PDF Summary:The Obesity Code, by Jason Fung

Weight loss—all it takes is eating less and moving more, right?

Not according to Jason Fung, author of The Obesity Code and head of the Intensive Dietary Management Program. For decades, experts have believed that obesity was a matter of excess calories—that is, that people get fat from overeating. Fung challenges this view, laying out a comprehensive theory of obesity as a hormonal disorder that causes overeating, rather than the other way around.

In this guide, we unpack Fung’s theory of obesity, which he developed after observing the weight gain that insulin caused in his diabetic patients, along with his solution for overcoming it. In this guide, we’ll show why dieting doesn’t work and why exercise actually has little impact on weight loss. We’ll explore how the body processes food, learn why not all carbs are bad, and discover which foods to avoid at all costs. In addition, we’ll compare Fung’s ideas with perspectives from similar books on obesity, including Why We Get Fat and Salt Sugar Fat.

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This effectively compels you to overeat in order to store fat until you reach the weight your body “thinks” it should be at. Thus, Fung argues, obesity isn’t caused by overeating—overeating is caused by obesity. The term “obesity” refers to the hormonal problem, and the resulting fatness is the main symptom.

(Shortform note: If we take Fung’s above assertion as true, then our common perception that “obesity = being fat” is wrong. Further, the notion that obesity means being fat likely contributes to the stigmatization of obese people. If, on the other hand, we accept that fatness and overeating are symptoms of an underlying disease, public perception of obesity could change. As an analogy, consider if we shamed cancer patients for being sickly. Doing so seems absurd—those conditions result from the cancer, so there’s no reason to stigmatize them.)

Several factors contribute to the chronically high insulin levels that raise your set weight and cause obesity. They fall into two categories: what you eat, or diet composition, and when you eat, or meal timing. We’ll explain how metabolism and insulin mechanisms work under healthy conditions, and then we’ll contrast that with the disruptive conditions that cause obesity.

How the Body Processes Foods

Fung explains that when you eat, your body breaks down the three main macronutrients—carbohydrates, fats, and proteins—into their constituent parts. Respectively, these are glucose (a basic sugar), fatty acids, and amino acids. (Shortform note: Avoid confusing these with micronutrients, a category composed of vitamins (organic molecules made by plants and animals) and minerals (inorganic compounds that can’t be broken down). The body needs both for healthy function, and many micronutrients participate in the metabolic processes Fung describes.)

Glucose is the body’s primary energy source, found in any foods with carbohydrates—apples, bread, carrots, cake, rice, and many others. In response to increased glucose in your blood, your pancreas releases insulin. Insulin is a hormone, a molecule that transmits “messages” within your body, and it has a few key functions.

• Step #1: Insulin delivers glucose to your cells for energy. Much like a “key,” insulin signals cells to open up and take in glucose for energy.
• Step #2: If there’s excess glucose, insulin takes it to the liver. There, the liver converts the glucose into glycogen, a large string of glucose molecules, for short-term storage.
• Step #3: If there’s still excess glucose, it becomes fat. The liver converts any remaining glucose into fat for long-term storage.

(Shortform note: In addition to insulin, multiple micronutrients contribute to the above processes. Vitamins B2 and B3 help in deriving energy from food, while vitamin B7 helps cells metabolize glucose and vitamin B6 helps convert stored sugars into energy for use throughout the body.)

Fung continues to say that a couple of hours after you’ve eaten, your insulin levels drop and return to normal. At this point, your body has processed the glucose, or food energy, from your last meal, and it’s stored away any excess. You’ll first use the glucose in your cells for energy, running your body on what you’ve eaten. When that blood glucose runs out, your liver converts its stored glycogen back into glucose and sends it into the bloodstream for use as your second energy source.

If you continue not to eat, your glycogen stores will run out and your body will turn to its third energy source—your fat stores. Beyond 24 to 36 hours, your body will convert stored fats (triglycerides) into glucose and fatty acids, which most of your body can use, as well as ketones, an energy source for your brain. (Shortform note: Fasting isn’t the only way to activate ketosis and gain the benefits of ketones. Many people use the ketogenic diet, reducing carbs to 5%-10% of their calories and eating around 60% fats, in order to keep blood glucose very low and induce ketosis without fasting.)

Under normal, healthy conditions, Fung says you should cycle between states of feeding and states of fasting, and this will keep your insulin and blood sugar levels balanced. In turn, this keeps your weight balanced—normal insulin levels keep your set weight at a healthy point. (Shortform note: For most of history, we spaced out our meals enough to keep insulin levels balanced. One major factor that changed this was the rise of fast food, which encouraged Americans to eat on the go—effectively spiking insulin levels throughout the day by allowing people to eat whenever.)

Poor Diet and Frequent Eating Unbalance This System

Fung explains that this system falls out of balance when we eat the wrong foods and when we eat too frequently. Both of these activities increase insulin levels. High and persistent insulin levels cause insulin resistance, a condition wherein the body needs more insulin per cell to successfully deliver glucose for energy.

Insulin resistance is a self-perpetuating feedback loop: The more resistant you get, the more insulin your body produces. The more insulin your body produces, the more resistant you get. And as you become insulin resistant, your chronically high insulin levels raise your set weight, causing obesity.

To emphasize that insulin causes obesity, Fung explains that you can make someone fat by regularly dosing them with insulin. As Fung witnessed, you can see this in diabetic patients who take insulin daily—weight gain is a ubiquitous side effect.

Diagnosing Insulin Resistance

Though insulin resistance has few obvious symptoms, it tends to accompany diabetes or prediabetes. Diabetes, a condition that hinges on the malfunction of insulin systems, has symptoms including:

• Extreme appetite, even after eating

• Tingling sensations in the extremities (possible nerve damage)

• Persistent fatigue

• Susceptibility to infections

Doctors often test for diabetes and prediabetes with A1C tests (measuring your average blood sugar levels over two to three months) or with a fasting blood glucose test (measuring your blood sugar levels after eight hours of not eating). Perhaps the most characteristic sign of insulin resistance is acanthosis nigricans, a condition wherein skin around the back of the neck and armpits darkens and becomes “velvety.”

Let’s break down how diet composition (what you eat) and meal timing (when you eat) raise insulin levels, cause insulin resistance, raise your set weight and, ultimately, cause obesity.

Diet Composition—What You Eat

The first main factor that contributes to chronically high insulin levels is your diet’s composition, or what you eat. According to Fung, processed carbohydrates—specifically wheat and added sugars—contribute heavily to obesity. They cause insulin to spike sharply upward, and they promote overeating despite lacking nutritional value. Added sugars also cause fatty liver and insulin resistance, further contributing to obesity.

Additionally, Fung advises against consuming low-quality meat and dairy, such as those from industrial farming operations. Further, he explains that while not all dietary fats are problematic, trans fats and refined vegetable oils pose considerable risks.

(Shortform note: One common criticism of The Obesity Code is that when it comes to what we should eat, Fung has nothing new to say. For instance, Gary Taubes made much the same argument—that fattening carbohydrates are the main culprit of obesity—six years earlier in Why We Get Fat. In addition, Fung doesn’t clarify what constitutes low-quality meat. Consensus holds that while, for example, grain-fed and grass-fed beef have nutritional differences, both are generally healthy when eaten in moderation within a balanced diet.)

Below, we break down each of these food types.

Processed Carbs

Highly refined carbs are starchy sugars that have been stripped of any fiber, fat, protein, and micronutrients that you’d find in the whole food. (White wheat flour is the prime example.) Because they’ve been processed into a fine powder, the digestive system rapidly absorbs them. This rapid absorption causes a sharp spike in blood glucose levels, which in turn spikes insulin levels. These spikes underlie the development of chronically high insulin levels. Foods such as white bread, snack cakes, crackers, chips, and pastries all have this effect.

In addition to causing insulin spikes, processed carbs encourage overeating. Fung asserts that because refined carbs have been removed from the whole-food context from which they originate, our bodies have no satiety signals associated with them. In other words, they lack the protein, fat, and fiber that help you feel full. This is why you can snack for hours without feeling satiated—refined carbs aren’t filling. Meanwhile, they provide little to no real nutrients.

(Shortform note: In Salt Sugar Fat, Michael Moss reports that major food producers—including General Mills, Kraft, Kellogg, and Unilever—have intentionally engineered processed foods to produce the effects Fung describes. By manipulating the chemical structures of salt, sugar, and fat, these corporate giants have optimized foods for taste and profit. Recounting a 1999 meeting of top CEOs from the major players, Moss describes one executive’s belief that consumers “bought what they liked, and they liked what tasted good.” Unfortunately, what “tastes good” has come at the expense of nutrition and public health.)

The Danger of Added Sugars

In addition to the problems found in all processed carbs, Fung explains that added sugars, such as table sugar and high fructose corn syrup, will severely damage your health.

These sugars are dangerous because they contain fructose, a form of sugar that originates from fruits and vegetables. While every cell in the body can use glucose for energy, only the liver can process fructose. When you consume a high amount of fructose, such as from soda or candy, that sugar goes right to your liver. As Fung describes, a high, concentrated amount of fructose quickly overloads your liver, which races to convert it into glucose and fat.

(Shortform note: While fructose is deadly when concentrated, health experts generally agree that you can safely enjoy fruits. Whole fruits contain fiber and water that balance out their sugar content. Since fruit is filling, it’s difficult to eat so much that you suffer negative effects from the sugar. However, fruit juice and smoothies can disrupt this balance: They strip away the fiber and make it possible to consume much more fruit than you would if it were whole. So in general, stick to whole fruit and avoid juices, especially those with added sugars.)

Before long, high fructose consumption causes fatty liver, a condition wherein your liver is overstuffed with fat and sugar. To compensate, your liver becomes insulin resistant, requiring more insulin per unit to continue taking in fructose.

As we’ve explained, insulin resistance contributes to chronically high insulin levels: Resistance leads to increased levels, which leads to more resistance, and so on in a self-reinforcing feedback loop that drives obesity. So added sugars, especially fructose, contribute to obesity. Note that this isn’t a problem of excessive calories, but a problem with the nature of the food.

(Shortform note: Fatty liver occurs in two varieties: alcoholic fatty liver disease (AFLD) and non-alcoholic fatty liver disease (NAFLD). The disease progresses through four stages, becoming increasingly severe until it culminates in cirrhosis, a serious condition of liver scarring and failure that can cause jaundice, brain fog, nausea, and weight fluctuations. Studies on rodents have shown that high fructose corn syrup causes fatty liver; however, a 2021 study on pigs, a larger mammal, found that a four-week diet of 60% fructose did not induce fatty liver.)

Meat and Dairy Also Stimulate Insulin

While refined carbohydrates are a main concern, Fung explains that in fact, all foods raise insulin levels. This occurs because of two hormonal mechanisms: the incretin effect and the cephalic phase effect. Put simply, your body releases insulin in anticipation of eating as well as when food reaches your stomach. Proteins stimulate far more insulin release than dietary fats, and vegetable proteins stimulate far less insulin release than meat and dairy. Given this, Fung says that we need to consider the meat and dairy that we eat.

(Shortform note: In most cases, people with diabetes lack the incretin effect. This is because in diabetics, the pancreas no longer responds to GIP, a key incretin hormone, so insulin doesn’t release as normal. Studies have found that bariatric surgery may restore the incretin effect, though drug-based therapies do not.)

Meat: According to Fung, meat is generally associated with weight gain. One study he cites found that for each additional daily serving of meat, subjects gained one additional pound. Fung speculates that this may be due to the industrial production of meats—that is, animals fed with corn and grains, rather than their natural diets, produce meats that are less healthy for consumption. (Shortform note: Grain-fed beef, for example, does have a higher proportion of saturated fat than grass-fed beef. Additionally, factory-farmed meats are often treated with antibiotics and drugs—in effect, eating grain-fed beef means eating the meat of an unnaturally obese cow. Some believe that this is intuitively unwise, though the science is still out.)

Dairy: In contrast to meat, research finds that dairy products such as whole milk and cheese do not cause weight gain. In fact, large studies found that some dairy products, such as yogurt, may even promote weight loss. Fung suggests that this is because dairy is very filling: While you could eat a pound of chicken, you’d be hard-pressed to eat a pound of cheddar.

(Shortform note: Other sources report that the science on dairy is mixed. Studies seem to swing back and forth: One reports that dairy does not contribute to weight loss unless it’s part of a calorie-restricted diet; another reports that fermented dairy such as yogurt might protect against heart disease. In the end, most experts agree that dairy is a good source of calcium, protein, and dietary fat, yet it’s not strictly necessary for a healthy diet.)

Bad Fats and Good Fats

According to recent research, there’s little relationship between obesity and the amount of dietary fat you consume. Studies have found that saturated fats, long thought to cause heart disease, may actually have a protective effect on obesity since they’re highly satiating. In addition, Fung advises avoiding refined vegetable oils, such as canola, soy, and corn, as well as trans fats, such as margarine. Each has been linked, respectively, to inflammation and heart disease.

(Shortform note: Two of the most important fats to consume are omega-3 and omega-6 fats. We need each in the right amount, though today we tend to eat far more omega-6s. Fatty fish, nuts, and eggs are a few of the best sources of omega-3s, while omega-6s are found in everything from soy and corn to red meat, poultry, eggs, and some seeds.)

Meal Timing—When You Eat

The second main factor that contributes to chronically high insulin levels is meal timing, or when you eat. Fung explains that chronically high insulin doesn’t result from insulin-spiking foods alone—it also requires constant insulin stimulation. When insulin levels are both high and constant, your body’s cells become insulin resistant. That kicks off the self-perpetuating loop of chronically high insulin that drives obesity.

Constant insulin release occurs when we eat too frequently. Insulin levels increase in response to food, and they decrease between periods of eating. If you leave little or no time between periods of eating, you never allow your insulin to return to normal levels. The more frequently you eat, the faster you’ll create insulin resistance in your body.

(Shortform note: Additional risk factors for insulin resistance include smoking and drinking alcohol in excess. Chronic cigarette smokers appear to have a higher insulin response and take longer to clear insulin away, while a rat-based study found that binge drinking as often as one time monthly decreased insulin sensitivity. This effect persisted for 54 hours, and the researchers speculate that this is due to impaired function of the hypothalamus, which communicates with insulin to coordinate insulin sensitivity and set weight.)

In addition, eating constantly keeps your glucose and glycogen stores full, preventing you from burning your fat stores as fuel. Fung explains that neither snacking nor breakfast are strictly necessary.

Snacking: According to Fung, snacks have little or no nutritional value and no place in a healthy diet. Snacks are often made of refined carbohydrates, such as wheat and sugar, so they spike your insulin levels and contribute to obesity. Fung suggests that “Big Food” companies created snacks to generate more profit, and that they’ve disrupted the traditional “three square meals” eating pattern that allowed insulin to drop between meals.

(Shortform note: The Hartman Group, an organization that tracks trends in consumption of foods and beverages, reports that snack culture is shifting toward healthier, “fresher” snacks. Legacy companies still hold a large share of the market, but consumers increasingly want healthier options and more diverse flavors. This isn’t just packaged snacks—restaurants and other grab-and-go style establishments now offer quick, casual “snacks”—think smoothies, acai bowls, burritos to go—that occupy a “gray area” between legacy snacks and sit-down meals.)

Breakfast: Much like snacks, Fung contends that breakfast foods are unhealthy and contribute to obesity. Like snacks, many breakfast foods are made of cheap, processed carbs, and “Big Food” pushes the notion that “breakfast is the most important meal of the day” in order to profit from unaware consumers. Research shows that breakfast is inessential to good health, and Fung recommends skipping it or replacing carb- and sugar-heavy breakfast foods with whole, unprocessed foods such as salmon or a salad.

(Shortform note: In addition to marketing breakfast as essential, which Fung notes, the creators of breakfast cereals used religious moralizing to push their wheat-based cereals. James Caleb Jackson, who invented the first breakfast cereal, and John Harvey Kellogg, originator of Kellogg’s breakfast cereals, were Seventh-Day Adventist preachers who believed that a strict vegetarian diet and whole wheat cereals could counter ill health. Further, cereal became known as a reliable kickstart to the workday, such that people associated breakfast with increased productivity, another nearly moral imperative to American society.)

The Effects of Chronic Stress

In addition to what and when you eat, Fung explains that chronic stress contributes to weight gain. When you’re under stress, your body releases cortisol. Cortisol temporarily raises blood glucose levels, which triggers the release of insulin. Chronic stress causes constant cortisol release, which causes constantly elevated blood glucose, which in turn causes increased insulin levels. And as we’ve explained, chronically high insulin drives obesity.

Given this, Fung argues that chronic stressors, such as sleep deprivation and work-related psychological stress, contribute to obesity. To offset these stressors, Fung recommends practicing active stress reduction. Get at least eight hours of sleep nightly, since research shows that missing even a few hours increases insulin resistance and contributes to weight gain. You can also try practicing mindfulness meditation, yoga, or other mind-body activities that lower stress levels.

(Shortform note: Aside from the immediate increase in stress, sleep deprivation has numerous downstream consequences. The American Psychological Association reports in their 2013 survey of stress in America that adults get an average of 6.7 hours of sleep, less than the 7 to 9 recommended hours. Sleep-deprived adults are more likely to feel irritable, overwhelmed, or apathetic, and they’re also more prone to snapping at spouses or children in angry outbursts. Moreover, many report feeling depressed, unfocused, and sluggish. In effect, sleep deprivation leads to a self-perpetuating feedback loop of stress and personal stagnation.)

How to Overcome Obesity

Now that we’ve described Fung’s theory of obesity, let’s look at how he recommends overcoming it. In short, beating obesity means changing what you eat as well as when you eat—mirroring the argument that diet composition and meal timing contribute to high insulin, thus causing obesity.

In practice, this means eating a healthy diet composed of whole foods and engaging in regular, 24- to 36-hour fasts. Whole foods help you feel full and don’t spike your insulin, while regular fasting lowers your insulin levels long enough to reverse insulin resistance. When insulin resistance reverses, your set weight will drop and your body will adjust by burning off fat until you reach that weight.

The NOVA Food Quality Guidelines

In Fast. Feast. Repeat., Gin Stephens offers much the same solution as Fung, albeit in a less prescriptive way. Stephens stresses that restricting your diet makes it more difficult to stick to, and that meal timing is ultimately more important than what you eat. However, this doesn’t mean that food quality is unimportant. Stephens suggests using the NOVA classification system, a set of guidelines devised by researchers from the University of Sao Paulo, Brazil:

• Category 1: Unprocessed foods, including whole fruits, vegetables, and meats

• Category 2: Processed culinary ingredients, such as salt, flours, butter, and oils

• Category 3: Processed foods, including bread, cheese, pickles, and canned foods

• Category 4: Ultra-processed foods, such as sugary cereals, ice cream, soft drinks, and “instant” meals like cup ramen

Stephens advises sticking mainly to categories 1 through 3, though she allows for selective enjoyment of your favorite category 4 items. In contrast, the researchers suggest avoiding them altogether, if you can.

Change What You Eat

Fung outlines several diet changes that reduce your insulin levels and combat obesity. In general, a healthy diet involves a high amount of healthy fats, a moderate amount of healthy proteins, and a moderate amount of healthy carbohydrates.

Change #1: Avoid added sugars. Forego sugar-sweetened drinks such as soda, refrain from snacking on sugary snacks such as candy, and avoid sugary breakfast foods. Feel free to enjoy quality desserts, like bakery-made cakes and cookies, on special occasions, but not every day.

The alternatives: Instead of consuming sugar, choose unsweetened drinks and foods. Snack on nuts and fruits in moderation, and choose unsweetened coffee, tea, or water. For breakfast, choose healthy foods such as a vegetable omelet. For dessert, enjoy a square of dark chocolate with cacao content over 70%.

(Shortform note: While Fung takes a nearly zero-tolerance approach to added sugars, Gin Stephens argues in Fast. Feast. Repeat. that you can enjoy treats more often. She recommends that you “delay, don’t deny”—that is, wait to eat your favorite desserts rather than avoiding them altogether. If you keep them to, say, the weekends, you can still lose weight.)

Change #2: Avoid refined carbohydrates, primarily wheat. Flour-based foods spike insulin levels and drive obesity. By removing them from your diet, you increase your chance of beating this disease. According to Fung, refined wheat products have no place in a healthy diet. While whole wheat retains some protein and fat, it’s still a highly refined powder that will spike insulin.

The alternatives: Instead of consuming refined carbs, choose whole carbs and eat them in moderation. This means many vegetables—such as tomatoes, broccoli, and zucchini—and healthy seeds and grains. In particular, Fung recommends quinoa, chia, and beans, all of which contain carbohydrates, fiber, protein, and various micronutrients.

(Shortform note: One doctor advises against certain fruits and vegetables. In The Plant Paradox, Steven Gundry argues that lectins, compounds found in some fruits and vegetables as well as meat, dairy, and certain medications, are at the root of many modern health problems, including obesity. He asserts that lectins are toxic and lead to long-term health consequences such as joint pain and obesity. His “PPP” diet involves removing from your diet any foods that contain lectins—including potatoes, grains, beans, and seeds such as chia and quinoa.)

Change #3: Choose healthy proteins and fats. Fung recommends eating 20% to 30% of your diet as protein. In his view, high protein diets are difficult to follow, since isolated protein is difficult to find in whole food forms and forces you to rely on artificial substitutes, such as protein powder. Choose grass-fed meats, wild-caught seafood, and whole, vegetable proteins. For fats, choose unprocessed fats including olive and coconut oils, natural lards, and fatty foods like avocados. Healthy fats stimulate insulin the least and may protect against obesity.

(Shortform note: While Fung asserts that saturated fats have been unfairly demonized, the science is not conclusive on their health effects. To play it safe, experts recommend limiting saturated fats from meat, eggs, and dairy and instead consuming unsaturated fats from plant sources. When it comes to protein, research links consumption of plant proteins to reduced risk of stroke, cancer, and heart disease compared to consumption of animal proteins.)

Change #4: Eat foods that suppress insulin spikes. Some foods, such as those high in fiber, can offset the insulin stimulation caused by carbohydrates and proteins. Fung recommends consuming whole, fibrous foods such as fruits and berries, seeds, and whole oatmeal. In addition, vinegar and fermented foods, such as kimchi, reduce insulin spikes and help you feel fuller. (Shortform note: While fermented foods are known for their health benefits, they aren’t free of downsides. For instance, kimchi has the potential to develop harmful bacteria and has been linked to outbreaks of E. coli. In addition, it has high sodium content that may not be appropriate for everyone.)

Change When You Eat

Fung argues that intermittent fasting is the key to beating obesity. Combined with a healthy diet, intermittent fasting heals insulin resistance—here’s how it works.

In the short term, most diets produce weight loss. However, dieting alone can’t fix obesity, because dieting doesn’t lower your set weight. Once you fall beneath that set weight, your body will compel you to eat and regain any weight you’ve lost. To lower your set weight, Fung recommends intermittent fasting. Intermittent fasting is an eating pattern wherein you intentionally stop eating for a stretch of time. Research has shown that regular fasting reduces insulin resistance, lowering your set weight and allowing you to steadily lose fat.

(Shortform note: In Fast. Feast. Repeat., Gin Stephens explains that long-term dieting causes “diet brain,” a mindset condition wherein you become disillusioned with dieting as a solution to weight loss. She agrees with Fung that intermittent fasting is the solution to obesity—specifically, she recommends Alternate-Day Fasting and suggests trying various lengths until it feels right for you. While Stephens is not a health professional, she’s run several online support groups that have helped thousands of people overcome obesity.)

Fung explains that when you fast beyond 24 hours, insulin levels fall and your body begins to derive energy from your stored glycogen and fat. Beyond two to three days, ketosis kicks in: Fat becomes your body’s main energy source, producing three energy molecules—glucose, fatty acids, and ketones—that fuel your body and brain. (Shortform note: Stephens calls ketosis your “fat-burning superpower,” and she encourages people to embrace fasting as an enjoyable lifestyle rather than self-deprivation. In Stephens’s view, the benefits of fasting vastly outweigh not eating as often—and she says that in time, you won’t even want to eat all the time.)

According to Fung, fasting is a time-honored practice that humans have used to maintain good health for thousands of years. During our hunter-gatherer past, food was periodically scarce. To handle this, our bodies adapted to store fat as a back-up energy source in times of need. Since then, physicians in ancient cultures, such as ancient Greece, have lauded fasting as a powerful cure for many illnesses. Many religions prescribe fasting as a regular “purification” practice, and over three billion Muslims, Christians, and Buddhists fast as a normal part of life.

(Shortform note: Some researchers argue that we shouldn’t romanticize the idea that our ancestors fasted for health. The natural environment provided an abundance of food during most seasons, and fasting for days or weeks was rarely necessary. Further, our ancestors wouldn’t have fasted unless necessary, such as if famine or disaster struck. Voluntary religious fasting began after the agricultural revolution, when we had the certainty of stored food.)

While some worry that fasting is unhealthy, Fung explains that these concerns are misplaced. He argues that fasting does not cause your body to consume your muscles for fuel, damage your metabolism, deplete your blood sugar, or deprive you of essential nutrients. In contrast, studies show that fasting regulates blood sugar and electrolytes, produces an increase in human growth hormone that conserves your muscles, and increases your metabolism once ketosis kicks in. For these reasons, Fung asserts that anyone can fast successfully and reap powerful health benefits.

(Shortform note: Some bodybuilders use intermittent fasting to accelerate muscle gain and stay lean. This happens because fasting changes your body composition, or the ratio of muscle to fat. Your body preferentially burns fat while pumping up human growth hormone, and some believe that leveraging this to stay lean while building muscle is the optimal way to work out.)