How Fasting Affects Your Physical Body: The Pros and the Cons!

Posted On January 4, 2023 By Peter In

In both the Old and New Testaments, God powerfully rewarded those who fasted: “Those who fast get healing” (Isa. 58).  Jesus said, “those who fast get power over demonic oppression” (Mk 9).  When Daniel fasted, God gave him understanding (Dan. 9). When Jesus started his earthly ministry, one of the first things he did was a 40-day fast. There are crazy rewards promised for those who fast!

I knew a lot of people who had done 40-day water-only fasts like Jesus. So, I thought: Surely, I can do three days, right? But by day 2, I was already losing my STINKING MIND! The headaches, the exhaustion, and the mood swings made me completely useless! I found myself thinking: “God, is this really how you want me to worship you?” I was making my wife so miserable that, even she began thinking: “God, are you sure you want my husband to fast?”

I started wondering: “Do I simply suck at this! Am I just an unspiritual person?” But, looking back, the truth was far simpler: I was just doing it all wrong.

As two small examples, back when Jesus fasted, I can guarantee you:

(1). He probably wasn’t as addicted to caffeine like I was.

(2). He probably wasn’t as addicted to sugar and carbs as much as I was (i.e., he naturally lived a ketogenic lifestyle). So, his body wasn’t going through a massive withdrawal like mine.

But I share these things because, I wasn’t being reasonable about what my body was capable of doing! In fact, I really had no clue about the physiological affects of fasting (for better or for worse!) And I’ve discovered over the years that there are a lot of Christians who avoid this incredible discipline for the same reasons. So, if you’re unfamiliar with things like “Keto-flu” or acidosis, keep reading because you are soon to discover there are many ways to minimize the intolerable headaches and side-effects and maximize all of the incredible physical and spiritual benefits!

Setting aside all of the spiritual benefits, check out what science is discovering about fasting!

Medical Benefits of Fasting

  • Fasting is a Natural Anti-depressant: Did you know, people who do water/juice fasts have dramatically increased amounts of seratonin, dopamine, epinephrine, norepinephrine and other substances that have a strong regulatory influence on metabolism and mood.[1]
  • Fasting Dramatically Reboots Cell Regeneration: Research shows that our bodies have all sorts of damaged or elderly cells that are soaking up energy. So, when we fast, the body automatically kills off the weak cells as well as boosts the production of healthy stem cells. In fact, it’s the unhealthy cells (like cancer cells) that tend to die off when we starve them of sugars and other processed foods. One scientist discovered that a simple fast before chemotherapy has a huge reduction on chemo-side-effects and increases the impact of the chemo.[2] Thus, the next benefit shouldn’t surprise us either…
  • Fasting Decreases Inflammation: This process of “cell-rebooting” caused by fasting is so powerful that, a simple 5 day veggie fast (once/ month) was proven to have a bigger impact on inflammation than the majority of drugs on the market.[3] Indeed, fasting for more than 4 days tends to increase adrenaline, noradrenaline and glucagon – key hormones which help regulate inflammation and immune system functions.
  • Fasting Decreases Autoimmune Issues, Heart Disorders, Gut disorders, Skin Problems, Endocrine problems and Joint Disorders and Lung Disorders[4] For example, when ppl with bronchiole asthma fasted, 70% of them were completely healed after 14 days! Research found that fasting tended to purge out histamines (the allergen that causes bronchiole spasms). In fact, after 12 days, there was almost no histamine left to create inflammation. So, many researchers are now suggesting that fasting might be one of the top solutions for ppl who struggle with allergies & auto-immune disorders. And I haven’t even mentioned any of the weight-loss benefits!

So, why aren’t we hearing more about these benefits?

Well, think about it! Fasting can be hard! I LOVE food. Even more, I’m kinda lazy! I’d rather take a pill – which doesn’t require much sacrifice. And, at the risk of sounding conspiratorial, big pharmaceutical companies need us to keep using their chemical cocktails as it’s hard to make money off people when you cure significant portions of the population with simple fasting & food regimens!

But, it suffices to say, there’s a lot of reasons why people don’t want to hear about fasting. Indeed, every time I teach on it, some people write me letters as if I’m harming people.

So, let’s go there: Can Fasting Be Dangerous?

Absolutely. Any time you mess with your body, your health can become complicated. Indeed, anyone who does a fast longer than fourteen days needs to do some serious research on dehydration or electrolytes – which is where most people get into trouble. (And I’ll cover some of these basics below).

For example, a lot of people get concerned that fasting can burn muscle and weaken their hearts – both of which are true. If you are a child or a person on certain strong medications, it’s possible to reach this depleted state in a matter of a week.

But here’s how you can know if you’re “at risk.” Research shows, the average person (155lbs) can go 40 days, (water only) before their bodies start to burn muscle and weaken their hearts. Notice I used the word “average“. The golden rule is this: When your body fat goes below 3%, fasting can have a negative impact on health. But, let’s be honest:  Many Americans could probably go 50 days before we reached 3%! (I’m just sayin!) Indeed, there have been medical studies on morbidly obese people who’ve gone over a year with no food. So, if you’re wondering if it’s possible to survive with absolutely no food whatsoever, the answer is yes – depending on how much body fat you possess. But is it healthy? Of course not. Your body is designed for food! And your brain needs electrolytes to work.

For example, if you have diabetes (or you’re on some intense meds that can affect your blood sugar), fasts can be quite dangerous. But, keep in mind, the opposite can also be true! It can ironically be dangerous to NOT fast! Indeed, there are certain medical fasts that can ironically almost cure certain types of diabetes! The same is true with eating disorders. If you have a complicated relationship to food, a fast can be a destructive tool. But, under the supervision of an expert, it can also help people with eating disorders to overcome their compulsions.

Thus, it suffices to say: Don’t write off fasting. And don’t ignore the risks either!  Make sense?

But with all the basics covered, allow me to get practical and explain why most of our bodies freak out in the first few days of fasting.

(1). Caffeine withdrawal: Many of us are addicted to caffeine. Thus, when we fast, it’s common to get massive headaches. A lot of times, I tell people: “Don’t try to break your coffee habit while fasting…unless that’s the whole point of your fast.” Weaning yourself off caffeine is a big deal all by itself. So, don’t bite off more than you can chew (pardon the expression).

(2). Dehydration & Electrolytes: When you aren’t eating food, it can be easy to skip water as well – which results in the physiological equivalent of a hang-over. So, when you fast, grab a bottle that will help you measure your water intake. And read up on the appropriate amount of water you should be drinking (for your size/weight).[5] (Roughly 127 oz/day for men; 90 oz/day for women). I’m also a huge fan of electrolyte tablets (like Nuun Sport)[6] as I need to keep my brain sharp while I fast. Most of my job is intellectual. And if my brain doesn’t have electrolytes, I’m borderline useless. However, if you fast often enough, you’ll find that the ketogenic state caused by fasting can ironically sharpen your mind by day five – which I’ll explain below.

(3). Caloric Intake makes you Tired: If you’re anything like me, I need to prioritize extra sleep and rest. It’s common for me to take naps when I do water-only fasts. Most people have experience an energy trough around 2pm (or later in the aft, if you’re a night-owl like me). Thus, if you decrease caloric intakes, it makes you downright drowsy at those moments. But you can compensate for these troughs too.

(4). Keto-acidosis Headaches are REAL! But thankfully, there are a few solutions to stop them:  When people are addicted to carbs (which is most Americans), your body will go through a predictable purge in the first three days of fasting – which is why our bodies tend to freak out in the first four days of a fast.

Here’s a quick physiology lesson on why:

Day 1: Your body basically burns glucose (sugar) – So, without more sugar, you’re extra tired

Day 2: Your body switches to protein; But…

Day 3: Your body makes another switch: Instead of protein, it changes to fat.  I.e., Your body knows it’s not smart to feed off of muscle; so, your liver starts converting fat-stores into ketones.

For example, many of you have heard of ketogenic diets (the ones that burn away the tough fat on our thighs & bellies.)When you’re doing a water-only fast, ketosis usually starts within 3-4 days, which is good because ketosis not only burns fat but can make you super sharp mentally! Ketones (which come from fat) are a great clean-burning energy source for your brain. So, after doing hundreds of fasts, I’ve learned that I feel brain-dead for the first few days. Yet, once my body breaks into ketosis, my brain suddenly becomes super sharp.

But here’s the catch: If your body isn’t accustomed to switching into a ketogenic state, it can cause “Keto-Flu” right around Day 3 – a painful withdrawal that is also called Keto acidosis. It’s the trauma of your body relearning how to process energy from a new source. For a diabetic, this can quickly turn into a lethal diabetic coma. But for most of us, the common result is often just a bad head-ache – along with aches and pains that can feel like the flu (hence, keto-flu). And it can last between 24-36 hours. Ugh! This means, somewhere in between day 3 and day 5, fasting can be quite miserable. But here are a few hacks.

Beating Keto-Flu

(1). Your first time is the worst. But each time your body does this shift, it makes the transition quicker. Over time, it’s common for people to have zero keto-acidosis symptoms. It’s a sign that your body is no longer addicted to carbs and is becoming efficient at burning fat. And although this process sucks, your body is going to need to get good at this no matter what as you get older and your metabolism slows down. So, why not start now?

(2). Try switching to a Keto-Diet the week before you do a water only fast. A water only fast is generally like biting off two big elephants in one week. Your body is switching into a ketogenic state AND eliminating calories all at once. So, it’s not a surprise that many people feel like a zombie in the first week. So, to avoid this, try eating Keto-friendly foods for a week. You might still get a mild headache around day four. But, this is way easier to endure if you are getting more calories and electrolytes. It transforms day 3 into a small hurdle instead of an insurmountable wall of pain.

In my opinion, the simplest way to eat keto is Atkins frozen dinners. Obviously, there are hundreds of keto-friendly foods at the grocery store (pizza, lasagna, stir-fry, etc). But remember: if you want to guarantee a “ketogenic state” for your body, the golden rule is this: The average person needs to stay under 20 grams of net carbs a day for three days.[7]I.e., You can eat lots of ketogenic foods and still not be guaranteed to be in “ketosis” (a fat-burning state). But if the goal is merely to prep for fasting (and weight-loss isn’t your goal), then give your body a few days to wean off of carbs.

(3). If you still get a nasty keto-acidosis head-ache during a fast, then chill out and eat something small – like a bowl of soup. As the saying goes: “You gotta walk before you can run.” There’s no sense in making this miserable. Indeed, God wants you to love this discipline. And being crabby defeats the purpose (see Isaiah 58).

(4). Don’t feel guilty if it’s harder than you expected. There’s been several times when I was around Day 3-5 of a water only fast when suddenly, I’d wake up at 3am and BINGE on food:  sandwiches, cereal… anything I could get my hands on! The next day I’d feel like a total sinner – as if I had completely failed God.

But here’s the truth: God isn’t surprised when we struggle with fasting. Indeed, what honors him most is that you’re attempting to do it. Just get back on your bike and start riding again! Because, I can promise you, it will get easier with each time you do it!

In fact, whenever I do extended fasts, I love to plan out “break days” – days where I can eat whatever I want. And after doing dozens of these over the years, here’s what I learned:

(A). Break days never satisfied me as much as I expected! I was obsessing over pizza on my upcoming break day. Yet, once I finally ate it, it gave me a momentary feeling of pleasure followed by heart-burn and a sick feeling.

(B). Ironically, most break days made me feel gross, which made it easier to fast in the days after. Once I ate the food I was lusting after, I started to realize how much my mind would overhype the binge. Ironically, many of my easiest stretches of “water-only fasting” occurred after a stupid binge – because I was freshly aware of how my mind was playing with me.

(C). I learned that cruddy food will always be there. Fasting is never a permanent state. You will inevitably eat snack cakes again. (Hopefully, less of them haha). But, chill out about it. Stop obsessing over food. Thus…

(D). Let your mind throw a fit. But know that these little tantrums will get shorter and shorter. Over time, these 30-minute pity-parties will turn into a 5 second moment of grief:

“Oh Well… so I can’t eat ice-cream with my kids!  It’s actually no big deal.” Every time I fast, there’s always that tough moment when someone breaks out my favorite dessert! It’s like the devil is taunting me!!! But, the little tantrums my brain would throw got smaller and smaller. I started to see these moments as no big deal! Even more, the joy of self-discipline would often eclipse these moments of temptation.

And when you have self-discipline from fasting, I promise you, it will start to pour into the other areas of your life! Many of you will suddenly feel like you can overcome ANYTHING by the power of the Holy Spirit – which is true!

And that’s what I want for you!

So, challenge yourself with fasting but don’t over-do it! Don’t try a 7 day water-fast right out of the gate. Go slow! Start by skipping lunch once a week. Then eat fruits/veggies for a day (a.k.a., a “Daniel Fast”). Then eat Keto for 3 days. Then skip dinner and lunch the following day (a full 24 hour day). Mix it up!

In some ways, I find that doing one type of fast for 21 days tends to get “too easy.” Sometimes, I focus on God twice as much when I mix it up. So, don’t be afraid to customize your fast.

The goal isn’t to suffer (or merely to lose weight – although that’s an admirable goal too). Rather, your primary goals should be these: (1) To practice flexing your self-denial muscles while (2). Growing closer to God. And be sure to set a “break-through goal!” I.e., Fast for someone’s healing. Ask God to give you wisdom as a result of the fast. And watch what God does.

Every year, our church has a crazy miracle story that comes out of our fast. So my hope is this: I want YOU to be the next miracle story. But you need to start somewhere!

Again, I have a lot of fasting resources [here]. But one thing is for sure: The more you practice this discipline, the more you will love the results! So don’t give up if your body freaks out. Rather, learn about your body, so you can fully devote it to the Lord.


[1] In fact, the week FOLLOWING a week-long fast is one of the greatest feelings in the world!

[2] The University of Southern California’s “Longevity Institute” proved that fasting might be one of the best therapies to combat cancer. See Dr. Valter Longo research and Prolon diet here:

[3] As a person with Rheumatoid Arthritis, my anti-inflammation medications can easily cost thousands each month. So, this was a significant discovery for me.

[4] Check out the documentary called: “Fasting” on Amazon Prime – Directed by Doug Orchard – 2017 – He interviews many of the top medical experts on the benefits and dangers of fasting. Indeed, you’ll find a treasure trove of medical citations from this.


[6] You can get electrolytes from any sports drink. However, many sources of electrolytes are also filled with sugar – which messes up ketosis (fat-burning). Thus, I prefer low-carb tablets. If weight loss or keto-acidosis head-aches aren’t an issue for you, then sports drinks and soup broth can both be a great source of electrolytes – keeping your brain sharp.

[7] The goal is to focus on “Net Carbs” – I.e., Total Carbs minus Dietary Fiber. Ie., A meal might have 40 total carbs – including fiber and glycerin. But once you subtract these, there may only be 9 grams for the entire meal. This matters because, in order to guarantee that your body stays in a ketogenic state, you need to ingest less than 20 grams a day (if you want to lose weight). If this isn’t a goal, then, you have ten times the options. But remember: Keto-friendly foods do not guarantee ketosis. There are many “keto-friendly meals” that have 30-50 net carbs which is well above the 20 grams/day threshold.

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