Dave Asprey (0:00)

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Dave Asprey (1:25)

Fasted for three days and here's what happened. I actually starved to death. You probably tried a 16 hour fast. You skip breakfast. You feel good about yourself. Repeat. It's not that hard. It drops your insulin a little bit, burns some stored sugar. But that's exactly the problem. At 16 hours, you're mostly still burning sugar. Your body hasn't flipped the real switch into deep fat burning yet. It's only after 24 hours, 36, 48, even 72 hours without food that the real transformation begins. Because in three days without food, your body can do what no supplement, no diet and no workout plan can achieve. It can turn back your biological clock. Your body has an anti aging switch and a three day fast is one of the best ways to turn it on. But if you do it wrong or you do it too often, you can damage yourself in a couple minutes. I'll tell you exactly how to do a fast and how often. But first you need to understand that none of this matters if you don't know the one thing that's secretly destroying your body right now. The problem starts with how people think about aging. Most of us think wrinkles, fatigue and brain fog past 30, just things that happen. You get older, you get weaker and there's nothing you can do about it. Doctors don't question it either. They measure the number of birthdays you've had instead of the actual condition of your cells. And because of that, decline becomes the default diagnosis. So what do you do? You might reach for creams and treatments, hoping they'll somehow make you younger and feel better. And yeah, they can give you a surface level boost. Some of them actually work. You'll get smoother skin. But those things rarely reach where aging actually begins. Aging doesn't start in your skin. It starts inside your cells. Your cells run on tiny little batteries or engines called mitochondria. They turn food and oxygen into energy. But the problem is, modern life is really hard on mitochondria. Stress keeps your cortisol high all day, and it breaks down your repair systems. And bad sleep means your cells don't get a chance to repair DNA. And if you're eating constantly, having snacks all the time, late night meals, it keeps your insulin high all the time. And when insulin stays high, your body can't switch from storing mode to repair mode. So damaged parts of your metabolism build up. Broken mitochondria stay broken. Inflammation spreads to your brain, your organs, your immune system attacks the wrong things, and it misses real threats. Even healthy eaters feel it. You're working harder, but you're getting less energy. You feel tired all the time. Your brain fog is just unending. And you look older than you should. And that's because your cells are stuck. They can't repair themselves. When you never stop eating, the damage keeps stacking up faster than your body can clean it out. And that's the real cause of what causes aging, inside and outside. So you need to give your body a break from food and let it repair itself. We're talking about a deep metabolic pause to restart repair. Not another pill, not another diet trend, but an actual break, long enough for your body to flip the switch back on. When you do this, your cells activate a process called autophagy, which is when your built in repair switch turns on. Insulin goes down, inflammation goes down, growth hormone goes up to preserve your muscle, and it can repair your tissues. Your mitochondria even start rebuilding. The enzymes that repair your DNA start working again. And in just a few days, the brain fog lifts, gets your energy back, and the engines inside your cells start running like they're supposed to. But most people won't actually experience those benefits with a standard 16 hour fast. And that's because of one common mistake. When you stop eating, your body doesn't just shut down, it moves through multiple Fuel phases. In the first few hours after a meal, you're in the fed state. Your body is busy breaking down food, sending sugar from the meal into your bloodstream and using insulin to move that sugar into cells for quick energy or to store it as glycogen in your liver and in your muscles. You're basically running on the fuel you just ate. Around three to four hours in, you shift to the early fasting state. Blood sugar and insulin levels drop because there's no new food coming in. And your body starts burning the glycogen that your liver stores and turns that into glucose, which keeps your brain and muscles powered. And as those stores of glycogen go down, your body starts looking for a backup plan and slowly begins to break down fat, releasing fatty acids into your bloodstream. And that's where a little bit of fat burning begins. Because by about 18 hours without food, most of your liver glycogen is gone. And that's when your metabolism starts to pivot into a fasting state. The 18 hour mark is when you actually do burn more fat. And your liver turns those fats into ketones, which is a really good fuel for your brain. And your brain loves ketones. That's the moment people talk about when they say fasting flips the fat burning switch. But here's the problem. Fat burning doesn't kick in until after your glycogen levels have dropped. And that's why ending a fast after 16 hours is better than not fasting for 16 hours, but it does cut the process two hours short. You've just started dipping into fat free energy, but you haven't stayed long enough to get the full benefits of it. So if your goal is to maximize fat loss or metabolic benefits, breaking your fast too soon means you step out right as the good stuff is just starting. So let me walk you through what happens to your body when you fast for three days in a row. And if that sounds like an enormous lift, it's not as hard as it sounds. Like we just talked about, the first day of a fast is all about burning through the easy fuel. Your body has stored away sugar. Your liver empties its glycogen tank, your muscles empty their glycogen tanks, your insulin levels drop, and your body starts tapping into fat. And that's also the roughest day. Your hunger signals are loud, your mood can swing, you get hypoglybitschy, you get hangry, and you're probably a little bit tired and foggy unless you artificially boost your ketones which makes it painless. You can do this by putting butter and MCT in your danger coffee. Or you can use something called kinetic, which is a ketone drink. Use code Dave at Kinetic ketones are like a premium fuel for your brain. And many people notice their energy stabilizes and their minds get sharper when they're in ketosis. Can happen through the things I just mentioned, or it can happen just by naturally waiting. But if you don't want a day of feeling hangry, that's a hack. Butter, an MCT in your danger coffee or a ketone supplement. But behind the scenes, something important is happening. Your hormones are shifting, growth hormone rises to protect your muscles, and your metabolism begins preparing for a deeper reset. And by the second day, your sugar tank is fully empty and your body makes a powerful shift. It's going to run mostly on fat and ketones and possibly some muscle if you're not doing something to preserve your muscle. Ketones are like premium fuel for your brain. And many people notice their energy stabilizes and their mind sharpens when they're using ketones as fuel. And deep inside your cells, autophagy kicks into high gear. When you do this, inflammation starts to fall, your body starts to break down old, weak immune cells, and it makes room for fresh new ones. You might have felt deprived on the first day, but now on the second day, you strangely feel calm and clear. And by the third day, the magic peaks. Ketones are at their highest and your brain feels completely steady and focused. And more energy goes into your brain than normally does because your body's not worried about digesting food. Stem cells start to wake up and it lays the foundation for a stronger immune system. Once you do start eating again, your mitochondria, they become more efficient, more effective at making the fuel of life. And in your brain, ketones help spark BDNF release, which is a growth factor that sharpens your memory. Oh, and ketones are anti inflammatory too. So to put it simply, this is when fasting shifts from just burning fat to actually upgrading your biology. You feel lighter, clearer, and more in control than you have for years. And you are not obsessed with food. You don't even think about it. So 16 hours without food, it burns sugar. 72 hours without food turns on your body's full longevity anti aging switches. A three day fast reverses aging, sharpens your brain, and it activates stem cells. And it's not even in the same category as a standard intermittent fast. But even a three day fast is useless if you're doing it the wrong way. In fact, if you follow the wrong process, your body can start burning muscle instead of fat and a careless fast can do more harm than good. If you skip electrolytes, things like sodium, potassium, magnesium, it can give you headaches, dizziness, heart palpitations and cramps. You don't want that.