Eve Mayer
Science. It’s an intimidating word to me, and always has been. Even in high school, where I steadily earned As and Bs without much effort, I was haunted by the one D on my report card in honors chemistry.
This fear and inability to understand basic science helped keep me fat for twenty-four years. Today, I’m no longer obese, and science doesn’t terrify me. Have I suddenly become a scientific savant? Heck, no! Sometimes I still get confused by the simplest terms. But something has changed, and I know that part of the reason I shed so many pounds and got healthier was that I became curious about what was going on in my body.
I’m going to talk to you about the science of fasting in the way I wish someone would have spoken to me: in very simple terms. Dr. Fung and Megan will continue with deeper, more detailed explanations about what happens in your body after you eat, but allow me to describe my own personal experience—and how learning about metabolism, digestion, hormones, and more has helped change my life.
For years, I was fat, and I didn’t want to be. I also struggled with prediabetes, infertility, allergy issues, sinus infections, joint pain, bronchitis, and pneumonia. I went to one doctor after another to solve my problems, and because I’m a hard worker, I did what they said. I ate fewer calories. I worked out. I took medication. I went to therapy. I ate more fruit and vegetables. To get my slow metabolism to speed up, I consumed smaller meals more often. I had lap-band surgery twice, and later I had the size of my stomach permanently reduced with the gastric sleeve procedure. Results varied with each of these actions. But, most often, I lost weight, only to have it return later. Through it all, I never uncovered the root of the problem.
I thought my body was broken. Then I tried a new approach.
At the beginning of 2018, I cut sugar out of my diet and greatly lowered my carb intake, and an astounding thing happened. I was no longer hungry all the time. It was a dramatic—and more than welcome—change, and suddenly I wanted to know why and how my new approach had worked. When I read The Obesity Code, a light came on in my head, and I realized:
Being fat is a problem with my hormones. What I eat and when I eat affects my hormones. Therefore, if I change those two things, I can lose weight.
Doctors had told me I was prediabetic and that there were issues with my insulin, but they had never explained to me what the heck that meant. Why is insulin important? What does insulin do in the body? What is insulin resistance? Why do diabetics take more insulin if increasing insulin is a problem? Why am I taking metformin other than the fact that it will help me not go from prediabetic to diabetic?
Dr. Fung’s approach explained everything to me. Now I understand that my body can either be focused on storing energy or burning energy. But not both at the same time. When I eat often, my body becomes busy packing away energy as fat. When I eat less often, my body has more time to burn energy—and fat. Fasting allows my body to focus its efforts on using energy instead of storing energy. I still have warehoused energy in my body in the form of excess fat. My metabolic and digestive systems are completely capable of using that fat as energy—but not unless I give them the opportunity by not eating for a period of time.
I believe this science because of how I feel. My health issues are gone. I am not prediabetic, I rarely get sick, I take no daily medications, and I feel like a million bucks. When I eat often, I feel hungry, tired, and down—and this is because of my hormones, not because I ate a certain amount of food. Fasting is sort of like a good night’s sleep. I go to bed to let my body and mind rest. Overnight, my body can focus on replenishing and repairing rather than the million things it has to do when I’m awake. Sleeping also allows my mind to process everything that has happened and sort out what to do with all that information. It’s an efficient period of bodily rejuvenation—just like fasting.
If you are fascinated yet overwhelmed by the science of fasting—like I was—take it easy and be kind to yourself. Fasting is worth exploring whether you want to lose three pounds or three hundred pounds, or simply improve your health. The best scientific proof you can ever get is trying it and feeling the changes in your own body.
Jason Fung
There are so many reasons to make fasting a part of your life. From a purely medical standpoint, many diseases are caused in part by excess body fat. Being overweight increases your risk of heart disease, stroke, and cancer. Losing weight increases your high-density lipoprotein (HDL) or “good” cholesterol levels and lowers your triglyceride levels, which helps reduce the risk of those same diseases. Excess weight may raise your blood pressure, lead to or aggravate arthritis, disrupt your sleep, cause back pain, cause liver disease, and more. Type 2 diabetes, which is closely related to increased body fat, is also the number one cause of blindness, kidney disease, nontraumatic amputations, and infections. As a nephrologist, I’ve seen forty-year-olds with type 2 diabetes go into renal failure and need dialysis, which is a life-sustaining treatment they endure for the rest of their years. I’ve seen fifty-year-olds with type 2 diabetes develop poor circulation in their legs and require amputation. I’ve witnessed more people with type 2 diabetes lose their vision than I can count. Yes, in those cases, losing weight would have greatly improved my clients’ health and helped them avoid the diseases and side effects that diminished their quality of life—or, tragically, ended their lives entirely.
But, as Megan discussed in her introduction, there are many people who are not overweight—as defined by body mass index (BMI)—and are still metabolically unhealthy. And there are many people who are overweight—again, defined by BMI—and are metabolically healthy. So, while weight is not the entire story, fasting has been shown to help decrease the prevalence of many metabolic syndromes, including type 2 diabetes.
I know this may be overwhelming, or worse, too good to be true. How can skipping a few meals—or even just one meal a day—make such a difference in your health? Our client Natasha offers an example of just how beneficial a fasting lifestyle can be.
Natasha was diagnosed with type 2 diabetes in early 2012. Even though she’d tried altering her diet, exercising, and taking metformin (a drug prescribed to treat diabetes), almost nothing worked. She didn’t lose weight from her small, 5-foot frame; metformin made her miserable; and her blood sugar spiked even when she ate a tiny amount of carbs.
Natasha had tried fasting, and she liked it, but she was afraid to fast longer than a day. Her fasting coach eased her fears about extended fasting, and, today, Natasha refrains from eating for forty-two hours two or three times a week. Her blood sugar level is now within a normal to prediabetic range, and she’s down to a size 2. She looks great. She feels great. And, best of all, her health has been restored. Fasting has changed her life.
I know there are those of you out there who are scared to fast for a few hours. Even the idea of cutting out something as simple as snacks may make you feel anxious. But, if you’re like Natasha, it could just be fear that’s holding you back. Or, perhaps, you don’t understand the science behind why fasting works, and what it can do for you. Knowledge is power, so allow me to explain how the foods you eat impact your body, why they may lead to hormonal fluctuations that cause weight gain and chronic disease, and how fasting can help.
The second that food enters your mouth, your body begins the hard work of turning that food into cellular energy. However, the path isn’t always easy or straightforward, and if you eat the wrong foods or consume them in excess, your body may develop problems.
The body’s endocrine system includes a vast network of glands that release hormones into the bloodstream to regulate all the body’s functions, including sleep, metabolism (the conversion of food to energy for cell function), reproduction and sex drive, mood, hunger, and more. When we eat, the pancreas—a narrow, six-inch-long organ that sits behind the stomach and is part of both the endocrine and digestive systems—secretes the hormone insulin. Insulin signals to the rest of the body that food is now available to process into energy, and this food energy (calories) needs to be stored away for the future.
The body stores food energy in two different ways: as sugar and as body fat. Sugar is available for quick energy, while fat is kept in reserve, available to burn when our body doesn’t have blood sugar at the ready. Let’s talk about sugar first, as the stable regulation of blood sugar—also known as glucose—is one of the central benefits of fasting.
One of the easiest ways to spike your blood sugar is to eat carbohydrates, which, chemically speaking, are chains of sugars. When we eat carbohydrates, some of this sugar is used by cells in the kidneys, liver, brain, and more. If there are carbs left over, they’re stored in the liver as glycogen, another chain of sugar. We’ll come back to glycogen in just a minute.
The other way our body stores energy is body fat. When we eat dietary fat (found in all kinds of plant and animal foods, from potato chips to red meat to milk), the individual fat molecules, called triglycerides, are absorbed directly into the bloodstream and delivered to fat cells. If we eat too much glucose and exceed the body’s ability to store it in the liver as glycogen, the liver converts this glucose to triglycerides. The triglycerides then feed fat cells.
These two systems of energy storage—glycogen and fat—are complementary. Glycogen is easy to use and simple for the liver to process, but the liver has limited space to store it. Body fat is harder to get to and more difficult for the liver to break down, but it offers the advantage of unlimited storage space (as anyone worried about the rolls of fat on their belly knows all too well!). Think of glycogen as a refrigerator. You can easily put food into it and take it out at a moment’s notice, but you only have so many shelves. Think of body fat like a basement deep freezer. It’s harder to get to, harder to cook the food in it (because it’s frozen), but it’s huge and almost never full.
As I stated previously, insulin is the hormone that signals your body when it’s time to convert food into energy. But its job doesn’t stop there. Insulin also regulates the body’s glucose levels, making sure they don’t spike or plummet. It does so by helping to extract glucose from the blood to store it in the liver as glycogen or in the body as fat. Because the body needs fat for protection, warmth, and energy in times of famine, insulin also prevents us from using too much body fat as a source of energy.
If your insulin levels are high, the body will put food energy into storage, both in the fridge and the freezer. Problems begin, however, when your pancreas goes into overdrive, secreting too much insulin. How does this happen? All foods, which contain a variety of macronutrients (protein, fat, and carbohydrates), will stimulate insulin production to some degree, but certain foods are more effective than others. The worst offenders in this regard are refined carbohydrates, like white bread, sugary drinks, cakes, and cookies.
If we eat lots of sugar or carbohydrate-rich foods too often, as is the case with the typical Western diet, where people regularly eat six or seven carb-heavy meals or snacks a day, our insulin levels will spike. High levels of insulin tell the body to keep trying to store food energy, preventing us from burning our fat stores. We are, in essence, continuing to restock the refrigerator, while wondering why our basement freezer is bursting at the seams.
Eventually, when there is too much insulin flooding your system, the cells in your pancreas that produce it can no longer respond, and your blood glucose levels become high. If they stay high, you can now call yourself one of the estimated 500 million people in the world with type 2 diabetes.
If you become diabetic, your symptoms may include increased thirst, fatigue, blurred vision, hunger even though you’re eating more than you normally do, frequent urination, tingling, pain, or numbness in your hands or feet, or cuts or bruises that are slow to heal. But you may not have any symptoms. Many people discover they’re at risk of diabetes or already have it only after they have a blood test.
There are several tests doctors use to determine if a person has diabetes, but I’ll talk about two of them because many of my clients have them regularly—and many see their results improve dramatically after they try fasting.
The first test is the A1c test. This simple blood test measures what percentage of your hemoglobin—a protein in red blood cells that carries oxygen—is covered with sugar. A1c measures average blood sugar levels over two to three months, so one carb-heavy meal won’t necessarily impact the results. People without diabetes have low A1c levels, between 4 percent and 5.6 percent. If your A1c levels are between 5.7 percent and 6.4 percent, you’re at risk for developing diabetes, often referred to as prediabetic. And if your levels are over 6.5 percent, you have type 2 diabetes.
The other test is called the fasting plasma glucose test, or FPG. This test measures blood glucose levels at one point in time, and it’s given after you’ve fasted for eight hours, typically in the morning. A high result—indicating you have diabetes—is anything over a level of 126 mg/dL. A prediabetic level is between 100 and 125 mg/dL, and anything under 100 mg/dL is considered normal.
If your test results are in the prediabetic range, you’ll need to adjust the foods you eat and perhaps consider some of the medications I’ll discuss below. But remember to focus your health efforts not just on diabetes. Midrange A1c or fasting glucose levels also mean you’re at risk for heart disease, stroke, cognitive difficulties, or insulin resistance (a disease in which your body doesn’t respond well to insulin and raises your blood sugar).
In addition to weight loss, exercise, and diet modifications—typically a diet low in sugar and carbs—the most common treatment for diabetes is prescription medication. Metformin is the gateway drug for diabetes treatment, and it works by limiting the amount of glycogen your liver converts into glucose, as well as helping your body use insulin more productively. Other drugs—including sulphonylureas—help the body produce more insulin or become more sensitive to it, excrete glucose into the urine, or slow digestion. Typically, the last resort for diabetes treatment is insulin, given by subcutaneous injection.
However, it’s disheartening—to say the least—that fasting is not recommended by the health community. Why? Because more than any drug or diet modification, fasting helps control your insulin. Type 2 diabetes is essentially a disease of too much sugar and too much insulin. What decreases sugar and insulin? Fasting. When your insulin is in check, your blood sugar stays in check, your weight stabilizes or decreases, and your risk of developing any number of chronic health conditions goes down.
If I could sum up fasting in one sentence, I would say this: fasting regulates your hormones. It’s more than a diet; it resets your body’s internal controls, allowing it to burn the right amount of energy to keep you alive.
When we don’t eat (fast), insulin levels fall, and this signals our body that no more food is available. In order to survive, the cells then draw upon the stored energy supply, either in the form of glycogen or, if that’s been fully expended, fat. This is the reason we don’t die in our sleep every night, or why we can live a few hours—or a few days or more—without eating. The body has a wondrous ability to store food energy, then find it in either the refrigerator or freezer to burn it.
Therefore, it figures that the most logical solution to keeping our blood sugar levels stable, allowing the body to continue to use its stored reserves of energy, is to fast. By not eating, we allow insulin levels to drop, which tells the body that food is no longer available and that it’s time to eat some of the food in the fridge (glycogen) or freezer (body fat). Weight loss and preventing type 2 diabetes—as well as a host of chronic conditions I’ll outline in the next chapter—are about correcting the underlying hormonal imbalance that caused obesity. This hormonal imbalance, again, is an insulin level that stays high for too long.
But what about metabolism? Doesn’t fasting kill it, as many of you have heard? For that matter, what is metabolism? Our metabolism, or basal metabolic rate (BMR), is the amount of energy (calories) required to keep our bodies alive while we are at rest. BMR is the measure of what we need to keep our body’s very basic functions—such as brain activity, circulation, and digestion—chugging along. If you have a high metabolism, your body burns energy more efficiently, and you tend not to put on weight rapidly. If it’s lower, weight loss will be more of a struggle.
Our BMR is not fixed. Our bodies may increase or decrease BMR by 30 to 40 percent depending on our diets, level of activity, age, body temperature, and more. But, from a dietary perspective, the most significant determinant of BMR is insulin.
The body only exists in one of two states: the “fed” state, after we’ve eaten, and the “fasted” state, when we have not eaten. In the fed state, insulin levels are high, and the body wants to store food energy as sugar or fat. Our metabolism is humming. In the fasted state, when insulin levels are low, the body wants to burn stored food energy. So, we’re either storing calories or burning calories, but not both at the same time.
If we elevate insulin levels (by eating foods that stimulate insulin) and keep them persistently high (by eating constantly—say, by consuming six or seven snacks or meals per day instead of three), then the body must stay in the “fed” state. The body stores calories because those are the instructions we’ve given it. If all the calories are going into storage, then there are fewer calories to use, and therefore the body must slow down its energy expenditure, or BMR.
Suppose we are eating 2,000 calories per day and burning 2,000 calories per day. We neither gain nor lose body fat. We now reduce our calories to 1,500 by eating high-carb, low-fat foods six or seven times per day, as many health professionals urge us to do. Insulin levels stay high but calories drop. Now, the body cannot burn body fat stores because insulin is high, and we are in the “fed” state. With only 1,500 calories coming in, the body must reduce its calorie expenditure to 1,500 as well. We cannot make up this caloric deficit because insulin prevents us from burning fat. We are in “fat storage” mode. This is the dirty little secret of the low-fat diet. At first, the weight comes off, but as our BMR drops, the weight plateaus and then eventually returns.
What happens during fasting? A study of four consecutive days of fasting—that is, four full days without any food to eat—showed that BMR increases by about 10 percent. Yes, metabolic rate increases when you don’t eat. Why? We know that fasting decreases insulin but increases counter-regulatory hormones, so called because they run counter to insulin. If insulin falls, these hormones go up. If insulin rises, these hormones go down. The counter-regulatory hormones include noradrenaline (responsible for stimulating muscle contraction and heart rate), growth hormone (stimulates cell growth and regeneration), and cortisol (the so-called stress hormone, responsible for triggering motivation and action). If noradrenaline increases, then metabolic rate is expected to also go up.
The increase in BMR is likely a survival response. Imagine that you are a caveman. It’s winter, and there is nothing to eat. If your metabolic rate decreases, that means that for every day you do not eat, you get a little weaker. This makes it that much harder to find and hunt for food. It’s a vicious death spiral. As you get weaker, you’ll be less likely to find food. As you don’t find food, you get weaker. If this is what happens to your body, you would not have survived. Your body is just not that stupid.
Instead, your body switches fuel sources. Instead of relying on food, you turn to stored food (body fat), and your body does not shut down. It ramps up by increasing noradrenaline, cortisol, and the other counter-regulatory hormones. You power up by using a different fuel source. Concentration increases. Focus increases. So BMR increases during fasting. If you maintain BMR during weight loss, as opposed to burning 500 calories less per day, that is a huge advantage.
So, the key to the energy balance equation of “Calories In, Calories Out” is not the number of calories we eat and the exercise we do. That is virtually irrelevant. The key is to control hunger and maintain basal metabolic rate. In order to do that, we must eat foods that increase satiety hormones and keep insulin (fat-storing hormone) low. Fasting provides the hormonal changes necessary to successfully lose weight in the long term. Hunger decreases while BMR is maintained. And guess what? Fasting has been used for thousands of years, during which time obesity has been no more than a passing footnote in the pantheon of human illness.
Megan Ramos
When I think about all the clients I’ve met over the years, I can’t even count the number of conditions that fasting has helped improve. Diabetes and obesity are the two obvious ones, but I also think about a woman named Marta, who suffered from acne, PCOS, joint pain, asthma, allergies, gall bladder disease, restless leg syndrome, mood swings, reactive hypoglycemia, fatigue, heartburn, Hashimoto’s, and sleep apnea—in addition to being overweight and receiving a devastating diagnosis of type 2 diabetes. Until she saw success with fasting, Marta didn’t know that all of these conditions can be related—or that fasting could fix them, as it did for her.
But what about the more serious illnesses, such as cancer or Alzheimer’s disease, that Marta might have developed in the future if she kept up her unhealthy lifestyle? There are now credible and compelling scientific studies showing that the science behind fasting doesn’t end with obesity, diabetes, and blood sugar regulation. Fasting is a lifestyle that may prevent a host of chronic conditions that seem to have nothing to do with the food we put in our mouths. Fasting can have significant benefits for your brain, your mood, your risk of cancer, and more.
The brain is a remarkable, complex, and resilient organ, and it’s one that’s not impacted negatively by fasting. So, if you’re concerned that fasting will cause you to be mentally slow, dull, or foggy, worry no more.
Fasting may even help your brain. I say “may” because unfortunately, no authoritative studies exist on fasting’s impact on the brain. However, two human studies—one that measured brain activity after a twenty-four-hour fast and one that measured it after two days—established that reaction time, memory, mood, and general function were not impaired by fasting. And in a study of rats who were put on a fast, the mammals improved their scores of motor coordination, cognition, learning, and memory. In addition, they showed increased brain connectivity and new neuron growth. Now, I know rats aren’t humans, but these results echo what so many of my clients say: that fasting makes them feel sharper.
Evolution also provides some clues to how fasting can help your brain. During times of severe caloric restriction, the organs of many mammals shrink in order to survive. But there are two exceptions: the brain and the male testicles. Obviously, the testicles stay the same size so that the males of the species can continue to attempt to mate, but what about the brain? Think about how you’d feel if you were starving. You’d want to be sharp and focused so you could search for food, right? That’s what happens with most mammals. Conversely, when we eat too much, we may experience brain fog, or what’s often known as “food coma.” Consider how you feel after a huge Thanksgiving dinner: lethargic, dull, and able to focus only on the thought of a nap.
The most encouraging research I’ve seen are the animal studies that demonstrate that rats who were subjected to fasts showed fewer symptoms in models of Alzheimer’s, Huntington’s, and Parkinson’s disease. Fasting induces autophagy—a cellular process that helps the body clear out old or damaged cell parts—and, in these studies, rats on a fast saw a decrease in the accumulated proteins that are a hallmark of Alzheimer’s. Imagine if fasting could prevent, treat, or even reverse these heartbreaking degenerative neurological conditions? Lives could be saved, suffering reduced, and we’d save tens of billions of dollars in healthcare costs.
Cancer is the second-leading cause of death worldwide, killing about 10 million people every year. One out of six people will die from it. Many cancers develop due to genetic factors, unintended toxic exposure, viruses, or some other, often unknown cause. For the most part, these unfortunate cases are difficult to prevent. But there are promising studies that show that cancers that were previously thought to be unavoidable may be preventable, in part, through fasting.
One of the keys to these findings, as with type 2 diabetes and obesity, is insulin. If you extract breast cancer cells from tissue, it’s quite simple to grow them in a lab. If you add glucose, epidermal growth factor (EGF), and insulin, they multiply rapidly. If you then take away the insulin, they die. Let me repeat that: breast cancer cells proliferate with high levels of insulin and die without it. What lowers insulin levels? Fasting.
The second reversible link to cancer is obesity. A 2003 study released by the American Cancer Society highlighted the findings from 900,000 US men and women. From 1982 to 1998, these people were tracked every few years to determine who had died and how they’d died. At each interval, their BMI (body mass index) was also factored. While all were free from cancer at the start of the study, after sixteen years, just over 57,000 of them were dead from cancer. Shockingly, for those with a BMI over 40, the death rates from all cancers combined were 52 percent higher for men and 62 percent higher for women. BMI was positively associated with death from esophageal, colon, rectal, liver, gallbladder, pancreatic, kidney, non-Hodgkin’s lymphoma, multiple myeloma, breast, stomach, prostate, cervical, uterine, and ovarian cancers. Researchers concluded that being overweight or obese accounts for 14 percent of all deaths from cancer in men and 20 percent for women. The evidence was clear: obesity is a major risk factor for cancer. What helps you lose weight? Fasting.
Finally, autophagy may slow down cancerous growths or prevent cancer from occurring—a finding that shocked scientists, who had previously believed that autophagy increased cancer growth. A 2019 study published in Nature concluded that autophagy played a major role in killing certain cells linked to cancer. When autophagy is stopped, these harmful cells can continue to replicate, fueling the growth of cancer. What causes autophagy? Once again, fasting.
Metabolic syndrome, also called Syndrome X, is a group of conditions that meet three of the following five criteria: abdominal obesity (as measured by waist circumference), hyperglycemia (type 2 diabetes), high triglycerides, low HDL, and hypertension.
The common factor among these conditions is that they all involve an excess of insulin. When insulin is too high for too long, the body stores more body fat than it needs to. Cells become overloaded with glucose, and they become resistant to insulin. Glucose from the blood can no longer go into the cells, and blood glucose levels become elevated. This is the disease known as type 2 diabetes. When the liver is overloaded with glucose, excess sugar gets stored as fat, and fatty liver develops. Attempting to unload all this extra fat, the liver exports the glucose into the blood, which causes blood triglyceride levels to increase and HDL levels to decrease. In short, excess insulin causes a series of problems that collapse, one by one, like dominoes.
Since metabolic syndrome is a disease of too much insulin, lowering insulin levels is critical to reversing it. Refined carbohydrates cause the greatest increase in insulin, so eating a diet low in refined carbs and sugar is a great start. Because all foods contain a mix of protein, carbohydrate, and fat, eating even those foods that are healthy will raise your insulin level somewhat. This is why fasting is so effective for treating metabolic syndrome. When you refrain from eating, your insulin levels drop and remain at a lower baseline.
Clearly, fasting helps to stabilize blood sugar. But having stable blood-sugar levels is only one of many benefits of a fasting lifestyle. As we will soon see, it can do wonders for the mind as well as the body.