by Dave Asprey
I’m calling Delilah eccentric, but I wouldn’t blame you if by now you’re thinking that maybe I’m a little eccentric, too, for choosing a shaman instead of a therapist or for heading off into a cave in the middle of nowhere guided by a woman I’d never met before. It does seem a little out of the ordinary to seek inner peace this way—except that it’s not. Whether it’s to mark the end of childhood, to celebrate a religious holiday, to take a spiritual retreat, or simply to escape from the bustle of everyday life into the calm of nature, people will often travel to an isolated location in search of a life-altering experience. These outings commonly include fasting or other ways of breaking with routine. People do it in all walks of life, all around the world, and they regularly return a lot better than when they left.
Put more bluntly: We are all eccentric in our own superficial ways, yet we are all profoundly the same. We each seek our own, idiosyncratic path toward the same goals of controlling our hungers and overcoming our cravings, cravings for anything we want but don’t have. That is exactly what makes fasting so powerful. Unlike many diets (or even some specific fasting plans you might have heard about), the program of fasting I’m talking about is not one rigid set of rules designed for one kind of person. It is a universal process of self-improvement that applies in specific, individual ways to every single person, based on biology, psychology, and even spirituality. It’s more than what’s (not) on your plate.
A LONG MENU OF FASTING OPTIONS
To understand what fasting does, let’s begin with a clear picture of what fasting is. People use the term in many different ways, so we’ll start by breaking it down to its essence. Its meaning is just two simple words: going without.
Note that I didn’t say “going without food,” because there are many ways to go without:
Sobriety is going without substances.
Meditation is going without thinking.
Solitude is going without other humans.
Sabbath (or Shabbat) is going without working.
Abstinence is going without sex and sexual release.
All of these are forms of fasting. They all involve turning away from something that people routinely feel they cannot live without. I know plenty of guys who say that they feel as though they’re going to die if they go too long without an orgasm. Or maybe it’s porn, a glass of wine, a piece of chocolate, or a busy day of feeling useful at the office. It may even be something that doesn’t seem like an addiction at all, such as going to the gym. Whatever it is you think you need, fasting is about deciding that you are in charge of it—about summoning the internal fire to say “no.”
I did many different forms of fasting in that cave, all at the same time. That’s why it was scary. When you go without, it creates space in your mind to examine the things you think you depend on and to discover whether that dependence is truly what you thought it was. For instance, our dependence on oxygen is real . . . but most people freak out when their lungs are empty after about ten seconds, even though they know that they can go one or two minutes without oxygen. You can actually fast from oxygen. It’s called hypoxic training, and it can increase endurance. Athletes often train2 in high-altitude locations such as my hometown of Albuquerque, New Mexico, or Chamonix, France, to experience the benefits of doing with less oxygen, and the most powerful are now exposing themselves to brief periods of no oxygen, which drives superhuman biological changes. There are also ways to “fast” from oxygen by controlling your breathing, which can greatly enhance a meditative state; you’ll learn more about that in chapter 7.
The same is true of food and drink: we do need those things, but we think we need them way before we actually do. (That goes for sex, companionship, work, and many other things, as well.) Examining your actual needs versus your perceived needs up close exposes how much power you actually have over your body and your behavior. You don’t need to go into a cave to do that kind of self-evaluation. You can just carve out the quiet time to cross-examine your internal story about the things you are certain you can’t do without. You will quickly discover that your certainties are not based on reality. Fasting from food, for instance, teaches you that you really don’t need French fries. Each little step helps set you free.
While we’re discussing what fasting is, I also want to clarify what fasting is not. It is not suffering. Although you may be uncomfortable the first few times you do it, fasting eventually becomes joyous and then transitions to . . . nothing consequential at all. Once you discover that you can do without, you gain power and control. When you end a fast from anything, it makes that substance or that experience much more appreciated. It causes heightened pleasure and brings easy gratitude into your life.
My ten-year-old son, inspired by the things he watched me do in the course of writing this book, recently decided he would try his own twenty-four-hour fast, with only a little black coffee in the morning. He was determined to pull it off, and he did. He turned down the fasting hacks you will learn about here, because he wanted to see what it was like to go it alone. At the end, he said, “Daddy, you’re right. Fasting really is the best spice. That food I had for dinner was the best-tasting meal I’ve ever had!” Seeing the immense look of accomplishment and self-confidence in his smile made the father in me happy. My son felt that way because fasting puts you in control of the things you thought you couldn’t live without. Fasting creates gratitude for things you probably took for granted. It’s that simple—and that complicated.
There’s another part of the definition that is absolutely crucial for you to know. Fasting does not mean eliminating something from your life completely. When athletes are hypoxic training, they are limiting their access to oxygen in a carefully controlled way; an uncontrolled oxygen fast is called suffocation. Even the most devout observer of the Sabbath will leap into action if he or she sees that someone is injured, because there are reasonable and unreasonable ways to give up work. So it is with dietary fasting. We normally think of fasting to mean going without food entirely, but it is much more flexible than that. Fasting, in all of its forms, is most effective if it is responsive to your circumstances. An uncontrolled fast from food is simply starvation.
Which brings us to a final, crucial part of the definition. Fasting is not just one thing. There are many forms of dietary fasting. Have you ever heard of a dry fast? That’s when you go without food and water. Have you ever heard of a dopamine fast?3 My friend Cameron Sepah, a psychologist at the University of California San Francisco, created that concept. A dopamine fast is basically a break from all the instant gratification stimuli in your life, from shopping to gaming to alcohol and drugs. Or have you heard of people who go into a cave and stay there for a week or two in absolute darkness? That is fasting, too, from food and even light. Anytime you reduce the inputs to your body, you may be fasting.
I’ve been fasting regularly for more than ten years. Through a lot of wide-ranging research and experimentation I’ve found that the best way to fast regularly—no matter what you’re going without—is a method called intermittent fasting.4 It delivers remarkable benefits to your body and your mind. It is surprisingly easy to do, because it can be customized to the way you eat now, improving your life no matter what your style. It also painlessly opens the door to longer fasts.
The basic principle of intermittent fasting is that you toggle between short periods of doing without and periods of returning to your baseline behavior. This idea has been catching on lately, so you may have seen books and articles promoting specific ideas about the right way to do intermittent fasting. Some people cite scientific research in support of the so-called 16:8 intermittent fast, built around sixteen hours of going without. Others argue that an eighteen-hour fast might be somewhat better. Or twenty-four hours, based on yet another study. Then there are fasting-mimicking diets, in which you’re allowed to eat so long as you limit yourself to a careful selection of foods, like the ones outlined in my Bulletproof Diet. A fast that includes food may sound like an oxymor
on, but it’s a real thing and it works. It’s an important part of the fasting tool kit. I’ll explain more about that later.
The bottom line is, there’s no clear-cut rule of how long those “periods without” need to be, as long as they give you what you seek. In fact, obsessing over the rules runs counter to the big-picture goals of what fasting is supposed to achieve. We can therefore stop focusing on the details of the fast and instead pay attention to the shape of what fasting does for you. The details are just that: details.
HOW FASTING HACKS THE BODY
So what does fasting do for you?
One significant benefit is that fasting regulates your insulin levels. After you eat, your body breaks down carbohydrates from food into a sugar called glucose, which is one of the primary molecular energy sources in the body. The glucose level in your bloodstream goes up. In response, the pancreas secretes insulin, a hormone that acts as a kind of metabolic switch. The insulin attaches to cells in your body and causes them to fuel up on glucose. Finally, your body unleashes other hormones, including cholecystokinin and leptin, to signal that you’re full and to convince you to stop eating.
That is how the system is supposed to work, but the modern Big Food diet can overwhelm those delicate biological mechanisms, which evolved in a world where bad fats didn’t exist and honey was the only sweet thing available most of the year. Companies are under constant pressure to sell you the cheapest sources of calories, and those sources are almost always the least desirable from a health standpoint. To boost the appeal of those budget calories, the companies blend them with artificial flavors, artificial sweeteners, and whatever else they can to make the resulting product taste good. It’s not as though there’s anyone evil out there who wants you to be sick. These are just businesspeople doing what businesspeople do, trying to maximize profits and minimize costs. From the consumer’s point of view, though, the lack of evil intent is beside the point. All that matters is the result: supermarket shelves jammed full of processed foods that are mismatched to human metabolism.
If you eat a processed diet full of refined sugar and low-cost carbs, your body can’t always keep up with the flood of calories you’re consuming. The ramped-up flavors and sweetness in processed foods also tend to scramble the digestive system’s normal “stop-eating” signals, compounding the problem. When there is more energy coming into the body than there is going out, the extra glucose will be stored as fat. At the same time, your pancreas will be overworking in a frantic attempt to keep things in balance. Eventually your body may become insensitive to insulin; that’s a leading cause of type 2 diabetes.
Fasting slows down this insulin-glucose cycle and gives your body a rest. Such a rest period is especially welcome if you are eating low-quality foods, which is why fasting benefits everyone regardless of his or her normal diet. Basically, consuming less of the bad stuff for even a brief period helps. While you’re taking a break between meals, your body will also draw on its stored reserves of sugar and fat. Your glucose levels will remain stable, while your insulin levels will drop. Adrienne Barnosky, an endocrinologist at Duke University School of Medicine, and her colleagues confirmed that intermittent fasting helps prevent insulin resistance.5 There is persuasive clinical evidence that fasting helps prevent leptin resistance as well, which matters because leptin resistance is the first step of insulin resistance. As long as you don’t eat anything that raises your blood sugar, you can get most of the same benefits even while eating; that’s the principle behind a fasting-mimicking diet.
Another benefit of fasting: It triggers autophagy, a self-cleaning (literally “self-eating”) process in the body. Through the normal wear and tear of living, your cells gradually clog up with accumulated toxins, pathogens, misshapen proteins, and dead cell parts. All of that microscopic garbage can impair the normal operation of your cells, and can even make it so that they don’t divide and reproduce correctly. Autophagy is a whole package of biomolecular tools that constantly sweep through the body, collect the trash, and deposit it into tiny digestive vessels called lysosomes.6 Autophagy is essential for keeping your cells in good working order.
A growing number of studies indicate that triggering autophagy also helps slow the aging process, reduce inflammation, and enhance your body’s performance overall. Researchers still don’t totally understand why fasting boosts autophagy, and most of the studies have been done on mice, not on humans, but the biological mechanism seems to operate much the same way across the animal kingdom: when the body isn’t busy moving sugar and storing fat, it devotes more resources to basic maintenance. One recent study at the Scripps Research Institute in La Jolla, California, found especially pronounced cleanup of the neurons in the brain during fasting.7
More and more studies are finding that fasting makes your body work more efficiently and cleanly at the molecular level, in a complex set of ways that we are still exploring. For instance, a group of Japanese biologists reported in 2019 that a fifty-eight-hour fast—in people, not in mice!—increased the blood levels of forty-four different compounds that are involved in the chemical pathways that break down fat and control the structures of proteins.8
Intermittent fasting has been shown to influence a potent antiaging molecule known as nicotinamide adenine dinucleotide, or NAD. In its activated form, it’s known as NAD+. The deceptively simple-sounding job of NAD+ is to shuttle electrons around so that the chemical reactions in your body can proceed smoothly. That little molecule works hard: it enables your cells to generate energy; it helps repair the damage to your DNA that happens all the time; it keeps your proteins properly shaped, which is key to avoiding mental decline; and it protects your cells from oxidative stress, one of the relentless processes at the heart of aging. Intermittent fasting increases the levels of NAD+ in your blood. Seriously, you want that.
Every time I think I’ve caught up with the literature on the biological benefits of fasting, something new catches my eye and surprises me. A recent study out of MIT9 found that fasting for twenty-four hours substantially improves the ability of stem cells to regenerate. It appears to promote the growth of new nerve cells in your brain and to expand the brain’s ability to adapt to stimuli. Fasting even does good work on your microbiome, the ecosystem of bacteria that live in your gut. When these bacteria are deprived of food, they secrete a hormone called fasting-induced adipose factor; this hormone instructs the body to stop storing fat and start burning it instead.
In almost every instance, these fasting benefits don’t require going without food entirely. In 2014, I wrote about a special kind of intermittent fasting created to make it painless to ease into intermittent fasting.10 It led, in large part, to all the people who used it collectively losing an estimated 1 million pounds. It’s similar to 16:8 fasting but with a key difference: In the morning, while you’re taking a break from solid food, you drink a cup of Bulletproof Coffee, which contains fats that help people not feel hungry while they’re going without the stuff that they usually eat but keep insulin and protein metabolism silent. “Butter MCT coffee intermittent fasting” sounded silly, so I named it Bulletproof Intermittent Fasting a decade ago. Because it works, it has stood the test of time, and there are hundreds of thousands of views of the videos I’ve done about it.
With this version of intermittent fasting, the type of fats that you consume matters. Corn oil, soybean oil, canola oil, and seed oils contain unstable fats that can contribute to inflammation and other undesirable effects. Grass-fed butter and MCT oil are much healthier. (MCT stands for medium-chain triglyceride, a group of fatty molecules that are relatively small and therefore easy for the body to absorb and process for energy.) These are the types of fats that go into Bulletproof Coffee, and they have been staples of my diet for the past decade.
Bulletproof Intermittent Fasting is a way to turn on the insulin-stabilizing and autophagy benefits of fasting while managing the other, less pleasant biological effects of fasting. I’m talking, of course, about the hungry/hangry f
eeling you get, especially when you are new to the fast and haven’t yet adapted to it. That’s the reptilian part of your brain talking. You can either use a bunch of energy to struggle with it to remain in control or tell it to shut the hell up with a cup of creamy coffee and save the energy for something better. Either way, your body thinks you fasted.
I suggest that you start fasting in the way I have developed through much research and trial—a way that works for almost everyone. Wake up in the morning and drink a cup of Bulletproof Coffee: black coffee, a dollop of grass-fed butter, and a teaspoon or more of C8 MCT oil. It’s the best latte you’ll ever drink. C8 MCT oil is a flavorless extract of coconut oil that has the ability to suppress hunger and increase energy in your body. The quality fats will keep you full until lunchtime. At the molecular level, you will continue autophagy and fat burning. At the personal level, you won’t miss your familiar breakfast at all. Instead of using up precious willpower to fight back against your hunger response, you are biologically pressing the “off” button on your food cravings.
Bulletproof Coffee hacks the feeling of hunger during fasting by boosting the level of ketones in your blood. Ketones are another important part of the biology of fasting. If your body isn’t getting enough glucose from what you’re eating and drinking, your liver and muscles use all the carbohydrates stored in your body, then prepare an alternate source of energy by converting fat into smaller molecules—ketones. The ketones circulate through the bloodstream and go to your muscle cells and other body tissues. This state is called ketosis; your body is literally burning fat as its primary fuel source.
