How Long Should You Do Intermittent Fasting? Here's What Happens

I've written multiple books about intermittent fasting, metabolic autology and stronger by stress. I've practiced and researched intermittent fasting for over 14 years. I've tracked hundreds of blood markers and have seen that intermittent fasting is a consistent contributor to getting better results. However, I don't think that all fasts are equally beneficial. The interesting thing is that your body doesn't experience the same benefits throughout the fast. Different things happen at different times. In this video, I'm going to walk you through exactly what happens during a fast, hour by hour, based on the current research and my own experience. Before we go through the timeline, I think it's important to actually understand what happens inside your body when you stop eating. Your body is constantly switching between two metabolic states. The third and fastest state. When you eat, your blood sugar rises and your pancreas releases insulin. During this state, your body is focused on anabolism, which simply means building things. Building muscle, refilling glycogen, making new proteins. But eventually, the meal is digested and your blood sugar comes back down. Insulin falls, and that's when your body begins switching into a fastest state. Now, glucagon rises, that tells the body to use the energy that you stored. Instead of burning calories coming from your last meal, your liver begins breaking down glycogen. The same thing also happens inside your cells. Two important nutrient sensors begin changing. mTOR goes down, AMPK goes up. You don't need to remember those names, but you do need to remember what they do. Think of mTOR as the growth switch. When there's food around, mTOR tells the body to build and grow. AMPK does the opposite. It's the repair switch. When energy becomes scarce, AMPK tells the body, conserve energy, burn stored fat, recycle damaged proteins, and repair old cells. One of the ways it does so is by increasing autophagy, which we'll talk about later in the video. Now, neither of these states is better than the other. You don't want constant mTOR stimulation, but you also don't want constant AMPK activation. You need both. You need to build muscle when you're eating, and you need to break down and repair old, broken cells. But the problem is that you can't achieve the repair process if you're eating all the time. And you can't build when you're not eating. Optimal health isn't achieved by being only in one state or the other. It's achieved by cycling between them, which is called metabolic switching. Alright, so let's talk about what happens after your last meal. The first four hours is the transition from being in a fed state into a fasted state. As soon as you finish eating, insulin gradually begins falling, and glucagon begins rising. By around four hours, these hormonal changes become much more noticeable. The insulin response from your previous meal has largely disappeared, blood sugar has returned close to baseline if you're metabolically healthy, and your body begins relying less on incoming food and more on stored energy. The main source of energy now becomes liver glycogen. Your liver stores approximately 80 to 120 grams of glycogen, which acts as your body's reserve fuel between meals. Most people don't notice any significant changes in this state. That's because your body is just switching from the food that you ate to stored energy. But this metabolic switching is allows the other steps to also occur. After around 12 hours, liver glycogen has decreased substantially, and your body begins relying more heavily on fat for energy. As fat oxidation increases, your liver also starts producing ketones. Ketones are molecules made from fatty acids that can be used by both your muscles and your brain, although the muscles and brain will always use some glucose. This shift is one of the defining features of fasting. Instead of relying on glucose, the body becomes more reliant on fat and ketones. This process is called metabolic flexibility, and it's one of the main reasons intermittent fasting improves metabolic health. Research suggests that poor metabolic flexibility is implicated in diabetes, obesity, and insulin resistance. However, people get metabolic flexibility wrong. It has two parts, going from fair to faster state, which is the low liver glycogen and increase in fat burning, but also going from a faster state to a fair state, shifting into an anabolic state of repair with higher insulin and low glycogen. You need both for optimal metabolic flexibility, as you can be metabolically inflexible both ways. If you can't switch over to burning ketones after you've stopped eating, that's a sign of bad metabolic flexibility. But if you can't switch over from burning fat to burning glucose and lowering the glucose again, which requires you to be insulin sensitive and produce insulin, then you're also not metabolically flexible. The metabolic flexibility is achieved by being able to switch both ways, not just one or the other. Personally, I think that most of the day-to-day benefits of intermittent fasting are achieved in this state. If your goal is to just improve your metabolic health and insulin sensitivity, then fasting for around 12-16 hours is the sweet spot. There's plenty of evidence from human trials showing that time-restricted eating or intermittent fasting for 12-16 hours improves blood sugar, blood pressure, weight loss, and insulin sensitivity. A 2025 study found that 8 weeks of 16 day intermittent fasting reduced resting heart rate by almost 9%, while simultaneously increasing heart rate variability. The participants saw an average 6.6 beat reduction in their heart rate and 9-12 point increase in HRV, which is significant. At the same time, blood pressure decreases as your body is in a parasympathetic state. Most people can just stop here and do daily time-restricted eating, which involves having an earlier dinner. That's the best way to do it. By around 18 hours, ketone production increases further and blood ketones become measurable in many people. Your brain is now obtaining a meaningful proportion of its energy from ketones instead of glucose. Another important process that begins occurring is autophagy. Autophagy literally means self-eating. It's the process by which your cells break down damaged proteins, dysfunctional mitochondria, and other worn-out cellular components so they can be recycled into new ones. Autophagy is also involved in degradation of viruses and bacteria, or even amyloid proteins in the brain. You can think of it as your body's recycling and quality control system. Disabled autophagy is considered a hallmark of aging, and it influences all other hallmarks. Aging is characterized by the accumulation of junk material inside cells that autophagy is supposed to remove. The big misconception is that you need to fast for several days to increase autophagy. That's not true. The truth is that autophagy is happening to a certain degree almost all the time. The question is how much of it is active. It's been shown in humans that 16 hours of fasting already upregulates markers of autophagy. That's because fasting gradually reduces liver glycogen, which then suppresses the mTOR pathway that inhibits autophagy and allows the activation of AMPK, which then increases autophagy. So the faster you deplete your liver glycogen, which can be accelerated with exercise, the faster you also go into autophagy. So you don't need to fast 3, 4, 5, 7 days. So that's why I aim around 16 hours of fasting every day. I finish my meal around 5, 6 p.m. in the evening, and then my next meal is 10, 11 a.m. That gives me plenty of time to go through some of the liver glycogen, which probably already gets depleted way faster than 16 hours, but this is just a convenient way of how I do it. As a bonus tip, it's better to go to bed on a slightly emptier stomach because that also allows autophagy to occur faster. Most of the autophagy occurs in your sleep, so you don't want to have a massive meal before bed because it's going to inhibit autophagy, not to mention your sleep quality. By the way, if you want to know whether your health routine is actually improving your health, I created a three-minute health percent assessment. You put in things like your blood markers, fitness metrics, body composition, and it shows you your health percentile, what's your bottlenecks, and gives you a program to follow to get to top 1% health. The link is down below. After 24 hours, the processes of fasting start to ramp up even more in a dose-dependent manner. You go into deeper ketosis and use more fat as opposed to glycogen. Your blood sugar drops even lower. You increase more autophagy, and you increase more growth hormone. For example, it's been shown that 18-24 hours of fasting increases growth hormone by about two-fold above baseline. During the fastest state, the body releases more frequent pulses of growth hormone as well. The reason is that within 24 hours, you've fully depleted your liver glycogen, and your body is producing more ketones. Now we're talking about longer than 24 hours of fasting. In this state, you've also slept in a faster state, which will result in significantly more autophagy because of the reasons that I mentioned. Going to bed and sleeping in a faster state inevitably increases more autophagy. Now, I don't think that most people need to fast this long. Most people can just stick to 16 hours of fasting and get most of the health benefits, but sometimes, if it's convenient, then you can still do 36-48 hours of fasting. At 36 plus hours, growth hormone will keep increasing more, although not exponentially higher. It'll just result in greater spikes and more frequent spikes of growth hormone as a way to reduce muscle breakdown. This process has been seen to occur up until five days of fasting, after which growth hormone plateaus. At two days of fasting, growth hormone is up three-fold, and at day five, it's five-fold. This is important to keep in mind. If you're not eating anything for over 24 hours, then your body greatly increases protein breakdown, which then suppresses mTOR and suppresses muscle growth. That's why you can't fast many days all the time. It will inevitably lead to more muscle loss. The only saving grace at this point is growth hormone that tries to put the brakes on the muscle breakdown. The five-fold increase in growth hormone at day five doesn't mean that you're going to build five times more muscle or something like that. Even if the growth hormone increases five-fold by day five of fasting, you're still losing more muscle compared to if you would have been eating regularly. So eating is still more anti-catabolic and more anabolic than fasting, even if fasting increases growth hormone by five-fold. If you do a 36 to 48 hour fast every once in a while, then you're not going to lose any muscle. You're just going to rebuild it quite easily. However, if you're someone who has low muscle mass or you're older with low bone density, then it might be more harmful than beneficial to fast this long. Losing muscle and bone density after the age of 60 is more harmful than beneficial. Autophagy still has a beneficial role in muscle preservation that's been shown in research. However, it doesn't have to entail fasting for 3 to 5 days. The daily 16 to 24 hour fast, done consistently, might be better than a 3 to 5 day fast, at least for most people, and definitely for muscle preservation. When it comes to 72 to 100 hours of fasting, then there isn't much more research on this. In terms of physiology, then the same processes occur. Growth hormone spikes more frequently and plateaus at day five. Autophagy keeps increasing, but more isn't necessarily better, as the body needs to obtain energy from somewhere, and it will do it by salvaging cell material. At some point, it can become excessive and potentially more harmful. Ketosis keeps increasing. The body will start covering the vast majority of its energy from body fat. However, your insulin sensitivity decreases, in proportional to that. The deeper into ketosis you go, the lower your insulin sensitivity is, at least for the time being when you're in the ketosis state. With good metabolic flexibility, you would regain your insulin sensitivity as you exit ketosis. But with bad metabolic flexibility, or if you've been fasting for a very long time, then in the short term, your insulin sensitivity will drop. And of course, you're gonna keep losing weight if you fast for 72 to 100 hours, because you haven't eaten anything for several days. A 2023 review of human trials concluded that prolonged fasting for 5 to 20 days results in moderate weight loss of 2 to 10% of total body weight. However, about two-third of that is lean muscle, and only one-third is fat tissue. So that's not really the most optimal split of fat loss. Yes, you will lose muscle on any diet, even if you eat a lot of protein, when you're trying to lose weight. But if you were to do, let's say, 16 and 8 intermittent fasting, you could easily lose 50% fat and 50% muscle, or even 60% fat and 40% muscle. So you can make the weight loss more advantageous to you, with a slightly shorter intermittent fasting window, and with a better macronutrient split. So the super long fasts are only beneficial if you have a lot of weight to lose, like you're 40-50 kilos overweight. If you're already lean, you have 5, maybe 10 kilos to lose, then going for these super long fasts for weight loss purposes isn't that beneficial, because you also lose a lot of muscle in the process. One of the most interesting discoveries comes from the research of Walter Lungo and his colleagues on mice. They found that fasting for 2 to 5 days in mice and fasting patients reduces white blood cell count. However, after starting to eat again, the white blood cells return. Hematopoietic stem cells also become activated, which then produces new immune cells. Lungo and colleagues propose that fasting helps to remove older immune cells, while refeeding stimulates regeneration of the immune cells. Now, fasting for 2-5 days in mice is a lot longer in humans, probably like a 7-day fast. However, what it does suggest is that longer fasts might have some benefits for the immune system. Now, Lungo has created his own fasting method called the fasting mimicking diet, that supposedly mimics some of the benefits of fasting by keeping insulin, mTOR and IGF-1 low. All of the pathways that get activated during eating. So theoretically, you could mimic the benefits of a 7-day fast by following the fasting mimicking diet. The protocol is quite simple. You eat 5 days low-calories, low-protein, plant-based diet once a month. Day 1 is 1000 calories, day 2-5 is 700-800 calories. And after that, you return to normal diet. There are about 10 human clinical trials on the fasting mimicking diet. Across the studies, the fasting mimicking diet has been shown to lower biological age scores based on the blood marker test pheno-age by 2.5 years, reduce insulin resistance, lower liver fat, improve immune system markers, reduce IGF-1, which might be beneficial for cancer risk, reduce body fat and visceral fat, activate autophagy-related pathways, and improve cardiovascular risk factors. So now let's talk about how long should you fast. The optimal fasting duration depends entirely on what you're trying to achieve. If your goal is to just improve your metabolic health, insulin sensitivity or even sleep, then the 16-hour fast tends to be good enough. If your goal is to increase fat oxidation, go into deeper ketosis for some reason, or to get more autophagy, then fasting for 24-48 hours will give you more of that. But you can already go into mild ketosis and already increase autophagy with a 16-hour fast, if you're metabolically flexible. So as a healthy person, you need to fast shorter to get the benefits. Whereas if you're overweight, diabetic, insulin resistant, with bad metabolic flexibility, then you need to fast for a lot longer to get those benefits. For me, the best sweet spot is around 16 hours of fasting. Sometimes I go for 18 hours, sometimes it's 14 hours, doesn't matter. It's in that ballpark. And I always try to stop eating 4-5 hours before bed. That's the most important part. If your goal is sleep quality, insulin sensitivity and even autophagy. Because if you're only doing some sort of longer fasts, one meal a day or whatever the case is, but you see that your muscle mass is going down, your visceral fat doesn't change, your blood markers doesn't improve, then you're not really improving your health. You can only see your health improving if your health markers are improving. So you want to follow a fasting protocol that works for you in improving your health markers in the more positive direction. If you want to see how your health actually stacks up, take my 3-minute health percentile assessment in my top 1% health community. It puts your biomarkers, fitness and body composition markers into perspective, gives you your health percentile and shows you what you should focus on improving next. Click the link below to get started.