by Dave Asprey
By the straight, Ancel Keys–style accounting of the CICO diet, I should have gained about 20 pounds. I was hoping I’d gain only 3 pounds or so. That would have poked a big enough hole in CICO that I’d feel vindicated. The results were even more startling: I actually lost weight. I felt amazing. I found that I could sustain my high-calorie diet for months and months and months, to the point where I was just sick of eating. It’s actually quite hard to eat 4,500 calories a day, chomping on big steaks all the time and putting extra butter on everything. After a while, I grew weary of all that eating, but I felt good. Excess calories aren’t good for you even if you can handle them, however.
The experiment proved to me how powerful intermittent fasting could be, even with Bulletproof Coffee, if you apply it to your regular diet, regardless of what it is. Probably you’re not eating 4,500 calories a day, but if you are, intermittent fasting will help you out. If you never wanted to restrict your calorie intake—or if you tried it and failed—fasting offers a stealth way to reduce calories as a by-product, not an end goal, of improving the way you eat. And if you are already restricting calories, intermittent fasting can give you the same benefits but in a healthier and happier way.
Here’s one more crucial distinction between dieting and fasting: CICO diets force you to build your life around rigid rules; they make you accountable to the calorie-counting police; intermittent fasting encourages you to play with your methodology. It’s safe to play around this way, and it’s fun to do so. You are in charge of yourself.
There may come a moment when you say, “I thought I was fasting, but it didn’t really work today. I got really hungry, so I gave in and ate a potato chip.” When that moment comes, you will respond one of two ways. In the CICO philosophy, you would punish yourself: “Today I failed, so I will do twelve minutes of penance on the treadmill to ‘pay for’ those calories.” That reaction doesn’t do you any good. All it does is make you feel bad about yourself. Plus, there’s no real way to pedal away a bag of potato chips.
In the intermittent fasting philosophy, you turn that perceived failure into a victory. Instead of telling yourself that you’re a loser and wallowing in your lack of success, tell yourself that you’re having a potato chip fast. You ate only one potato chip all day long—just one!—and that was the entire amount of food you ate that today. It turns out that you can eat just one, and you’ll be less hungry than if you eat ten.
A lovely member of my family who looks suspiciously like my wife, Dr. Lana, was doing a five-day fast but had a teaspoon of ice cream on the third day. You know what? Two days later, when she finished her five-day fast, it was still a five-day fast. She had gotten all the benefits, and she felt great. Never toss away a phenomenal moment of personal success. Fasting is a superpower. Build yourself up with the right biohacks—what you eat, when you eat, how long you fast, how you sleep and exercise—and you will discover that you can do all sorts of crazy stuff. The superpower is already inside you, waiting to be set free. But you sure won’t set it free by fixating on lists of calorie counts.
Every person reacts differently to fasting, based on his or her age, weight, and genetic predisposition. Still, there are predictable similarities in how the human metabolism reacts to a period of time without ingesting food. It’s at this basic, biochemical level that intermittent fasting gives you totally different results from CICO diets. This is the origin point of your superpower and of all of the biohacks that facilitate it. This is the microscopic pathway leading to your best self.
THE STAGES OF FASTING
STAGE ZERO OF FASTING BEGINS THE MOMENT YOU FINISH YOUR MEAL.
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You can think of the time from a meal until three hours after that meal as “Stage Zero” of fasting, or as the anti-fast. This is a normal eating pattern for most of us: you eat breakfast; three hours later, you eat lunch; a few hours after that, you eat dinner. You will feel pretty normal at this stage, since your body is still busy digesting the last thing you ate. But even during the anti-fast, you can do things to help train yourself for fasting. The most obvious one is to avoid constant snacking between meals (especially snacking on the nasty processed foods that are lying around a lot of offices and homes). There’s actually a lot going on between meals, and you want to get out of the way and let your metabolism do its thing.
In those immediate postmeal hours, your body is digesting the carbs, proteins, and fats from your food, turning them into amino acids, fatty acids, and above all, glucose. Your pancreas releases insulin, which will be used to transport all that new glucose into cells, where some of it will immediately be used to produce energy and protein synthesis in your muscles. This period of time is known as the growth period or anabolic period, since the nutrients your body needs are all readily available. Anabolic literally means “building up.” Your body is drawing on the energy and raw materials from the digested food to build up the essential molecules in your body. Some of the surplus glucose will be combined with water and stored as glycogen, a starchlike molecule that is an efficient way to store energy, or turned into adipose tissue. Bad news: every molecule of glycogen carries two bloat-inducing molecules of water that effectively hide your abs. Those fitness models you see on magazines? They’ve all been fasting so they have no glycogen-water love handles.
This is also when your hunger hormones kick in. Ghrelin is the primary hunger hormone, activated to tell your body it’s time to eat. Leptin is the antihunger hormone, which tells you when you’re full. Ghrelin levels decline after a meal as leptin levels rise. CCK creates a short-lived sensation of fullness after eating and slows the emptying of the stomach to aid digestion.
A great many diet plans advise you to eat every three hours to keep your metabolism humming along at warp speed so you can lose weight. If you follow that advice, the minute you hit the three-hour mark and you start feeling the slightest bit hungry, you answer the ghrelin response and eat something. If you don’t, cravings kick in, your blood sugar level starts to drop, and you yell at someone nearby because you feel like crap. (You are feeling hypogly-bitchy!) Your biggest question at three hours isn’t whether to wait a few more hours before eating but what type of calories you’ll be putting in your body for your next meal. This is a bad idea because your body never gets a break, and the constantly high blood sugar level created by constant snacking ages you.
If you feel hungry three or four hours after your last meal, it’s because you ate kryptonite foods, you didn’t eat good fats, you didn’t eat enough, or your metabolism hasn’t been trained to shift easily between burning sugars and burning fats—most likely all three. You have two choices: eat quickly, but do it so you will be satisfied for hours afterward, or face the music, experience some real discomfort, and fast anyway. Fasting after eating a meal is the hardest way to start a fast, and I don’t recommend it. You already fast for eight hours every night, which is why starting a fast in the morning is easiest.
STAGE ONE OF FASTING KICKS IN FOUR TO SIXTEEN HOURS AFTER YOUR LAST MEAL.
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This is the beginner’s fast, or the 16:8 intermittent fast. Now you are beginning to break with the stereotypical three-meals-a-day eating pattern. There’s an old adage that if you place a frog directly into a pot of boiling water, it will leap out immediately, but if you place it in warm water and then slowly turn up the heat, it will remain in the pot even as the water boils. This is an apt metaphor for the relationship between people and fasting. You are likely not ready, as a fasting beginner, to go even one full day without food. Do it too suddenly, without the right tools, and you may find yourself dreaming of eating that frog before you give up on fasting entirely. Given enough time, though, a daylong fast is not difficult at all. You just have to turn up the heat slowly.
The whole intermittent fasting process begins in this four- to sixteen-hour window. All the energy coursing through your body after a meal has been put to use. Now the body switches over to stored energy. Glucose is still the prime fuel, t
hough now you are tapping it in the form of glycogen, which has to be pulled from your muscles or your liver.
Skipping breakfast will get you through the three hours after dinner, a night of good sleep, and up to 11:00 a.m. before breaking your fast for real at midmorning. This is known as 16:8 intermittent fasting (sixteen hours without a meal, then an eight-hour eating window before starting the cycle again). It is one of the most common rhythms of intermittent fasting. There are a lot of chemical changes happening inside you at this point.
During a 16:8 fast, your blood glucose levels drop, leading to a decrease in insulin secretion in the pancreas. You may begin to experience the sensations of hunger, light-headedness, and agitation that often accompany low blood sugar, especially if you have never fasted before. After twelve hours without food, your blood glucose level will have dropped by about 20 percent. The hormone known as glucagon is secreted to activate glycogen breakdown, which provides more glucose. As your body starts to use up the glycogen in your muscles, the body starts releasing adrenaline and cortisol, which free up extra energy from the protein in your body for emergencies. You will need less sleep and feel extra energy, though it’s possible you will also feel somewhat cranky.
One you master the Stage One fast, you’re going to realize that you’re just not hungry at 11:00 a.m. anymore. The gnawing feeling is gone. A colleague can put bagels right in front of you at work and you won’t have to fight the urge to snatch one. You simply won’t crave them. Most people who practice Stage One 16:8 intermittent fasting several days a week for a month find it easy to extend to advanced Stage One, in which you eat an entire day’s worth of food in two meals between about 2:00 and 8:00 p.m.
STAGE TWO OF FASTING IS THE ONE-MEAL-A-DAY FAST, OFTEN ABBREVIATED AS OMAD.
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Once you pass the sixteen-hour mark, your body realizes there is very little glucose to be found and begins the full transition to fat burning. Doing so sets into motion a complex interaction of hormones and chemicals that enable your body to use fat as a fuel source, a key step in training your body to be metabolically flexible.
Technically, OMAD is just a twenty-four-hour fast: you have a meal, and you don’t eat until you have a meal at the same time the next day. Calling it OMAD somehow gives it a little more attitude, like a fast with tattoos and cool hair. The abbreviation sounds like “nomad.” If calling your twenty-four-hour fast OMAD makes you feel good, then go with OMAD. It’s kind of sexy to say, “I’m OMAD today.” You can do the Zoolander hair flip for added effect.
A twenty-four-hour fast causes the body to activate lipolysis, a process in the liver that breaks down fat molecules into fatty acids. This act of chemical transformation is regulated by a protein known as peroxisome proliferator-activated receptor-alpha, or PPAR-alpha, which activates the key genetic mechanisms needed to create, transport, and consume fatty acids. The fatty acids, in turn, are transformed into energy-rich ketones (technically, into ketone bodies) through a process known as beta-oxidation. There are three different types of ketones: acetone, acetoacetate, and beta-hydroxybutyrate, or BHB. They are all important to ketosis, the state in which your body is running on fat. Finally, the liver launches the ketones into the bloodstream. This state of ketosis occurs more quickly if you have already burned muscle glycogen earlier in the fast through exercise, which is why exercise is an important aspect of biohacking your fast. You’ll hear a lot more about this later.
Along with these chemical changes, your heart rate and blood pressure drop as your body switches into energy-saving mode. Overall, your basal metabolic rate, or BMR, becomes lower and more efficient. There has been a lot of recent scientific interest in the effects of OMAD fasting but so far only a small number of published, controlled studies. One of them, conducted by a group led by David J. Baer at the US Department of Agriculture, found lower levels of triglycerides and higher levels of HDL (“good”) cholesterol in the blood of healthy middle-aged adults who reduced the frequency of their meals.3
OMAD is a cornerstone of my intermittent fasting regimen, and it should be for yours, too. You might be surprised, then, by what I’m going to tell you next: doing OMAD every day is a terrible idea. Fasting purists get their hackles up when they hear that, but after ten years of answering people’s questions about fasting on my blog, I’ve lost track of the number of people who feel great on OMAD-style intermittent fasting, vow to do it every day, and regret their decision two to four months later when they have to climb out of a health hole they dug for themselves. Intermittent means intermittent! If you do OMAD every day, expect to see your sex hormone levels fall (this applies to both men and women), your sleep quality drop, and your hair thin. People over thirty-five usually feel these effects before younger fasters, but they are almost inevitable at some point.
For maximum impact, I recommend regularly changing not only the duration of your fast but also the style. You might try a high-protein breakfast with plenty of fat on Monday, OMAD on Tuesday, intermittent fasting on Wednesday, OMAD on Thursday, intermittent fasting on Friday. On Saturday you eat whatever the heck you want, then on Sunday you circle back to OMAD. You are making your body stronger by cycling into and out of lipolysis and ketosis. Typically, you would do your twenty-four-hour fasts by eating dinner only. But if you can, challenge yourself and skip dinner occasionally. When you eat again, it will be about thirty-six hours since your last meal, and you’ll push into the next stage of fasting.
STAGE THREE OF FASTING (36 TO 120 HOURS) IS A MORE ADVANCED TECHNIQUE FOR PEOPLE WHO ALREADY KNOW HOW TO DO SHORTER FASTS SAFELY AND COMFORTABLY.
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Let’s say you just went thirty-six hours, because you slept before you ate again. If you haven’t done an extended fast before, it’s hard to believe that this is even possible. But by the time you get here, you will learn that it’s not that hard. In fact, thirty-six hours is my favorite fast.
After twenty-four hours, ketones are your primary fuel source: you are fully in ketosis. But the brain runs on glucose, not ketones, so a process known as gluconeogenesis occurs. The body ingeniously turns fat, ketones, and amino acids into glucose—sometimes producing as much as 80 grams per day—keeping your mind sharp.
At a time when you might expect your hunger to be at an all-time high, your production of ghrelin hormone drops off, so you actually cease to experience hunger pangs. As your body taps into its fat reserves, it also clears out the toxins that are often stored along with the fat molecules. Your body is pumped by the changes in your metabolism and is screaming at you to keep going. If you’re feeling inspired, you could easily skip another couple of meals before breaking your fast. When you hit the thirty-six-hour mark and make the bold choice to go for two complete days without food, you’ll hear your body asking, “Is it actually possible that I can go forty-eight hours without food? I think I can do it.”
The thirty-six-hour fast is my favorite because it is vanishingly easy. I go to sleep (that’s eight hours of fasting), then have Bulletproof Coffee for breakfast so my energy is high and my blood sugar is low. Lunchtime rolls around, and I’m not hungry. I tell myself I might have dinner, which makes my body stop thinking about food. But at dinner, I tell myself, “Hey, skip dinner and sleep on it, and you’ll get another eight hours of fasting—that will be thirty-two hours!” When I wake up, I find that I don’t even want breakfast. I have no hunger at all. By the time I eat lunch, I really only had to skip one meal: dinner the night before. A thirty-six-hour fast without feeling denied or even very hungry is entirely possible.
Still feeling good? You may be ready to keep your fast going up to 120 hours—five days, or a full workweek. This is an advanced level that can take you into a spiritual fast (we will discuss this idea in more detail in chapter 7), but you should approach this with caution and only after you have become experienced with fasting.
At this point, most people enter a full state of ketosis, in which your body is breaking down its own fat for energy. It will also break
down small amounts of muscle to turn into glucose through a process called gluconeogenesis. Some long-term keto dieters say that this is a great state to be in, and it is for a little while. Your body will break down old proteins first! The problem is that it’s biologically difficult to turn protein into glucose, and you don’t want to be doing it for long periods of time.
By now, your body has entered prolonged fasting mode. You will feel less mentally grounded but bursting with energy. If you get a hunger pain, have some water with a dash of sea salt in it or coffee or tea. It will pass quickly. Your levels of glucose and insulin have dropped for an extended period, decreasing your risk of metabolic disease. Meanwhile, your cells have become more resistant to toxins and stress. Your ghrelin production continues to drop, eliminating your sensation of hunger. Your ketone production increases as the body’s demand for ketones rises. A pleasant by-product of this change is that ketones also help suppress ghrelin levels. This is not the painful fast you feared.
There are significant health benefits to these extended fasts. Your body will turn up autophagy, the recycling of cells, mitochondria, and cellular junk. Your liver will reduce its secretion of insulin-like growth factor 1, or IGF-1, a hormone that is structurally similar to insulin. Although IGF-1 is crucial to normal bodily functioning, elevated levels are associated with cancer. But you need to pay attention when coming out of an extended fast: if you’ve fasted for several days or a week, the meal you choose to break the fast should include protein, fat, vegetables (carbohydrates), and lots of fiber. This combination will allow you to strike a good metabolic balance while keeping your gut bacteria healthy.