How to Hack a 7 Day Fast in 24 Hours

What do you think happens to a healthy human body when you fast for an entire week? Not 18 hours, not 24 hours, a full 7 days. Now, that might sound intimidating, maybe even a little crazy, maybe even for me, but the fact is our species, Homo sapiens, was shaped by fasting. Just as we evolved with sunlight, nature, movement, whole foods, we evolved with fasting. And fasting isn't simple caloric restriction, it activates biological programs that many of us rarely, if ever, tap into in our modern lives. And in today's video, we're gonna review a study where researchers took healthy volunteers and fasted them for an entire week, water only. And what they discovered, especially around day 3, changed completely how I think about fasting. In fact, these data might explain why so many people underestimate the benefits of fasting And they might imply that most people who practice fasting stop right before the most impactful biological changes begin. So today we're gonna dive into the data and I'll even show you how to hack a 7-day fast in just 24 hours. So let's do like a hand into a bag of Doritos and get into it. We're changing the body's operating system, the software update, in meaningful ways. No food, No caloric beverages, not even coffee. Something clicked. A massive second wave of protein changes, higher resolution view of human biology, but I'm guessing you've never heard of it. It makes you feel like a zen monk. All right, a real quick announcement before we begin. I just wanna say thank you to the Stay Curious Metabolism Community, which is truly the lifeblood of this channel. Thanks to your support, we're doing a complete level up. I've invested in a dedicated studio space, brought in a professional design team, and we're building out podcast areas, permanent lighting setups, and just a much more capable production environment. So no more dorm bedrooms, childhood bedrooms. We're leveling this up, and none of this will be possible without the Stay Curious Metabolism community, which has grown to be a global leader and top best-selling science publication on Substack globally. I guess that's redundant, but I'm just excited. Thank you, and I wanted to share my enthusiasm for what's coming next because it's going to be big. But now on to today's intellectual quest. In this video, we're gonna start with a quick overview of the study design and the fasting timeline. What changed on days 1 to 2, then days 3 to 5, then days 5 to 7, and then during a refeeding period after the fast. And then we're gonna get into the study's most interesting finding, the day 3 threshold. Something happened around day 3 that the researchers and I weren't expecting, and it wasn't weight loss, it wasn't ketosis, we will come back to that. And then finally, in the last part of this video, we're gonna get to the real payoff. How to hack a 7-day fast in 24 hours. But first, make me a promise, pinky swear, don't skip ahead. Without understanding the data and the underlying physiology, the practical takeaways won't make nearly as much sense. But with that, part 1: the study design and fasting timeline. The researchers recruited 12 healthy volunteers and put them through a 7-day water-only fast. No food, no caloric beverages, not even coffee. No! Just water for an entire week. Now, these were healthy individuals. Average starting BMI was around 25.4 kilograms per meter squared. And to the credit of the 12 participants that were recruited, every single one completed the study, which is kind of impressive. Even without coffee, we have to give them props for that. Anyway, during the 7-day fasting period and then the refeeding period, the researchers drew blood samples. And they deployed high-tech metabolism measurements, a system capable of measuring 3,000 proteins simultaneously from the participant's blood. Think about it this way. It's kind of like fishing. Often in science, we cast out a single line looking for a specific protein, molecule, or maybe a hormone like insulin. Then we reel it back in. Oh look, we caught an insulin. Anyway, the technology these researchers use is more like dragging a massive fishing net behind a trawling boat. Instead of catching 1 to 2 targets— oh look, there's insulin, oh look, there's ghrelin— you capture a massive amount of biological signal that can then be studied. And as a result, the picture that emerged from this study was far more complex and far more fascinating than anything that had been published before. So let's chat about the timeline in 4 main buckets, then we're going to go into more nuance on the day 3 threshold. But first, days 1 and 2. This was the period of fuel switching. Basically, during the first couple days, your body burns through its stored carbohydrates, particularly your liver glycogen, and that drives a switch in metabolism from carb burning to fat burning. Now this requires an increase in the machinery responsible for fat metabolism, so proteins involved in fat transport, fatty acid metabolism, and ketone production to turn fats into ketones to feed your brain all surged in the first 1 to 2 days, first 48 hours. There's no surprise there. That's fasting 101. And critically, this 1 to 2 day window is where most people stop a fast. And you might rethink that after what we learn in the rest of this video, because at day 3, in days 3 to 5, we're at tipping point. After the initial fuel switch, researchers observed a massive second wave of protein changes that extended far beyond energy metabolism and dwarfed anything that was seen in the first 48 hours. So basically, if the first 2 days were about changing what your body burns, going from carbs to fat burning— smoked salmon and manchego cheese, it's so good— shifting your body from being basically a computer plugged in to one on a battery, day 3 was where the software update actually hit. You're no longer just switching fuel sources. The body appears to be undergoing a much broader systems-level remodeling. Proteins involved in immune function, inflammation, brain biology, blood clotting, cellular stress, cardiovascular disease, longevity, all began to shift. Of the roughly 3,000 proteins the researchers measured, 1,034 changed significantly over the course of the 7-day fast. And what's particularly striking to emphasize this point is most didn't begin to change until this day 3 threshold. And we're gonna get into it more in the next section, but let's first complete our timeline. Days 5 to 7 in the refeeding period, your body's hitting a new metabolic steady state. By the end of the week, many of the protein changes were approaching a new equilibrium. The body wasn't panicking. It had basically reconfigured and settled into a new metabolic state that, at the protein expression level, looked fundamentally different than the original baseline metabolic state. And notably, many of these late-stage changes were mapped onto proteins and pathways that had been independently linked to longevity, brain protection, neuroprotection, and reduced risk of cardiovascular disease and autoimmune disease. I'll get a little bit more specific later, but seriously, think about that. Over one-third of the 3,000 proteins measured changed significantly, and understanding these changes in more depth, both in this paper and in future research, will give us insight into how fasting physiology actually works and why it might matter for specific diseases. Again, we're gonna get into a couple of examples later specifically for the heart and brain, but let's complete this timeline, the refeeding period. During days 8 to 10, the researchers monitored participants for 3 days after the fast to see, as they gradually returned to normal eating, what happened, what changed, what didn't. Now, one of the biggest changes was a rebound in lean mass, like muscle mass. I know one question you're probably asking is, how much weight did people lose on the fast and how much of it was fat mass versus lean tissue, lean mass? Well, over the 7-day fast, participants lost 5.7 kilograms on average, which is 12.5 pounds. That's the equivalent of roughly 2 BMI points in this study, so their BMIs dropped around 25 to 23. That is a lot for one week. Now, looking at the raw numbers, they initially sound concerning because participants lost 3.6 kilograms of lean mass during the fasting period. However, there's a really important nuance here. The technologies we use to measure lean mass don't just capture muscle, They also capture things like organ mass, so if your liver shrinks, that's gonna be counted as loss of lean mass, water, glycogen, and other non-contractile components stored within muscle tissue. Basically, if lean mass is going down, it doesn't mean muscle mass is actually going down as much. Now, what makes this really interesting is what happened in the refeeding period. Because once participants started eating again, virtually all the lean mass returned within 3 days, while the fat mass remained reduced. So, by the end of the fasting and refeeding period combined, those 10 days, participants had lost 1.85 kilograms, or about 4 pounds of fat, but had retained basically all the muscle. So, in simple terms, the fat loss persisted where the apparent lean mass loss rebounded rapidly during the refeeding period, which I find very reassuring. But beyond that, one of the most fascinating findings wasn't actually what changed during the fast, but what refused to change after the refeed. Not all the proteins that shifted during the fast, particularly after the 3-day threshold, returned to baseline after the refeed. Some of those changes lingered, as if the body had downloaded a new software update during the fast that remained installed after food was reintroduced. And we will return to this software update analogy later, because I think it captures a really important concept. But for now, just file it in the back of your mind, and if you can't remember that until later, Your brain might need a software update. But now, let's do like an ATP synthase rotor spinning furiously in the inner mitochondrial membrane and excitedly turn to part 2: the day 3 threshold. As we reviewed, around day 3, something clicked. The body appeared to shift from simply changing fuel sources, carbs to fat, to updating itself in a much more profound way. And I really want to emphasize why this matters. In fact, it is the single most important takeaway from this entire paper because it helps resolve a lot of the debate around whether fasting actually has benefits beyond just reduced caloric intake. And to be clear, my opinion is an unequivocal yes it does. But the debate around fasting's benefit persists for two main reasons. First, most studies examine only a handful of endpoints and biomarkers, often focusing on things like weight or maybe a few metabolic measurements and sometimes under calorie-controlled conditions. And second, most fasting interventions last less than 48 hours, so you don't get to this day 3 threshold. If there's a shift in the body's proteome, the tens of thousands, maybe hundreds of thousands of proteins that ultimately determine how our bodies function, if it doesn't emerge until day 3, then if you cut your fast off at 24 or 48 hours, you're just not gonna see that signal. The studies we have done aren't designed to capture it. So what we're seeing here in this study is something fundamentally different, a much broader, higher-resolution view of human biology, looking at thousands of proteins that might change over a 7-day prolonged fast. And we're not just changing the body's fuel source, resources here were changing the body's operating system, the software update, in meaningful ways. So, what changed? Okay, hold on to your heat shock proteins because this is about to get dense. Let me highlight a few of the most interesting, focusing on two organ systems: the heart and the brain. One of the most interesting findings involved cardiovascular health. Researchers observed a change in a protein called HYOU1, which has been linked to heart disease in large human genetic studies. In fact, people with higher level of this protein, HYOU1, HYOU1, appeared to have substantially greater risk of cardiovascular disease. And during the prolonged fast, levels of this HYOU1— that's actually really hard to say— fell significantly, a change that points in the direction of better heart health. In fact, each single standard deviation decrease in HYOU1— okay, it's getting easier— is associated with a roughly twofold decreased risk of cardiovascular disease if you extrapolate from the human genetic studies. Now, this protein HYOU1 is interesting, but I'm guessing you've never heard of it. And honestly, that is the point. The point here is that we're seeing signals emerge that most of us don't know about, don't think about, and it's part of the universe of changes that go on behind the scenes. Just like when you get an iPhone software update, you don't know what the heck is going on in the background, you just benefit from the ultimate result. Except sometimes iPhone updates suck. Or you get that U2 album update that nobody wanted but it somehow ended up on your phone. Sorry, that wasn't supposed to be part of the fasting analogy. The point is, what we're seeing here is not just the change in the usual suspects, like cholesterol or blood sugar or blood pressure, but an entirely different, much broader set of biological changes that only become visible when we take a closer look. We're normally looking at the tip of the iceberg, now we're seeing the base. We're seeing here evidence of broad biological remodeling that touches nearly every organ system in the body. And if the heart doesn't excite you, let's talk about the brain. One of the most intriguing findings, even more so than HYOU1, was that fasting changed a protein signature in which for proteins involved in— let me try to do this without too much jargon— the structural scaffolding that helps hold tissue together. Think of it as the biological equivalent of the rebar inside concrete. And one protein in particular that caught my attention was— okay, little jargon here— TENASIN-R. "What does that mean?" Again, you've probably never heard of it. That's fine. In fact, that's good. That's the point. But TENASIN-R is noteworthy because it's largely specific to brain support structures called perineuronal nets. Sounds jargony, but really it just describes what it is. Peri means around, neuronal refers to neuron, no surprise there, and nets because they actually look like support nets around neural connections. These perineuronal nets surround neurons and contribute to the stability of these neural connections, which helps regulate the brain's ability to rewire itself, cement memories, and learn. And traditionally, it has been viewed that these structures are relatively stable, like glaciers. However, there's growing evidence that they may undergo active remodeling and recycling. That's really cool! It's like if we saw a glacier get up and dance. But why does this matter? Well, because when people talk about the neurological benefits of fasting or ketogenic diets for things like epilepsy, Alzheimer's, Parkinson's, we usually talk about fuel states—just changing from burning sugar to burning ketones in the brain. But these findings hint at something deeper. There are structural changes occurring within the brain itself. During the fast, researchers observed a progressive change in tenascin R levels that they interpreted as reflecting preservations and security of these perineuronal net structures. So quoting from the authors directly, "Our observations provide a potential new path to help understand the success of this ancient treatment regime." "fasting" on neurological disorders. That is a remarkable statement if you truly understand it. It suggests that some of the benefits that arrive from fasting for the brain are not just about changing brain's fuel supply, but altering the very structures that support our neuronal function. And again, that's just the tip of the iceberg. The researchers also observed changes in proteins linked to rheumatoid arthritis, inflammation, obesity-related diseases, aging, and a host of other biological processes. But now let's try to respect your time and dispense with the alphabet soup of proteins like tenasin-R, HYOE1, on Swap 70 and the rest. The real takeaway is that the body appears to be undergoing a profound biological rewiring, a software update during a prolonged fast. And what's especially interesting again is most of these changes start to kick in around day 3 and don't immediately disappear after the refeed. Even after a 3-day refeed, a substantial number of these protein shifts persisted. That's why I really like the software update analogy. When your computer or iPhone installs an update, It doesn't revert back the moment you restart it. The new program remains in place. Now, of course, we don't know how long these fasting-induced changes persist. The study only followed participants for 3 days after the fasting period during the refeed, not for weeks or months. But these findings do raise a fascinating and provocative possibility. If these beneficial adaptations linger, then periodic prolonged fasts might produce cumulative benefits over time. But that brings us to our climax and the part I know you've been waiting for: how to hack a 7-day fast in 24 hours. Well, the answer to that provocative question starts with another question: what is mediating the effects of fasting? Also, why do certain changes only click in at day 3? And what are the signals the body is sending to download that software update? It's not magic. The body needs to have somehow signaling molecules say, "Oh, we've hit this point in the fast, now we need to change." One of the signals are ketone bodies themselves. Ketones are made from stored body fat, primarily by the liver, and rise throughout a fast. But ketones are not just energy molecules for the brain. They're hormones, literal hormones that circulate throughout your blood and change how your organs work. They trigger signaling cascades in cells. They attach to proteins and change how those proteins work, and they change the expressions of literally thousands of genes to change the proteome in ways like we saw in this study. So, if we consider ketones themselves at least one signaling molecule responsible for the protein changes we're seeing in this study, the software update we talked about, then it's pretty reasonable to suggest you're going to get some of the benefits of a prolonged fast by getting ketone levels beyond a certain threshold. Does that make sense? And just to be clear, this is wholly consistent with clinical data and what we see in this study. So for example, fasting can help epilepsy. Ketogenic diets are meant to mimic fasting, which also help epilepsy. Or in this study, there were associations between ketone levels and certain variables. Like we talked about tenascin R, remember those perineural nets, those structures supporting neurons? Well, the higher ketone levels rose, the lower tenascin R levels fell, consistent with the stabilization of these critical brain structures in neurons. But now how do you accelerate the process? How do you do this much faster than a week? Well, now we have to establish our target thresholds. In this study, ketone levels passed 2.5 millimoles around day 3. So, we consider that one threshold for the tipping point: ketones of 2.5. But if we're being really ambitious, we'd think about day 7, where ketone levels reach 4 millimoles, which is quite high. So, how do you do that in just 24 hours? Edit Nick here in my kitchen to answer a common question that I didn't answer in the video about coffee and fasting. Yes, you can have coffee on a fast, particularly black coffee, no cream and sugar. I freaking love my coffee. Uh, you can see I'm pretty stacked with cold brew. That's not a product placement, I just like that cold brew. Also my peptides and apple cider vinegar and seltzer. So yeah, there's a biohacker influencer's fridge for you if you've ever seen one. But I also bring this up because I'm going to be doing a masterclass video on caffeine, different sources of caffeine, why they hit differently, energy drinks, things like Red Bull, Celsius, coffee, yerba mate, etc. So drop your questions below, and importantly, please subscribe and turn notifications on so you don't miss that. It's going to be very high yield and exciting. But now back to the main video. Where ketone levels reach 4 millimoles, which is quite high. So how do you do that in just 24 hours? Well, here's how. You start with a backdrop of a very low carbohydrate diet to get your body making some of its own ketones. As a general goal, getting your net carbohydrates— so total carbs minus fiber— below 25 grams per day is usually low enough even without protein restriction. And as an added tip and fun fact, if you focus a little bit more on unsaturated fat versus saturated fat, you're going to get those ketones even higher because unsaturated fats tend to be a little bit more ketogenic. Think about things like walnuts, pecans, olive oil, sesame oil. Now, I'm not suggesting you go hard on conventional seed oils— that's a topic for another time— but having more unsaturated fats is going to get your ketone bump a little bit higher to establish your baseline. Then, using that moderate ketogenic diet as a baseline, the idea is to do shorter fasts and add in exogenous ketones. That's exactly what it sounds like— ketones you consume to basically top off your ketone levels. So if your body making ketones, then you add some on top, and you can get above these thresholds pretty quick. Let me show you how it's done with a little metabolic demonstration and experiment I ran last weekend with an exogenous ketone product called Kinetic, which contains the identical beta-hydroxybutyrate your body makes. Basically, I was eating a pretty liberal ketogenic diet, about 75% of my calories came from fat with my main fat sources being olive oil and admittedly cheese just because I freaking love cheese, this is how I like to eat. Mmm, yeah. Then, I did a 24-hour fast and around hours 19 and 23, I added on an an exogenous ketone bump. These each contain 10 grams of ketone, and my ketone levels progressively rose to 4.4 millimoles, surpassing the ketone levels achieved in the study over the course of a week in just 24 hours. It's actually not hard to do at all. Now, some of you might be wondering why I'm using this product. Well, first, from a formulation standpoint, it contains the bioidentical D-beta-hydroxybutyrate ketone that your body makes, that your liver makes. Which ultimately ends up resulting in higher measured ketone levels than, say, something like a ketone salt or something like a 1,3-butanediol. And it's a heck of a lot tastier and less expensive than a ketone ester. I'm not going to go into a masterclass on exogenous ketones, but something like this is pretty effective. Also, from a consumer standpoint, I just like the versatility of the product. They have these canned sparkling beverages— strawberry peach is my favorite— and these little portable 2-ounce shots, which I take while traveling. Actually, as a little transparency note, So I did this little fast experiment when I was at, uh, one of my best friend's weddings. It was a lovely weekend. Shout out to Irina. And honestly though, traveling makes me a little bit anxious. So I like to pop these in to help me with a prolonged fast, even if it's 24 or 48 hours. They're really convenient and they taste pretty good. Tangerine. And importantly, they don't have any artificial sweeteners. They use allulose, and that's important to me. So if you want to try it, I did acquire a discount code for you, STAYCURIOUS, which will get you 15% off. You can grab the link below. I want to interject with another thought or observation drawn from personal experience, which is that different people appear to have different thresholds for subjective experiences associated with different ketone levels. What do I mean by this? Well, drawing from my personal experience, when I started playing around with a ketogenic diet, exogenous ketones, 7 years ago when I was doing my PhD, and this time for me it was for ulcerative colitis, I could tell what my ketones were based on how my stomach felt. Those are my symptoms of interest. And I knew, oh, based on how I'm feeling right now, my internal experience, my ketones are between 2.5 and 3 or between 1 and 1.5. And I was almost always right. And I've noticed for other people who have different underlying issues, they exhibit similar phenomena. So if you have mental health issues, depression, some people, if they measure enough, can get this sense where they're like, oh, based on how my mental health is, I think I know where my ketones are. So if you choose to play around with ketogenic diets Exogenous ketones in, you measure. Just try to be very observant of this because it's a very cool phenomenon if you can nail it. But zooming back out, I'm not going to sell you snake oil and say that this approach will mimic all the benefits of a 7-day fast. Obviously, a 7-day fast is a push and pull. It's not right for everyone, rather for preference and comfort reasons, or even for safety reasons. But that said, since we do know as a matter of fact that ketones mediate some of the benefits of fasting and are hormones that change how the body wires itself, pushing your ketone levels above these potential thresholds of say 2.5 millimoles or even 4 millimoles might capture some of the benefits of the body's rewiring, the reprogramming, the software update that align with better overall metabolic health and potentially longevity. And yes, there are at least some animal model data suggesting that a ketogenic diet and bumping up your ketones can extend lifespan. We don't know if this translates to humans, but those are the data we have. Oh, and I shouldn't forget to mention that cognitive feeling you get for having ketones elevated at 3 or 4 millimoles. It makes you feel like a Zen monk. It's so cool, even especially if you're a AAA++ neurotic ball of energy like me. It's just totally different cognitive state that you can absolutely feel. But now, as we wrap up the story, I do want to offer one quick cautionary note on how to break a fast, because it's an art, and it's an art that's easy to get wrong. Here's first my cautionary tale. After my first prolonged fast, I celebrated with several slices of pecan pie after not eating for days. That was a huge mistake. For context, I wasn't adapted to fasting and this 60-hour fast was actually situated around my first colonoscopy, so at the time I thought I deserved a sweet treat. I was wrong. Definitely not worth it. It felt like I'd eaten grenades. I'd never been so bloated in my life, and afterwards I sincerely thought about writing an apology note to my toilet. Anyway, here are a few a few high-yield principles for breaking a prolonged fast. Number one, start small, start simple. After a prolonged fast, parts of your digestive tract are just downregulated. That's normal, it's adaptive. It just takes time for everything to ramp back up. Second, prioritize easily digestible foods. Take 3 hard-boiled eggs with some salt for electrolytes and maybe a little bit of olive oil, or a bowl of full-fat Greek yogurt, or even just some bone broth to start. And avoid large amounts of fiber or roughage. If you decide to smash a bag of walnuts, I promise you, you will regret it. And finally, take it easy for the first day thereafter. If you fasted for 3 days or more, it's going to take a little bit of time, even after your initial warm-up meal, for your system to be ready for large meals again. So ease back into normal eating. But now wrapping up for real, the moral of the story is that fasting isn't just for caloric restriction or weight loss. It's about updating the body's software. In fact, I don't even think fasting is a great tool for weight loss specifically. There are more effective tools. And if you do want to learn about another trick to cut fat, particularly harmful visceral fat, even without caloric restriction, check out this video next. All right, I have something really exciting to show you. You ready? The studio is coming along miraculously. We still have work to do. I still need to mount the cameras, get all the tech set up. In fact, I'm dealing with a little problem. The tech store here, and I can't set it up yet because I'm dealing with an AC issue. It's actually really hot in here because my builder didn't put in a wet switch or a condenser pump, so there's a flood risk, and I'm trying to avoid a flood mixing with $25,000 in new tech equipment. So bear with me, be patient troubleshooting this, but it's only possible because of you. So thank you, because we're about to level up. There is so much to come. That's all because because of you. That's the golden plaque. Um, podcast, leveling up just the YouTube element of this. I have some other things I need to do first, like, um, I have a couple books. You'll see those coming in 2027 and 2028. Deals are all signed, a couple businesses. So this is the beginning of our Stay Curious journey, and thank you for making it possible. Sincerely,