by Dave Asprey
So why do so few people fast today, except as part of a religious ritual (or as part of a miserable, calorie-counting diet built around self-denial)? Mostly it’s because of the horrible emotions and feelings associated with fasting, especially with the first stages of fasting. You can blame that on your ancestors—or rather, on the evolutionary processes that shaped them. Despite the ready abundance of food in the modern developed world, there is a signaling process built into your brain that says, “Don’t fast, because fasting equals danger and fasting equals pain.”
Here is a message for you to send back to your brain: “That’s bullshit.”
It’s as important for you to fast as it is to eat a meal. Sounds insane, right? You have to eat in order to live. But you also have to fast in order to live well. Eating is effortless, but the ability to complete a fast is also effortless once you learn the skills and the mind-sets in this book. You can skip eating every now and then. You can look better and feel better and have the energy to do all the things you want to do—consciously, deliberately, freely.
FOOD ON THE BRAIN
Fasting makes you sharper. Well, of course it does. Think about what happens to animals when they don’t eat: they become fully attuned to their environment so they can find food. That’s another universal, fundamental evolutionary pressure, one that very much shaped what Homo sapiens is today.
As an illustration, I’ll tell you a story of an old friend of mine, a guy named Chris. He served in the army doing Special Forces–type long-range patrols in hostile territory. During training, his small team had to make their way across rough terrain wearing heavy backpacks, going for two or three days without any food. The point was to show them that they could do the seemingly impossible—schlep eighty-pound backpacks up mountains—while fasting. The training was intended to make the team feel as though they were starving and learn that the feeling wasn’t true. To show the team how fasting sharpened their senses, the folks in charge of the training would hang a cheeseburger high up in a tree at their destination.
Chris told me he could smell that cheeseburger from two miles away. Don’t believe it? Neither did I at first, but he swore it was true, and he’s an honest guy. “Absolutely, one hundred percent of us could smell the cheeseburger,” he said. “They didn’t tell us they were going to put the cheeseburger there. We just smelled it. We knew it was there, and we were drawn to it.”
That’s the remarkable power of your body and your brain working in unison. When you skip eating for a whole twenty-four hours, your senses sharpen, and your focus increases. The less toxic material you have in your bloodstream and lymphatic system, the higher your ability to reason. This is because the tremendous amount of energy that your internal organs normally require for digestion is now being shunted to the brain. Once toxins have been eliminated, the brain begins receiving healthy, toxin-free blood, allowing it to use its resources more efficiently. Your capacity to focus on anything you want to increases as well, because your brain is no longer distracted by food. You can focus on your work, you can focus on how you feel, you can focus on your meditation—you can focus on whatever you want. Intricate problems that once stymied you may seem easy to resolve.
Many people report that fasting provides them insights into themselves like they’ve never felt before. On a long fast, the feelings of emotional stability and euphoria can be life changing. Why do you think every great religious practice involves periods of fasting? Why do unrelated cultures all around the world link fasting with spiritual enlightenment? Because when you sharpen your brain, you sharpen all of your abilities.
Now, it’s important to manage your expectations. The first time you fast, your brain isn’t going to be all that concerned with a higher state of enlightenment. If things were that easy, humans would never have stopped fasting. Instead, you’re going to have to train yourself not to think about cookies or whatever your craving happens to be. Keep in mind that every thought takes energy. It takes electrons. In the same way that every app you run on your phone sucks battery power, every thought in your head sucks brainpower.
On a daily basis, studies show that about 15 percent of your thoughts are some iteration of “What’s for dinner? Do I need food right now?”8 If you’re on a diet, that number may be closer to 50 percent. Those food fixations are evolutionary relics that no longer serve a useful purpose, like your wisdom teeth or your appendix. (Actually, the appendix may serve a function in maintaining the vigor of your microbiome,9 so your cravings are less useful than your appendix!) By kicking up your metabolism and powering up your neurons with ketones, intermittent fasting teaches the brain not to waste time on obsessive food thoughts.
After you’ve done some of the basic fasting using the techniques laid out in this book, I guarantee that the amount of time you spend thinking about food will decrease. And if you also cut down on the foods that cause inflammation, you may find it remarkable how quickly you can shut down those thoughts entirely. There’s an old myth that humans use only 10 percent of their brains (totally untrue), which has inspired many fanciful stories about the astonishing things we could do if only we could tap into the full power of the modern, evolved brain. Well, here’s something that’s not myth at all but real and testable. You can cut down on food-related thinking and free up more time and energy in your brain for everything else—just through intermittent fasting.
Speaking of thinking, there are two primary types of cells in the brain, and they both influence our relationship with food and hunger. One is your neurons, the brain’s rock stars; everyone knows about those. But there are also the glial cells, or just glia, which are a lot less famous even though they are just as abundant in the brain: you have about 100 billion of each. The glia provide structural support, insulation, management, and nutrition for the neurons. Significantly, they also function as the brain’s immune system. The glia are the pruners and maintainers of the brain. When your glia get disturbed, that’s when inflammation in the brain happens. Disturbed glia can trigger pain and stimulate overeating.10
That’s not something you want.
Neurons and glia can both use glucose or fat for fuel, but they have radically different tastes. Glia run best when you have some glucose in your body for them to use, and they get stressed when blood sugar falls too low. Neurons run best when you have ketones from fat they can use as fuel, but they’re happy to use sugar as a backup. In nature, if you’re fasting, you have high ketones and happy neurons, and you feel great. If you’re eating anything with carbs, you have less-energetic neurons but good brain maintenance by happy glia. Which is better?
Remember that neurons are extremely energy hungry; they need lots of chemical fuel so you can think at your full capacity. If you’re really, truly hungry, your blood sugar’s low, and you’re not in ketosis. Here’s what happens: you can’t think clearly. Your reaction time is delayed, and you feel slow because your neurons aren’t able to get enough raw material to make electricity. But if your brain and body are running on ketones instead of glucose, something remarkable happens: the ketones have an inflammation-calming effect on the glia, because ketones are anti-inflammatory. Fasting gives you those ketones, which calms the glia, which improves the function of your neurons, which makes your thinking sharper.
At least, that’s the goal. If you’re doing a multiday fast and your body needs glucose for high-priority functions such as brainpower, you will enter a state called gluconeogenesis. Some proponents of the paleo diet and the dirty keto diet fetishize gluconeogenesis. The idea is that once you enter this state, your body will make all the carbohydrates you need from the proteins in your body. Technically, it is true: if you’re starving or on a really long fast, you can lose muscle, because your body will break down protein from your muscles to make carbohydrates. The problem is that turning protein into sugar is biologically very expensive. It also leaves all sorts of waste products in the body, including ammonia. Those waste products lead to inflammation, which in turn leads to crav
ings and the muffin top.
The dirty keto folks have a whole theory of why this is a good thing. And they’re right that brief periods of protein deprivation help promote autophagy, but you don’t need too much of a good thing. When you endlessly combine fasting and keto, you become metabolically inflexible because your cells don’t handle carbohydrates well. You’re better off to fast sometimes, be in ketosis sometimes (fasting from carbs), and be in carbohydrate-burning mode sometimes.
What you want to do is turn on that gluconeogenesis as fast as you can and get into ketosis as fast as you can. That is the goal of intermittent fasting, and Bulletproof Intermittent Fasting is designed to help get you there. When you burn ketones, you enable your glial cells to go efficiently about their brain maintenance and anti-inflammatory tasks—the tasks that evolution shaped them to do.
A TUNE-UP FOR YOUR MEMORY MACHINE
Intermittent fasting is crucial not just for your in-the-moment mental clarity but also for the long-term well-being of your brain. It’s extremely difficult to gather clean clinical data on the effects of intermittent fasting on human brain health; such studies take a long time and require separating the effects of eating habits from all the other things people do. But recently there has been a lot of highly encouraging evidence from lab animal experiments.
In study after study with rodents, intermittent fasting has been shown to increase memory, learning, and neurogenesis—the generation of neural cells. A team at the National University of Singapore reported in 2019 that intermittent fasting particularly leads to the growth of new neurons in the hippocampus, a part of the brain associated with learning and the conversion of short-term recall to long-term memory.11 Mark Mattson, the neuroscientist at Johns Hopkins University, has noted that fasting and exercise seem to boost a protein called brain-derived neurotrophic factor, or BDNF, which increases the number of energy-generating mitochondria in the hippocampus and also promotes neurogenesis.12 Intriguingly, Mattson and his colleagues have found that fasting elevates levels of another protein, SIRT3 (Sirtuin 3), which causes the mitochondria in the hippocampus to operate more efficiently.13 Sure enough, rats that undergo intermittent fasting perform better on learning and memory tasks.
Do these results apply to humans? I believe so. When I measured my hippocampal volume using a functional magnetic resonance imaging, or fMRI, machine, I was in the 87th percentile for my age. Given that your hippocampus shrinks over time, this is evidence that mine isn’t shrinking or has at least grown back. Since I had chemically induced brain damage from toxic mold in my twenties, the odds are that mine has grown back. I’ll take that!
If things go wrong in the brain, fasting seems to help there as well. Our friend Mattson and his colleagues ran yet another rodent study and found that fasting leads to faster recovery from stroke, apparently by taming inflammation in the brain and by speeding the repair of injured neurons.14
The jury is still out about to what extent fasting can protect against neurodegenerative disorders such as Alzheimer’s and Parkinson’s diseases, although a 2018 rodent study out of South Korea reported promising findings.15 One possible link is that ketones, unlike glucose, do not contribute to the buildup of destructive plaques in the brain. Valter Longo, a gerontologist at the University of Southern California, is filling in more pieces of the puzzle. His research has shown that fasting decreases the biochemical markers for diabetes (an Alzheimer’s risk factor), as well as cardiovascular disease and cancer. He concludes that fasting reprograms the brain’s metabolism and helps clear out malfunctioning cells, including the self-destructive immune cells that cause multiple sclerosis.16 High levels of BDNF are associated with a lower risk of developing Parkinson’s, Alzheimer’s, Huntington’s, and multiple sclerosis. And by suppressing inflammation in the brain, fasting also seems to improve blood circulation, which in turn helps preserve cognitive health.
This long list of health benefits may seem hard to believe. How can fasting do so many good things for you? The answer is simple: your body is already packed full of repair and rejuvenation mechanisms. Thousands of years—no, millions of years—no, billions of years of evolution have shaped them inside of us. Your ancestors have run the gauntlet of death and extinction. You and I are the survivors of an inconceivably brutal winnowing process. We wouldn’t be here if we didn’t have all of those health-preserving cells and molecules.
All that fasting does is remove the dietary impediments we put in our own way and allows us to take maximal advantage of the gifts evolution has given us. Fasting puts us in control of a 4-billion-year-old evolutionary process.
In many ways, the most important way fasting puts you in control of your brain is that it makes you feel better about yourself. By associating food with certain times of the day, family gatherings, and emotional comforting (why do you think we call it “comfort food”?), we give it overwhelming power over us. In the process, we turn a blind eye to the foods we’re eating—and the ways we’re eating—that do the opposite of making us feel comforted or nurtured. A food hangover is a very real thing. It is the price we pay for indulging in certain low-quality, inflammatory, toxin-laced foods just because we’ve been conditioned to crave them or to derive short-term comfort from them.
Try this experiment in food awareness: For one single day, pay close attention to how your body feels after each meal. Really pay attention. Are you sluggish after eating a certain food? Revved up? Do you have minor abdominal discomfort? Or worse, major abdominal discomfort? As you go through this experiment, pay close attention to mood swings and anxiety—sensations not often associated with food but just as common. Think about how your brain and body are responding.
Through most of the history of our species, it was unusual to encounter sugar-rich foods that would deliver rapid spikes of calories. Many of the ingredients of the modern diet have been cultivated by humans for only a couple hundred years or less—far too little time for our bodies to have adapted to them. Modern wheat is full of starch. Modern fruit is a sweet piece of candy compared to wild sour apples. Modern corn, bursting with sugar, didn’t exist in much of the world until recently. And canola oil was known as “rapeseed oil” and considered inedible until it was put through an elaborate industrial detoxification process. That doesn’t necessarily mean that modern fruits, vegetables, or grains are bad, exactly, but research shows that most of them contain more sugar than is ideal, and some of them (such as corn, soy, and wheat) really do harm your gut bacteria or your metabolism. Intermittent fasting helps reduce the potential for that harm, regardless of what you’re eating.
The inflammation that comes with eating manufactured foods also plays an important role in how our brain perceives emotional health. A glass of reconstituted orange juice is sugar water—nutritious sugar water, but sugar water nonetheless—that elevates your blood sugar in a hurry and brings you right back down in a hurry. Feelings of anger and increased hunger often follow. It goes without saying that sugar-sweetened soda does the same thing, just without the nutrition. But did you know that diet sodas can lead to feelings of depression? Modern artificial sweeteners mimic the taste of sugar so well that the receptors in your stomach have trouble distinguishing between real soda and diet soda, so they release insulin just the same.
So-called light salad dressing follows the same path, eschewing sugars and high-fructose corn syrup but using aspartame, an artificial sweetener linked to depression. Ketchup is loaded with sugar. Pasta and white bread are often made of highly processed white flour that quickly turns to blood sugar as you eat, resulting in energy spikes, depression, and anxiousness. Eating fried foods, pizza dough, cakes, cookies, and even crackers can lead to feelings of depression, as can eating candy, pastries, processed meat, and refined cereals. They are all out of sync with our dietary evolution. The more you practice fasting, the less you’re tempted by these low-quality offerings from Big Food.
In addition to its cellular repair mechanisms, your brain also contains higher-level emotional
repair mechanisms that are just waiting to be activated. After a period of fasting, the brain instructs the adrenal glands—two small triangular glands located in your lower back atop the kidneys—to release catecholamines. These chemicals are neurotransmitters, molecules that trigger focused neuronal responses. Back when our ancestors were mostly hunting for a living and often going days without eating, catecholamines served a vital survival function: they helped keep the hunter optimistic and prepared for the exertion of the pursuit. Catecholamines include several famous mood-altering and energy-boosting chemicals, including adrenaline, norepinephrine, cortisol, and dopamine. You may recognize these as the chemicals, or the chemical targets, that doctors often prescribe to battle depression and stress. Your body also uses them to stabilize your blood sugar should it fall too low during a fast.
How wonderful is it that your body produces these mood elevators naturally during fasting? This is the “go mode” I felt at the end of my vision quest (more about that soon). The same feelings of happiness and well-being are available to everyone, anytime, without pharmaceutical assistance.
Food is built into your evolutionary history. Taking control of your food consumption can be the key to your future happiness.
5
Fast for Better Sleep; Sleep for a Better Fast
At last I was heading into the solitary adventure I had yearned for from the start. Delilah drove me to a new trailhead and gave me directions to find First Woman cave, located about a half mile down a canyon.
