In The Obesity Code, Jason Fung argues that for decades, we’ve misunderstood obesity and, as a result, have consistently failed in our efforts to curb its spread. To solve this epidemic, he asserts that we need a new, comprehensive theory of obesity. Fung presents this theory, arguing that obesity is a hormonal problem that causes overeating and weight gain, rather than a problem of willpower or self-control. He contends that overcoming it involves improving your diet and fasting regularly.
Fung is a Canadian nephrologist (kidney specialist) and head of the Intensive Dietary Management Program in Toronto. After witnessing diabetes patients gain weight from insulin treatment, he began to explore type 2 diabetes and obesity, two conditions that often lead to kidney disease. The Obesity Code, published in 2016, received positive reviews and has been called one of the most comprehensive treatments of obesity yet written. Today, Fung is recognized as an expert on fasting and obesity. He continued his investigative medical writing in The Diabetes Code (2018) and The Cancer Code (2020).
In this guide, we’ll explain how the prevailing model of obesity falls short and how Fung’s theory surpasses it. Then, we’ll lay out his recommendations for what and when to eat, and we’ll explain his perspective on intermittent fasting as the key to overcoming obesity. In addition, we’ll compare and contextualize his work with other recent theories of obesity, including Why We Get Fat and Salt Sugar Fat.
In this section, we’ll start by sketching the history of modern medicine’s understanding of obesity, before explaining the dominant theory of weight gain. We’ll then break down how that model falls short and lay the groundwork for understanding Fung’s new model of obesity.
According to Fung, the physician Jean Anthelme Brillat-Savarin first identified the fattening role of carbohydrates as early as the 18th century. At the time, sweets, breads, beers, and starches such as white potatoes were typical European foods. Decades later, Englishman William Banting echoed Brillat-Savarin’s low-carb advice: After overcoming his own obesity by eliminating starches and sugars from his diet, he advised the public to do the same. This advice held consensus through the early 1900s, when calorie counting first came about.
(Shortform note: Brillat-Savarin, not a physician but a politician and lawyer with an interest in gastronomy, is best known for writing The Physiology of Taste. In the book, which has been called “the most famous book about food ever written,” he collects witticisms, anecdotes, recipes, and reflections on the appreciation of food and eating. Given that he was a mayor rather than a scientist, it’s all the more notable that he was one of the first to identify the cause of obesity—though it’s likely he didn’t understand the nuts and bolts of the issue.)
Around the 1950s, American health professionals became concerned with rising rates of heart disease. At the time, scientists believed that dietary fat was the problem, so doctors began recommending a low-fat, high-carbohydrate diet. However, Fung says, deaths from heart disease weren’t rising dramatically due to dietary problems—rather, an increase in the efficacy of vaccines and antibiotics had decreased deaths from other killers, such as tuberculosis and pneumonia.
Nonetheless, doctors blamed dietary fat. This was because in the same decade, early studies on the effects of starvation concluded that weight gain and weight loss depend on “calories in, calories out.” In this model, scientists reasoned that whatever caloric energy wasn’t used became fat, so your weight depended on how you ate (calories in) and whether you exercised (calories out). Given this, researchers believed that dietary fat—a high-calorie food—causes weight gain.
(Shortform note: While doctors began recommending low-fat diets to treat heart disease by the 1960s, they were already popular as a weight-loss method. With the invention and availability of the home scale in the early 1900s, Americans increasingly desired to lose weight and maintain the slim figures of their youth—a desire that goes against our natural tendency to gain weight as we age. This trend continued through the 20th century into modern times. Thus, doctors adapted existing low-fat strategies, rather than creating them from scratch.)
This view on low-fat diets became orthodoxy in 1977, when the US Senate Select Committee on Nutrition and Human Needs released the “Dietary Goals for the United States.” This document made official the notion that dietary fat caused both heart disease and obesity. (Shortform note: A record of the original document is still available in archived form. While it contains the misguided recommendations Fung discusses, it also appears to recommend against eating added sugars—which he doesn’t mention, and does agree with.)
Soon followed the “food pyramid” recommendations for Americans, with floury, starchy carbohydrates as the recommended foundation for a healthy diet. Fung suggests that in the buzz about the dangers of fat, people forgot the previous consensus that refined carbs cause weight gain. As a result, the low-fat era began in earnest, with food producers creating numerous low-fat options. At the same time, government-sponsored programs under the banner of “eat less, move more” attempted to quell rising obesity, with little success.
However, some experts began to argue in the 1960s through the 1980s that low-fat, high-carb diets actually cause weight gain, not weight loss. Notably, physician Robert Atkins advocated for a high-fat, low-carb diet that seemed to promote significant weight loss. Against this medical heresy, the American Heart Association (AHA) continued to push a low-fat, high-carb diet and, Fung suggests, took money from big food producers who profited from cheap, refined, carb-based foods.
(Shortform note: Today, the AHA still recommends a protein- and carbohydrate-based diet, while suggesting low-fat or fat-free foods. In the same recommendations, they explain weight loss as a matter of calories and suggest eating less and moving more. Decades later, the AHA still uses what are, according to Fung, outdated and ineffective recommendations. Meanwhile, the diet world has expanded. Atkins-style low-carb diets are still popular, while newer options, such as paleo and veganism, explore alternative ratios of proteins, carbs, and fats.)
Today, the prevailing view holds that obesity is a problem of calories in, calories out. Established decades ago, this orthodox view suggests that weight gain results from eating too much and exercising too little. So, Fung says, experts pushed the “eat less, move more” program on the American public.
On a technical level, we define obesity according to body mass index (BMI). BMI is a ratio of your height compared to your weight—that is, it approximates how wide you are compared to how tall you are. A ratio over 30 is considered obesity.
(Shortform note: While BMI has its uses, it has also received criticism for being an overly simplistic model of weight and health. Among other things, BMI measurements fail to account for body composition (how much fat versus muscle you have), age (it doesn’t work for kids under age 20), and differences in bone mass. Additionally, BMI fails to account for ethnic differences—for instance, South Asians tend to have a higher proportion of fat at a given BMI, while people of African heritage tend to have a higher proportion of muscle at that same BMI.)
Fung argues that the “calories in, calories out” theory fails to capture the full picture of obesity. Excess calories play a part, but they aren’t the root cause of obesity. More specifically, this theory fails to understand that obesity is a hormonal disease, rather than a problem of willpower or bad eating habits.
(Shortform note: Fung is not the first person to point out that “calories in, calories out” is flawed. In Why We Get Fat, science journalist Gary Taubes argues that because it’s an intuitive model—eat more, get fat; eat less, lose weight—we’ve resisted the possibility that it’s wrong. Taubes suggests that the medical establishment has failed the public by adhering to this paradigm and that people aren’t to blame for struggling with obesity.)
And, Fung says, failing to understand the root cause prevents us from beating this disease. He lays out five mistaken premises that hamper the current model of obesity.
Traditionally, experts have reduced all foods to calories, or units of food energy, and treated calories as the most important aspect of any food. However, Fung asserts that not all calories are the same. For instance, 100 calories of high-fructose corn syrup is quite different from 100 calories of salmon. Research supports this view, showing that different foods cause different metabolic and hormonal reactions in the body. Because obesity is a hormonal disease, this means that various foods have different impacts on obesity.
(Shortform note: In Salt Sugar Fat, Michael Moss says that the form a calorie comes in is what matters. Specifically, nutritionists have found that we can consume more liquid calories than solid calories before we feel full. This, Moss argues, is a key reason soda companies such as Coca-Cola became so successful—because consumers could drink multiple cans without much thought.)
As Fung explains, these variables in fact depend on each other. Your metabolism adapts to your caloric intake: When you eat less, your metabolism slows down to conserve energy. When you eat more, it temporarily speeds up, having more energy to burn. In other words, you can’t reduce calories in without slowing calories out, and you can’t increase calories out without increasing calories in. So “eat less, move more” can’t work, because you need to eat more in order to move more, and vice versa.
(Shortform note: Gary Taubes makes a similar point in Why We Get Fat: If you want to work up an appetite for, say, a Thanksgiving feast, you’d eat less to save room in your stomach, and you might engage in strenuous exercise to get hungrier. Yet these are exactly the instructions we’re given for weight loss—eat less, exercise more. Isn’t it strange, he says, that our prevailing theory of weight loss is exactly what we’d do to get hungrier?)
The notion that exercise leads to weight loss is a core tenet of the prevailing theory—it’s the “calories out” part. However, Fung asserts that we’ve misunderstood how the body uses energy: Exercise accounts for a percentage of total energy expenditure, but most energy goes to our “basal metabolic rate,” which includes beating our hearts and powering our brains.
Considering these other activities, exercise accounts for only around 5% of energy expenditure, so it’s too small to make a meaningful impact on weight loss. In addition, studies show that:
(Shortform note: In Why We Get Fat, Gary Taubes reasons that we’ve held on to the belief that exercise produces weight loss because it’s such a straightforward, compelling theory. However, he suggests that if it were true, the poorest individuals—many of whom work physically demanding jobs in factories, warehouses, oil fields, and so on—would be lean. Yet this isn’t the case. Statistically, the poor are more likely to be overweight, despite often working more strenuously than wealthier individuals. Taubes also points out that prior to the “exercise explosion” of the 1970s, many people considered exercise unhealthy. Then, the obesity epidemic accelerated around the same time as the “exercise explosion.”)
Fung explains that eating isn’t a conscious choice—rather, our hormones control when we feel hungry or full.
When you restrict calories to lose weight, your body senses the change and does two things to maintain your natural biological balance (homeostasis). First, it slows your metabolism to conserve energy. Second, it increases appetite by releasing ghrelin, a hunger hormone, and suppressing satiety hormones. You become tired and hungry, and your hormones all but force you to eat and regain the weight.
(Shortform note: In Fast. Feast. Repeat., Gin Stephens explains that when we’ve overeaten and/or dieted extensively, we lose touch with our appetite hormones. To tune back in, she suggests eating 80% of your normal intake and being mindful of how you feel. In time, you’ll learn what it feels like to be truly hungry or full.)
Because the “calories in, calories out” model assumes that people suffering from obesity are weak and slothful, it allows doctors to blame the victim instead of changing their treatments. For decades, Fung says, health professionals have shamed obese people for “failing” to lose weight. Yet it isn’t a personal failing—it’s that “eat less, move more” doesn’t work.
(Shortform note: In American culture, it’s long been socially acceptable to shame and marginalize people with obesity. We’ve long treated it as a moral issue and thus blamed individuals for lacking self-discipline, willpower, intelligence, or other nebulous virtues—today, though, it’s increasingly clear that you could no more beat obesity with “willpower” than you could any other chronic disease. Additionally, an analysis of the social stigma surrounding obesity concludes that, in effect, negative judgment of people with obesity is harmful to their mental and emotional health and hampers genuinely effective treatments.)
Here, Fung explains that it’s a mistake to assume that all unused calories go directly to fat storage. That is, the belief that fat storage works like a waterskin—the more you add without removing any, the larger it gets. However, research shows that various hormones such as leptin and adiponectin mediate fat storage. What this means, Fung argues, is that whether you store excess calories as fat depends on your hormones, not on whether you burn enough energy. Therefore, obesity must be a hormonal problem, not a calorie problem.
(Shortform note: Fung makes a point not to include animal studies in his book, but research on mice suggests that the gut biome also influences fat storage. Some bacteria harvest energy more efficiently from food in the digestive system, and this in turn leads to increased fat production and storage. Conversely, mice without the fat-accelerating bacteria resisted obesity even when fed a diet intended to induce it.)
Given the failings of the dominant theory, Fung contends that we need a new, comprehensive theory of obesity. When we better understand obesity, we can create more effective treatments and turn the tides on this epidemic. Fung highlights two problems with existing theories:
Standards in Medical Research
Developing a strong, well-supported scientific theory is no small task. Various fields have their own standards and best practices for experimentation, interpretation, and reporting of studies, and Fung shows his diligence in pointing out the above issues.
In general, the “gold standard” for scientific studies is the randomized controlled trial (RCT). As Matthew Syed explains in Black Box Thinking, an RCT involves randomly assigning participants to two groups, where one receives the experimental variable (such as a new drug treatment) and the other receives the placebo. Comparing the results of the two groups isolates the effect of the experimental variable. Without the control group, it would be impossible to determine the effect.
RCTs are often expensive and time-consuming, so they aren’t ubiquitous in medical research. However, other valuable testing methods provide distinct advantages—for instance, increasingly large databases of patient health records allow for data-crunching studies that can span years at a much lower cost than an RCT.
In this section, we’ll lay out Fung’s theory of obesity. We’ll start from the full picture and then break down one part at a time, until we’ve unfolded each aspect of the theory.
In short, Fung contends that obesity results from a hormonal dysfunction that centers on chronically high insulin levels. Chronically high insulin throws your body’s fat storage out of balance, disrupts your hunger hormones, and drives intractable weight gain. Here’s how.
(Shortform note: One systematic review of Fung’s central claim (as above) contends that current evidence does not strongly support it. In contrast, the reviewers say that his view is a minority view that is yet to be reconciled with the existing data on insulin and weight gain.)
Fat storage, Fung explains, hinges on the interaction between insulin and your “set weight.” Simply put, normal insulin levels stimulate your body to store a fixed amount of fat, depending on how small or large you are. That default amount of fat is called your set weight, and your body constantly works to maintain it.
If you fall beneath your set weight, your body will store calories as fat in order to reach it. If you go above your set weight, your body will increase your metabolism to burn off fat and reach it. This works similarly to cruise control in a car. Set at 55 mph, you can brake or accelerate but, when left alone, the car will always return to 55 mph.
(Shortform note: Fung explains set weight as a homeostatic mechanism. Homeostasis, which refers to self-regulating bodily processes that maintain balance within an organism, is the central concept of physiology. Normal homeostasis produces good health, and disrupted homeostasis leads to disease. Fung’s theory appears to be consistent with this principle.)
Fung explains that if your insulin levels become chronically high, your set weight increases. Returning to the car analogy, chronically high insulin levels increase the cruise control setting—locking you in at a “higher speed.” Set weight sets the amount of fat you store, so if your set weight becomes abnormally high, your body thinks that you have insufficient fat stores. To correct this, three things change:
(Shortform note: Fung acknowledges that we don’t yet know the exact mechanisms that control set weight. Some studies have shown that in mice, damage to the hypothalamus (a region of the brain thought to influence set weight) can cause a permanent change in body fat levels. Another intervention that can change someone’s set point is bariatric surgery, a range of procedures wherein surgeons remove parts of the stomach or reroute food past it altogether. This reduces food absorption and lowers the set point. Exact mechanisms remain unknown, though these surgeries seem to decrease appetite and normalize metabolism—the opposite effect of the changes Fung describes above.)
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.
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.
(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.)
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.
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.
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.)
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.)
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.)
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.)
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.)
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.)
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.
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.)
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.)
After laying out his theory, Fung explains that you can beat obesity by changing what and when you eat. In this exercise, sketch how you might implement his recommendations.
First, consider your diet composition (what you eat). Recalling Fung’s explanations of the main culprits—refined carbs and added sugars—list anything in your regular diet that might contain these ingredients. For instance, you might eat grocery store white bread or pasta sauce with added sugar.
Next, consider what you can swap them for. Fung stresses that healthy foods should replace unhealthy foods, and that you shouldn’t simply add them on top. Given this, how might you swap the above items for healthier options? For instance, you might learn a simple tomato sauce recipe to cut out the added sugar.
Second, consider your meal timing. Below, briefly sketch out when and how often you tend to eat. As an example, you might have breakfast, lunch, and dinner, plus a snack here and there.
Recalling Fung’s explanation of how frequent eating causes constantly high insulin, how might you change your meal timing to promote weight loss? (You might try intermittent fasting, or simply cut out snacks.)