Fung introduces three Golden Rules for weight loss: “avoid ultra-processed foods,” fast regularly, and “commit to a health mindset and healthy social habits” (xv). He argues that weight is primarily regulated by hormones and different foods trigger different hormonal responses; for instance, cookies stimulate insulin (fat storage), while eggs stimulate GLP-1 (fullness), making the hormonal “information” in food as important as its calories. Weight gain is not a problem of “overeating” but of “over-hunger,” which comes in three forms: homeostatic (physical), hedonic (emotional), and conditioned (environmental/social). Ultra-processed foods are designed to stimulate these hungers, and social environments heavily influence eating behaviors. Fung posits that successfully managing weight requires addressing all three types of hunger through its Golden Rules.

This chapter debunks the “Eat Less, Move More” strategy for weight loss, promoted by governments and health organizations. Citing experts such as the nutritionist Dr. Dariush Mozaffarian and the endocrinologist Dr. Eleftheria Maratos-Flier, as well as large-scale studies, the author asserts that diets based on reduced calorie intake are fundamentally flawed and have a failure rate ranging from 97.3% to 99.35%. Furthermore, they do not reduce the risk of obesity-linked diseases such as heart disease, diabetes, and strokes. When patients fail to lose weight by restricting calories, medical professionals blame the individual when it is the method that is at fault.

Fung argues that calorie intake is just one factor in weight loss or gain. Food provides both energy (calories) and information that triggers hormonal responses. Hormones dictate hunger levels, how many calories are burned, and how much fat is stored. The hormone insulin is the key regulator of fat storage. For example, 200 calories from cookies raise blood glucose, which triggers high insulin levels, signaling the body to store energy as fat. Meanwhile, 200 calories of eggs do not cause a blood glucose spike, and the subsequent low insulin levels allow energy to be released and fat to be burned. Therefore, different foods can have different fattening effects regardless of their calorie count. 

The chapter also dismisses exercise as a primary weight-loss tool. Fung notes that although more exercise benefits overall health, it is not a significant factor in weight loss. This is because as one’s physical activity increases, metabolic rate decreases while total energy expenditure remains the same. 

Fung asserts that most dietary advice is ineffective because it approaches the problem of weight gain from an overly simplistic perspective: i.e., weight is gained when we consume more calories than we burn. He argues that to address the root cause of obesity, we must understand why we feel “over-hungry.” The chapter concludes by introducing the three distinct types of hunger: homeostatic (physical need), hedonic (pleasure-seeking), and conditioned (habitual).

This chapter introduces the concept of a “body fat thermostat,” or body set weight. Citing experiments by Drs. Ethan Sims and George Bray, Fung explains that lean individuals who are forced to overeat will gain weight but effortlessly return to their baseline once they resume their normal diet. While Sims systematically overfed a group of prisoners, the obesity researcher George Bray deliberately gained weight by significantly increasing his calorie intake. Both Bray and the prisoners quickly returned to their normal weights once the experiment had ended. This phenomenon is explained by homeostasis, a core biological principle where the body automatically regulates critical systems to maintain a stable state.

The author states that body fat is similarly regulated. The body fat thermostat dictates a set amount of fat to carry, and if weight deviates from this set point, the body adjusts hunger and metabolic rate to return to it. Obesity is therefore not a disorder of overeating, but a problem of the thermostat being set too high. This set point is controlled by neurohormonal systems. The chapter concludes that since different foods affect these hormones differently, the key to lasting weight loss is to choose foods that adjust the hormonal thermostat downward.

Fung asserts that weight gain is controlled by the hormone insulin, which is primarily triggered by the rise in blood glucose after eating carbohydrates. The speed at which carbohydrates are digested and absorbed determines the magnitude of the glucose and insulin spike, and thus the amount of fat stored. Gastric emptying (the rate at which the stomach releases food after digestion) also plays an important role, as the swifter the process, the higher the insulin levels in the bloodstream. 

Several factors influence the speed of digestion. Processed and refined carbohydrates are digested quickly. Meanwhile, whole foods, larger particle sizes, and greater fiber content slow digestion, leading to a lower insulin response. The speed of absorption (the body’s processing of nutrients into the bloodstream) is also critical. The faster the enzyme amylase can break down the carbohydrates, the greater the insulin spike. The Glycemic Index (the speed at which a carbohydrate makes blood glucose rise) is a useful indicator of which carbohydrates to eat. Foods with a Low GI are digested and absorbed more slowly. 

Natural carbohydrates such as whole grains, quinoa, beans, lentils, steel cut oats, and broccoli take longer to digest, release insulin more slowly, and satisfy appetite for longer, promoting weight loss. Refined and processed carbohydrates, such as white bread, potatoes, white rice, and instant oats, are rapidly absorbed, triggering an insulin spike, greater hunger, and weight gain. Proteins and fats, dietary fiber, and resistant starches (carbohydrates that cannot be digested) do not contain glucose and therefore do not trigger raised insulin levels. 

Fung presents managing these factors to minimize insulin spikes as the key to controlling weight gain, regardless of total calorie count. He offers the following tips for weight loss:

• Eat fiber-rich foods, such as apples, avocado, carrots, oranges, and chia seeds, as they make the stomach feel fuller for longer. 
• Refrigerate and reheat rice and eat cooked potatoes cold. This process reduces their GI by disrupting the starch’s food matrix (its complex physical and chemical structure). 
• Choose pasta and parboiled rice over white bread. 
• Eat whole fruits, as pureeing and juicing fruits makes them more easily digestible, which can cause quicker blood sugar spikes. Berries have a lower fructose (fruit sugar) content than fruits like melon, grapes, and mango. 
• Drink viscous liquids like soup to satiate hunger.
• Avoid “naked carbs” by eating them alongside acidic (fermented or pickled) foods, fats, and proteins.
• Eat carbohydrates last during a meal, after fats and proteins.
• Eat the final meal of the day earlier to lower the glucose effect.

Fung uses the long-term failure of bariatric surgery as an example of why limiting calorie intake is not an effective long-term weight loss method. Although the surgery physically restricts calories consumed, it does not address the underlying problem of hormonally driven hunger.  

The chapter details several key hormonal systems that regulate appetite. Leptin, released by fat cells to signal fullness, is a mechanism that should turn the body fat thermostat down. However, in cases of obesity, individuals develop leptin resistance. Ghrelin, the “hunger hormone,” is secreted by the stomach when it is empty. Once the stomach is full, baroceptors switch ghrelin production off. However, ghrelin levels are often surprisingly lower than normal in people with obesity, demonstrating the hormone plays a relatively small role in thermostat regulation. Powerful satiety hormones released from the gut during eating, such as incretins (GLP-1), Peptide YY, and cholecystokinin, signal the brain to stop eating. Fung notes that the weight-loss drugs Ozempic and Mounjaro are highly effective because they mimic incretins, slowing gastric emptying and increasing satiety. 

Fung explains other factors that influence the body’s weight regulation. The sympathetic nervous system (SNS), responsible for the “fight or flight” response, prepares the body for action, increasing metabolic rate, releasing adrenaline, and suppressing appetite. For this reason, exercising can temporarily relieve hunger pangs. The parasympathetic nervous system (PNS) “prepares the body for rest, digestion and maintenance” (72) and turns down the body fat thermostat. The author explains that drugs such as beta-blockers and antidepressants often cause weight gain because they block the SNS. Meanwhile, certain drugs stimulate the SNS, leading to weight loss. Fung notes that nicotine, methamphetamines, and the “notorious” 1990s weight-loss drug fen-phen, while all having extremely unhealthy side effects, cause weight loss by stimulating the sympathetic nervous system.

The sex hormones testosterone, estrogen, and progesterone influence body fat percentage and distribution. Changes in these hormone levels can significantly affect hunger. For instance, perimenopausal women typically gain 2.25 kg due to a drop in estrogen—a hormone that decreases appetite. 

Thyroid hormones directly affect the body’s metabolism. An overactive thyroid increases metabolic rate, leading to weight loss, while an underactive thyroid has the opposite effect. Meanwhile, hypoglycemic hunger is a condition in which an insulin imbalance causes low blood sugar. 

Fung concludes by stating that “Weight-loss “experts” are […] obsessed with calories” (78). However, science recognizes the key role hormones play in weight management. Consequently, medical textbooks advise that obese patients should undergo tests for adrenal and thyroid function, as well as levels of insulin and sex hormones. Physicians also routinely consider the impact of medications such as beta blockers and antidepressants on weight gain.

This chapter argues that managing weight is about controlling hunger through food choices, not counting calories. Different foods with identical caloric values affect the body’s satiety hormones differently. The key to sustainable weight management is to eat foods that naturally make you feel full.

Fung recommends eating naturally occurring fats in moderation while avoiding the trans fatty acids found in products such as margarine. Foods rich in natural fats, including olive oil, butter, coconut oil, avocados, and almonds, stimulate satiety hormones. For instance, olive oil stimulates the incretin hormones GLP-1 and GIP, and the satiety hormone peptide YY. Foods labeled “low-fat” should be avoided as they often contain sugar and chemicals and lack the satiating properties of whole foods. Fung argues that dietary advice, beginning in the late 1970s, demonized fats and promoted refined carbohydrates, thereby contributing to rising obesity. 

Protein increases satiety hormones and metabolic rate. Therefore, a breakfast of Greek yogurt and eggs is a better choice than sugary cereals and toast. However, excessive protein intake is converted to glucose, so moderation is key. Fung does not recommend protein powders as their consistency does not satisfy hunger in the same way.

The chapter also suggests consuming bitter foods, such as kale, eggplant, arugula, and turmeric, as they increase the incretin hormone GLP-1 and reduce the hunger hormone ghrelin. Beverages like green tea, black tea, and coffee are also beneficial; they boost metabolism, decrease ghrelin, and can block some glucose absorption. Conversely, diet sodas and artificial sweeteners are discouraged. Despite being zero-calorie, they can increase hunger by tricking the brain and failing to trigger satiety signals, leading to compensatory eating and no net weight loss.

This section details three successful health transformations. Harry, a 45-year-old repeat dieter, adopted intermittent fasting and learned to manage insulin spikes by eating carbs last in a meal, ultimately achieving sustainable weight loss and increased energy. Cathy, a woman in her fifties with type 2 diabetes, changed her eating habits by eliminating snacking and focusing on whole foods. Through these changes and intermittent fasting, she successfully reversed her diabetes, stopped all her medications, and lost over 100 pounds. Helen, a 71-year-old with chronic arthritis, started a fasting protocol and learned that weight management involved more than calories. By reducing carbs, exercising, and journaling, she lost 180 pounds and now walks 3.5 miles every day and plans to continue teaching for another 10 years.