don’t fast unless you’re healthy enough to do it. Like I said before, any physician may talk you out of fasting because of how much body image abuse they’ve seen, but take their words seriously if they tell you fasting for any length may harm or kill you.

Let me know what symptoms and experiences you’ve noticed when you fast. What’s the most fascinating thing you’ve encountered? What’s the worst feeling you’ve dealt with? What’s the result you’ve received from God?

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