Is Fasting Safe? Debunking 5 Common Myths About Fasting and Human Metabolism
FASTING — MYTHS + METABOLIC SCIENCE
Is Fasting Safe? Debunking the Top 5 Most Common Myths
A science-backed breakdown of the biggest misconceptions about fasting — including “starvation mode,” muscle loss, blood sugar crashes, overeating, and nutrient deficiencies — plus what research and human physiology actually suggest.
Author
Craig McCloskey
Board-Certified Nutritionist
Fasting has a way of triggering strong reactions. For some people, it represents discipline, metabolic health, and a return to biological basics. For others, it immediately raises red flags - starvation, muscle loss, slowed metabolism, or blood sugar crashes.
What’s interesting is that most of these fears aren’t new. They’ve been recycled for decades, often without context, and frequently without a clear understanding of how human metabolism actually works.
Over the past several days, I’ve been in the middle of a multi-day fast. Not as a challenge, and not because I’m trying to “hack” my body. I’m doing it for the same reason humans have voluntarily fasted throughout history: to give my body a temporary break from constant digestion and allow repair, regulation, and metabolic flexibility to take place.
This article isn’t meant to convince everyone to do a long fast. It is meant to separate evidence from fear - and to walk through five of the most common fasting myths using both modern research and human physiology.
Fasting Isn’t a Modern Trend - It’s a Biological Necessity
One of the biggest misconceptions around fasting is that it’s a new invention born out of diet culture. In reality, constant eating is the modern experiment - not fasting.
For the vast majority of human history, food availability was unpredictable. There were periods of abundance, followed by periods where food simply wasn’t accessible. Missed hunts, failed crops, seasonal changes, and long winters all created natural cycles of eating and not eating.
Our physiology evolved within those cycles.
Only in the last century - largely following the industrial revolution - did the idea of structured meals, constant snacking, and near-continuous food access become normal. Three meals per day, plus snacks, wasn’t designed around metabolic health. It was designed around work schedules, factories, and productivity.
That context matters, because the human body is not designed to be digesting food all day long.
Eating is a catabolic process - it requires energy to break food down. Fasting, on the other hand, shifts the body toward repair. This is why sleep, illness recovery, and fasting all share overlapping metabolic pathways.
When fasting periods are extended even slightly - whether through overnight fasting, intermittent fasting, or occasional multi-day fasts - the body is given time to shift away from digestion and toward maintenance, cleanup, and metabolic regulation.
That’s the lens through which fasting needs to be understood.
“One of the biggest misconceptions around fasting is that it’s a new invention born out of diet culture. In reality, constant eating is the modern experiment — not fasting.”
Why Cultures Practiced Fasting Long Before Modern Science
What’s remarkable is that even as civilizations became more stable and food became more available, fasting didn’t disappear. It was intentionally preserved.
Across history, fasting has been embedded in medical systems, religious traditions, and philosophical practices. Ancient Greek physicians, including Hippocrates, used fasting as part of therapeutic protocols. Eastern traditions incorporated fasting into Ayurvedic and Buddhist practices. Even historical figures like Benjamin Franklin and Mark Twain commented on the restorative effects of eating less - or not eating at all - during certain periods.
None of these cultures had access to randomized controlled trials or metabolic labs. What they did have was observation over generations.
Today, modern research is finally catching up to explain why these practices persisted - and why many of the fears around fasting don’t align with how the body actually functions.
“For most of human history, eating was intermittent by default — and our metabolism evolved to handle periods of both feast and famine.”
Read the full transcript
I've been fasting the past 68 hours, which is a little less than 4 days, and I still have about a day to go. Today, we're going to be talking all about fasting, the benefits of it, should you do it. We're going to bust a lot of the common myths about fasting. We're going to dive into the science and the history of fasting. So, when you're ready, let's jump in. So, how do you know if you're somebody out there that needs to incorporate a fast into your regular routine? Well, if you are struggling with excess weight, digestive issues, if you can't miss a meal, or you can't go 3 or 4 hours without feeling hangry, you're struggling with skin issues or brain fog, that's a sign that you probably need to incorporate some sort of fast into your regular routine. Now, if you're somebody out there that's thinking about incorporating fasting into your regular routine, whether that's intermittent fasting, alternate day fasting, or a short three-day fast, or even a more long-term fast, 7 days or more, there's a ton of health benefits that come with all of these different types of fasts. And these benefits are all backed by incredible peer-reviewed science that we're about to talk about today. Like, fasting helps to lower inflammation. It improves skin and complexion. It can reverse aging and extends longevity. It can even potentially prevent Alzheimer's disease. It also accelerates the fat burning process, can stabilize your blood sugar, and improves mental clarity and concentration, regulates metabolism, and so much more. Now, before we jump into some of the common myths and benefits of fasting, I just want to touch on the brief history about it because fasting isn't a new invention or a new concept. The truth is, we didn't always have food readily available like we do today. You can go down to the grocery store. You can call up Uber Eats. Now we have food abundance. But for the vast majority of human history, we just didn't have food readily available all the time. And the truth is there was periods of feast where we had abundance of food. And then there was periods of famine where we didn't have food. We might not have had a successful hunt. We might not have had successful foraging. We might not have had a successful fishing trip. There was just a lot of periods where we just didn't always have food. And from this evolutionary standpoint, the concept of breakfast, lunch, and dinner, three meals a day, and snacking in between, this is a really new concept in our culture today. This was really born out of the industrial revolution when husbands had to go to work. The wives would stay at home and pack the lunches for the men. They would come home and have dinner ready. They needed that fuel in the morning to prepare them for a long day at work where we just didn't have that luxury. And kind of almost by evolutionary accident have we come to develop health benefits when we are in these periods of famine. Because remember this, eating is a very catabolic state. When we are eating food, we're kind of breaking things down. That's what catabolic means. We're breaking things down. But when we are in a fasted state, like you are sleeping, that is a repair process. Humans aren't designed to be digesting food 24/7. We need to ingest highquality nutrient-dense foods to allow our bodies to absorb that. And then we need to have a period of quote unquote famine, a fasted state in order to repair. Because fasting in and of itself is an anabolic process that just means to build back up. And the cool thing is when you can extend that fasting period just a little bit. I'm not talking crazy amounts, just a little bit, maybe just a few hours, you can reap some pretty incredible health benefits that we're about to talk about today. Now, as we evolved out of more huntergatherer times and into more civilized cultures where we had food readily available, cultures still wanted to practice fasting because they understood all the health benefits that came along with it. Every culture that you look at throughout time had some form of regular fasting protocol included into their regular dietary patterns. Even though food was readily available to them, they still voluntarily chose to engage into fasting protocols. But why would they do that? Why would they choose not to eat? Because they understood that it came with a ton of health benefits. Now, reiterating this point that fasting isn't a new concept because in our world today, we have food readily available. We have tens of thousands of new foods in the grocery store that have just been invented within the past few generations, the past hundred years. We have social media where everything is out there in the open. It might seem like restricting food is a form of an eating disorder in our current world today. But throughout time, I mean, cultures have always practiced fasting. And really, I've always said that you can't create a disordered food environment and then when you try and go back to eat how we've always eaten, say that that's the problem. Now, staying with the point that fasting isn't a new concept or invention, I want to run through a list of all the cultures and health practitioners throughout time that regularly practice fasting. Now, the first one is Hypocrates, the father of modern medicine. He was one of the early adopters of a fasting protocol for many individuals. He would often advise his patients for the treatment of obesity, a high-fat diet, and only eat once per day. Now, the Greek Orthodox religion also practice fasting. So much so that nearly 200 days out of the year, they would fast, which would mean they would eat less days than they would actually be fasting for. Then we have the Buddhist monks that would commonly fast all day afternoon and then not eat again until breakfast the next morning. Even in Ayurvedic medicine, which is very popular, they believed that fasting, engaging in a regular fasting protocol, could relieve the body of many different types of toxins. The Greek gods, the ancient Greeks practiced fasting and believed that they were better problem solvers and would be better solving puzzles because their brains worked better when they engaged in fasting. Even our own Benjamin Franklin, shout out to the $100 bill, Benjamin Franklin once wrote, quote, "The best of all medicines is resting and fasting." And our boy Mark Twain, he even said, quote, "A little starvation can really do more for the average sick man than can the best medicines and the best doctors." This is really powerful because all these people come from many different walks of life, different time periods, different countries, different backgrounds, but yet they all understood that fasting could come with incredible health benefits. And the thing that is amazing to me is that they didn't have the peer-reviewed science. They didn't have randomized control trials. They didn't have the modern-day science like we have today affirming these things. They went based off of firsthand experience, intuition, and time-tested practices. What I love to do is I love to blend modern-day science, highquality randomized control trials along with what humans have been doing the longest. That's how we can find out what's really close to being the truth. And the truth is, today fasting has incredible health benefits. There's some myths that we're about to jump into, but there's also a ton of health benefits. And if you're watching this right now, odds are you can engage in some form of fasting protocol. Whether that is a intermittent fast, a short day fast, alternate day fasting, or a three or four day fast like I'm currently doing, or even an extended 7 plus day fast up to maybe a couple weeks. Now, while in today's world, we have a profoundly sick society. People are struggling and they need answers. And fasting is often used as a tool to help people recover and get better that are struggling with illness. But I should note that throughout time, fasting wasn't necessarily used as a tool to help people that were struggling with illness. It was used as a tool for wellness. It was used to keep people from getting sick. Because I'm not struggling. I'm not sick right now, but I'm doing a fasting protocol because there's science that shows that it helps your immune system to come back even stronger and that it helps with a host of things. So, I'm doing this as a preventative measure. And if you're somebody out there that is not struggling with your health, but you're just looking to optimize your long-term health, then fasting is still something that I believe that you should regularly engage in. It's just finding the right fasting protocol for you. Now, think about this logically. When you are feeling sick, when you have the cold or a flu, what is pretty much the last thing that you want to do? The last thing that I want to do is eat a lot of food. Why is that? Because naturally in the wild, every animal doesn't want to eat a lot when they are feeling sick, when they're feeling run down. Because like I just mentioned when we are in a fasted state our body needs to cleanse and repair and build back up. If we are constantly ingesting food and other substances at that time then our body is not able to cycle through some of these repair processes that I'm about to talk about autophagy and apoptosis. That's why in today's world when we are sick we are told to just drink liquids or broths or soup like the chicken noodle soup moms or the Sprite or the ginger ale. We're told to just drink a lot of liquids. you know, eat light because this concept of fasting has just been passed down to us from generation to generation throughout time. Now, what's cool is that we have science today, peer-reviewed randomized control trials today showing why this is effective for helping us to get better. In a 2014 study, researchers found that by doing a 3-day fast, consuming less than 200 calories a day, this can actually reset some components of your immune system. Doing this three-day fast, it actually lowered people's white blood cell counts, but only to come back even stronger. This is a form of hormesis. Basically, what doesn't kill you makes you stronger. This is environmental hormmesis. So, it stresses you out just a little bit, but you're going to come back even stronger. That's why we fast when we're sick. And that's why I'm fasting now. So, that way I can basically get stronger, come back even better. What's up, guys? Real quick, if you're loving this content about fasting, then you are going to love these two supplements right here that can help take your fasting protocol to the next level. The first thing that I have is ancestral minerals. This is basically electrolytes that can help to replenish your body. You know, you're drinking a lot of liquids. You might be flushing a lot of things out. This is going to help keep you hydrated because you need electrolytes like magnesium, potassium, chloride, sodium, wildflower, bee pollen, and Atlantic kelp. This is going to help you to actually hydrate your cells because it's not actually the water that's hydrating, it's the electrolytes. And that's what ancestral minerals are. All in the most bioavailable forms from the most pristine sources. So, check that out. I'm also taking this grass-fed colostrum in powdered form from Ancestral Supplements because colostrum is incredible at helping the gut. And right now, I'm in a fasting protocol. I have everything cleared out of my digestive tract. I'm just doing juices, bone broth, water, electrolytes. That's it. And so, when I can ingest a little bit of colostrum, it's kind of like a nice G-code. There's very minimal, if any, calories in these little packets. and your gut is getting flooded with some nutrients in colostrum that are unique to colostrum alone that can help to repair the gut lining, help leaky gut, help your immune system, and so many more things. So, if you're looking to take your fasting protocol to the next level, give ancestral minerals and grass-fed colostrum a try today. Now, I want to talk about five common myths of fasting that a lot of people have. And I want to start off with number one. Fasting puts you in starvation mode. This is not true. Now, there's a difference between voluntary fasting, like I talked about, and then true starvation. When we're talking about this starvation mode, this is talking about your metabolism. It might cause your metabolism to just shut off, but that doesn't actually happen. In fact, there's science that shows that your BMR, your basil metabolic rate, actually doesn't change all that much. There was one study where participants fasted every other day. So, this is alternate day fasting for 22 days. This is a little over 3 weeks. And there was no difference in their basil metabolic rate, which is the amount of energy that our body burns to function optimally. Now on average in this study their fat burning increased by 58% and their carbohydrate oxidation which is their sugar burning capacity this decreased by 53%. This means they were becoming more metabolically flexible and learning how to use both sources of fuel properly because that's what we're ultimately trying to do. You want to be able to miss a meal and not have to rely on sugar as fuel. Because if you can miss lunch or miss dinner and feel good, that's what we're ultimately trying to get at is learning how to use our own body fat as fuel, learn how to use the fat that we're eating for fuel, and how to use sugar for fuel. And engaging in regular periods of fasting that's going to stress us out a little bit, that's going to help us to be able to use carbs for fuel and fat for fuel. Now, the second common myth about fasting is that fasting causes you to burn muscle. This is also not true. Did you ever wonder like why we have body fat to begin with? Why did nature, God, whatever you prescribe to, why did we get designed to have body fat in times of food abundance? It's because of this reason. So that way we have excess energy to burn when there's food scarcity. The problem is in our environment today, food scarcity never comes. Winter's never coming for us because there's always food available. So we need to give our bodies periods of famine because we have so much feast going on. Just a little bit. I'm not talking about doing a crazy fast, just a couple hours or a couple days here and there. Our bodies are so intelligent that it actually when it burns through our stored glycogen because we have stored glucose in our liver and we have stored glucose in our muscles. This is sugar. And when we burn through that, we start burning our own body fat. Takes about a day, 24 hours to burn through all that stored glycogen. But as soon as we are able to burn through that, then and only then can we burn our stored body fat as fuel. The problem is most people are eating every few hours and probably not the best foods and then they're never able to access their stored body fat. So, if you can engage in maybe just a 16-hour intermittent fast, finish eating at 6:00 at night and then not again until 10:00 the next morning, that is an incredible way and a really cheat code to boost your body's fat burning ability. Burning muscle can really be seen as a last resort. So, unless you're somebody out there with severely low body weight and severely low body fat, you don't have to worry about losing muscle. Now, you might lose the gains a little bit because a lot of that water retention is coming out. You're probably peeing a lot of water retention out, but you're not losing muscle mass. you're just kind of losing that water weight that's in there. And when we're talking about losing muscle mass during fasting, it's actually kind of the opposite because fasting, engaging in a regular fasting protocol, is one of the main things that helps to boost our growth hormone levels. There was a recent study in 47 people that found that HGH, human growth horn levels, increased fivefold during a 24-hour fast. Think about that. You fast, you're not eating, but you were recovering. You're going anabolic. You're building yourself back up. You were boosting your growth hormone all naturally just by something that's free. You don't have to pay for it. You just don't eat. There was another study where participants did alternate day fasting for 70 days over 3 months. And what the researchers found is that their weight reduced by 6% but fat mass decreased by 11% and muscle mass didn't change at all. So when they did alternate day fasting every other day for 70 days, they just lost body fat and not muscle mass. So you don't have to worry about losing muscle if you want to try a fasting protocol. Now a third common myth about fasting is that fasting is going to cause low blood sugar. This isn't true for the overwhelming vast majority of people because our blood sugar needs to be tightly regulated at all times. And kind of just like I was talking about, we have stored glycogen for a reason. So, if we're not bringing in food, your body has all these built-in reserves to access to keep your blood sugar steady at all time. And so, once your body goes through that stored glycogen, your stored liver glycogen, your stored muscle glycogen, then your body's going to turn to a process called gluconneogenesis. Gluco meaning sugar. Neoenesis is creation. So, you're literally creating glucose. And where do you think this comes from? This is coming from the backbone of body fat. So, this is what's happening. You're burning that stored glycogen and you're burning your own body fat which is actually in a way getting turned into sugar which is then able to keep your blood sugar steady. Now if you're somebody out there that can't miss a meal, you feel hungry if you miss a meal. Hangry, Betty White, hangry. If you're that Snickers commercial, if you're feeling rough because you can't miss that meal, this is a sign that you are not metabolically healthy or metabolically flexible. You should be able to miss a meal and not want to tear your neighbor's head off because you didn't get your food. So, if you're somebody out there that is struggling like this and you start a fasting protocol and you miss a meal and you feel blurry or hangry, it might take a little time for your body to kind of regulate itself. So, don't give up just yet. You know, keep going with it. I was like that when I first started fasting, but as soon as your body starts making ketones and you learn how to adequately use your own body fat as fuel, then you'll feel clear-headed. You'll have energy. You'll be able to miss a meal and you'll feel really good. Now, the fourth common myth about fasting is that it causes you to overeat. Now, listen. I've done many fasts in my life. I've been doing it for over a decade now and I routinely do intermittent fasting and never once have I overeaten the next meal because I just under I didn't eat. After this fast is over for me, I'm doing a 3 to 4 day fast. I know me, I've done this before. I'm not going to eat a ton of food. I'm probably only going to eat maybe half the amount that I normally would naturally just because my body's hormones are kind of getting readjusted. It's just naturally what happens. You don't make up for all the calories that you didn't eat, if that makes sense. Now, there is a caveat with this. If you go back, break your fast, and you start eating ultrarocessed foods like Oreos, pizza, chips, cookies, ice cream, you're going out and eating all these junk food, all these ultrarocessed foods that are designed to be overeaten, then yes, you're probably going to overeat because companies and scientists out there are literally in a lab creating these foods with a perfect combination of carbs, fats, salt, and sugar that cause you to overeat these foods. I mean, it's so hard to eat a baked potato, but if you add butter to that baked potato is probably a lot easier to eat. Combining the fat and the carbs together. But if you're just eating a high protein meal, nutrient-dense foods that have a lot of vitamins and minerals in it, whole food-based diet, you're not going to overeat. So, that's just a common myth. Now, the last common myth about fasting is that fasting is going to cause your body to become nutrient deficient. If you're doing just alternate day fasting and then you're replenishing your body with a nutrient-dense meal or you're doing a three or four day fast like me and then you start eating nutrient-dense foods again, you don't have anything to worry about. This is not going to deprive your body of micronutrients. Maybe in the short term, like I mentioned, you need this kind of repair process to have that anabolic processes build you back up and get stronger. So, you don't have to worry about becoming chronically nutrient deficient. There's more people out there that are eating a ton of ultrarocessed foods that are nutrient deficient than people that are eating a whole food-based diet and engaging in regular fasting protocols because ultrarocessed foods contain very little to no vitamins or minerals. So, if you're just alternate day fasting or intermittent fasting or doing fasts like I'm doing a 3 to 4 day fast once or twice out of the year, you have nothing to worry about as long as you're eating whole foods regularly throughout the rest of the year. All right, guys. I hope you enjoyed this content today where we talked all about the history of fasting, all the different myths that we debunked today. If you enjoy this content, then you are absolutely going to love part two where we talk all about the benefits of different fasts. If you want, check out that video, just click the link right here and I'll see you there.
Myth #1: “Fasting Puts You Into Starvation Mode”
This is easily the most common fear, and it’s also one of the most misunderstood concepts in nutrition.
True starvation is a chronic state of inadequate nutrition over long periods of time, typically in the context of severe calorie deprivation without body fat reserves. Voluntary fasting is not starvation - especially in individuals who are adequately nourished and metabolically functional.
What people often interpret as “starvation mode” is actually the discomfort of metabolic inflexibility - a body that hasn’t learned how to switch between fuels efficiently.
Which leads directly to the next myth.
“Short-term fasting is not the same as starvation. In well-nourished individuals, metabolism remains stable while fat burning increases.”
Myth #2: “Fasting Burns Muscle”
If fasting burned muscle by default, humans would not have survived seasonal food scarcity.
The human body stores energy for a reason. Glycogen (stored carbohydrate in liver and muscle tissue) is used first. Once that’s depleted - usually within about 24 hours - the body shifts toward fat oxidation. Muscle tissue is not the primary energy source unless body fat levels are extremely low and fasting is prolonged beyond adaptive limits.
It’s also worth noting that rapid weight loss during fasting is often misinterpreted as muscle loss when it’s actually water loss from glycogen depletion. Glycogen binds water, and when it’s used, that water is released.
Loss of muscle requires prolonged protein deficiency and insufficient energy reserves - not short-term fasting.
“If fasting automatically burned muscle, humans would not have survived seasonal food scarcity.”
Myth #3: “Fasting Causes Dangerous Blood Sugar Drops”
For most healthy individuals, blood glucose is one of the most tightly regulated systems in the body.
When food intake stops, blood sugar doesn’t simply crash. Stored glycogen is released from the liver. Once glycogen is depleted, the body transitions into gluconeogenesis - the creation of glucose from non-carbohydrate sources, including glycerol from fat tissue.
This is a built-in survival mechanism.
If someone experiences shakiness, dizziness, or extreme hunger when missing a meal, it’s usually a sign of poor metabolic flexibility - not that fasting itself is dangerous. The body hasn’t learned to efficiently switch from glucose dependence to fat utilization.
That adaptation takes time.
As the body becomes better at producing ketones and utilizing fat for fuel, mental clarity and energy often improve rather than decline. This is why many people report clearer thinking during fasting once they’re adapted.
Myth #4: “Fasting Leads to Binge Eating”
This fear assumes that the body responds to fasting with uncontrollable hunger - but that’s not what most research or real-world experience shows.
Where overeating does occur is when fasting is broken with ultra-processed, hyper-palatable foods designed to override satiety signals.
Breaking a fast with nutrient-dense foods - protein, minerals, whole fats - tends to produce the opposite effect: early satiety and reduced appetite.
This distinction matters. Fasting doesn’t cause overeating - food quality does.
“Fasting doesn’t drive overeating — ultra-processed foods designed to override satiety signals do.”
Myth #5: “Fasting Causes Nutrient Deficiencies”
Short-term fasting does not deplete nutrient stores in a well-nourished individual.
Micronutrient deficiencies develop over time, usually from diets high in ultra-processed foods and low in nutrient density - not from occasional fasting.
That said, hydration and mineral balance do matter during fasting. This is where electrolytes become important, especially sodium, potassium, and magnesium. Supporting hydration with mineral-rich electrolytes can help maintain energy, blood pressure, and overall comfort during fasting.
Many people also choose to include bone broth or small amounts of colostrum during longer fasts. These provide minerals, amino acids, and gut-supportive compounds without significantly disrupting the metabolic benefits of fasting — something I personally find helpful and sustainable.
When I’m fasting, two things I like having on hand are Ancestral Supplements Minerals and Grass-Fed Colostrum.
The minerals are an easy way to stay on top of electrolytes (especially sodium, magnesium, and potassium) so I feel steady, hydrated, and clear-headed - not flat and depleted.
And colostrum is one of my go-to “gentle supports” because it’s gut-first: it helps support the gut lining and immune function, which matters even more when you’re giving digestion a break. If you want to try them, you can use code CRAIG for 15% off.
The Role of Fasting in Immune Function and Repair
One of the most compelling areas of fasting research involves immune system regeneration.
It also helps explain why appetite naturally decreases during illness. The body prioritizes repair over digestion.
Fasting activates cellular cleanup processes like autophagy, where damaged proteins and dysfunctional components are broken down and recycled. This isn’t speculative - it’s a fundamental biological process that earned a Nobel Prize for its discovery.
“Fasting isn’t about deprivation — it’s about restoring the rhythm between nourishment and repair.”
Fasting as a Tool - Not a Rule
Fasting isn’t mandatory. It isn’t superior. And it isn’t appropriate for everyone at every stage of life.
But when applied thoughtfully, it can be a powerful tool for restoring metabolic flexibility, supporting immune resilience, and giving the body space to repair.
The goal isn’t deprivation. The goal is rhythm.
Periods of nourishment followed by periods of rest. Feast and famine — just as human physiology evolved to expect.
When fasting is framed through that lens, many of the fears surrounding it begin to dissolve.
