The Hunger Code: Resetting Your Body's Fat Thermostat in the Age of Ultra-Processed Food

by Jason Fung

THE SEQUEL TO THE MULTI-MILLION COPY BESTSELLER THE OBESITY CODE

"Jason Fung nails it again in The Hunger Code. This book clearly and compellingly explains why most Americans are overweight and gives us a concise, evidence-based way to manage our weight with as little hunger as possible."—Dan Buettner, New York Times bestselling author of The Blue Zones

The Obesity Code helped thousands of people lose weight naturally.

The Hunger Code will help thousands more lose weight—and keep it off—in an era of weight‑loss drugs and ultra-processed foods…without counting calories.

For generations, we've accepted the story that weight loss can never last—that as soon as we go off the diet or stop taking the medication, we revert to our old habits, regain the weight, and the calorie-counting journey starts all over again. Traditional medicine continues to treat the symptom of weight gain rather than addressing its root causes. Too often, healthcare systems intervene with quick fixes and short-term solutions, ignoring the factors that lead to sustainable weight loss that can last a lifetime.

What if the secret to long-term weight maintenance and better health isn’t just about what you eat—or even when you eat—but why you eat?

With the food industry's reliance on ultra-processed foods and the popularity of Ozempic and Mounjaro, understanding the forces behind why we eat is more important than ever before.

In The Hunger Code, New York Times bestselling author Dr. Jason Fung reveals the three powerful forces that drive us to
Physical HungerEmotional HungerSocial Hunger
Dr. Fung also introduces the concept of the body’s “fat thermostat”—a biological “set point” that regulates how much fat your body tries to maintain. Guided by hormones and metabolism, this internal system drives hunger and energy use, explaining why lasting weight loss requires more than just willpower …

With three Golden Rules and 50 actionable tips, The Hunger Code empowers you to recognize and respond to hunger appropriately. Learn how to slow digestion, break emotional eating cycles, and overcome social pressures to eat constantly, so you can maintain a healthy weight—from scratch, after fasting, or after using weight‑loss drugs.

Genres: Health, Nonfiction, Science, Audiobook, Self Help, Nutrition

288 pages, Hardcover

First published March 3, 2026

Reviews

[Donna · Rating 4 out of 5]

I've read this author before and I have also heard him as a guest on several podcasts promoting this new book of his. I like his logic and his overall message and I'd read anything that he puts out there.

In this one, he dives into hunger....what it is, how to keep it at bay, and what to eat to prevent frequent trips to the vending machines. He also expands on ideas he's introduced in his previous books such as the role of insulin in fat storage, fasting, diabetes, obesity, etc. He challenges much of what we've all been taught about nutrition since the 1980's. I love that.

What stood out the most is how much more he explores weight loss and how hunger can be controlled. This all seemed more detailed and more applicable for daily use. I loved that focus.

Now with all that said, I can see this book receiving some criticism. Given Fung's reputation for challenging the standard of care, he naturally has his share of hecklers. At times, I found myself wondering whether his intended audience was the people he wants to help or the critics he's trying to persuade. His humor and occasional language choices didn't always strengthen his message and may turn some readers off.

However, I still found this one worthwhile and I'd recommend this to anyone who is struggling with diabetes and the complications that go along with that. I hope this sparks more meaningful conversations about preventing disease and reversing it rather than simply managing it. So 4 stars.

[Lisa · Rating 3 out of 5]

I suppose nothing will compare to The Obesity Code. This companion book seeks to explain our struggles with hunger and what we know is the right path forward. Nothing was new to me. I know I am an emotional eater and prone to environmental hunger. The practical solutions are things I’ve been trying to achieve for years. It’s still a good read, just not as helpful as I hoped it would be.

[Cara · Rating 3 out of 5]

So repetitive! I swear, every single thing he said, he had to say it at least six times.

Maybe I’ve read too much on this topic, but I found this book really boring and not particularly helpful. The premise is that the real cause of overweight and obesity is stupidly simple: we’re all too hungry. Cool cool, that makes sense. So then I’m eagerly waiting for the part where he says how to be less hungry. Or better yet, as the subtitle advertises: how to reset my body’s fat thermostat! Yes please!

Well, first he has to spend about a million pages explaining how very fat we all are, like we didn’t know. Then he covers the types of hunger (homeostatic, aka real, physical hunger; hedonic hunger, which is eating to feel better, including emotional eating; and conditioned hunger, which is eating because it’s lunchtime or whatever). Fine.

Insulin, how your body stores or burns calories, the reason ultra-processed food sucks, how much harder it is not to be fat when our society is one giant bad food habit after another. Yeah, yeah, ok, can we please get on with it?

Finally, the last 30 pages or so are part 4: How to Manage Hunger. But even there, he’s just saying the same things he’s already said over and over throughout the book.

Here, let me save you some time: avoid ultra-processed food. Stick to two meals a day or less. Don’t eat stuff that spikes your blood sugar. Eat consciously, and choose food that’s filling and stays with you. Drink water, coffee, or tea, not sweet drinks. Go longer without eating, ideally fasting. Get enough sleep. Deal with your issues and have a more interesting life so you don’t do emotional eating anymore. Set up your habits to support you, not hinder you.

All he said about not being so hungry is: quit eating ultra processed foods, which are designed to make you hungry. Don’t eat carbs by themselves or you’ll be hungry again soon. And fast so you can see for yourself that hunger goes away if you ignore it.

Fasting does help reset your body’s interaction with insulin, so I guess that was how to reset your fat thermostat? But I used to do a 48-hour fast every week. I still got fatter than ever after I stopped, just like everything else I’ve ever tried.

In conclusion, I think this is an informative book if you’re new to the idea of low-carb and fasting to correct your relationship with insulin. But if you already know about that, the book is repetitive. To me, it doesn’t live up to its promises.


Notes
P. 175
Studies show that our expectations of food determine our perceptions of it more than the actual calories. People given an “indulgent” milkshake and told it was 620 calories had a bigger drop in ghrelin than people drinking an identical milkshake who were told it was “sensible” at 140 calories. Or people who ate a “healthy” protein bar were 60 percent hungrier than people who ate the same protein bar when labeled “yummy” instead of “healthy.” Same with identical breakfasts labeled “250-calorie sensible” vs. “500 calorie indulgent. Same with identical omelettes labeled as 2 eggs + 30 g cheese vs. 3 eggs + 60g cheese.

Some people who thought they were eating a healthy snack ended up more hungry than if they had eaten nothing.


People eat 30% more veggies if you call them tasty-sounding names like Bang Bang broccoli or Zesty sweet potatoes.

This reminds me of eating with my ex, who was always trying to get me to lose weight. He’d feed me a bunch of dry, flavorless carbs and say it was “healthy,” and as soon as I got away from him, all I wanted was cheese or cream or something. Give me the antidote! (He was also big on lots of carbs right at bedtime, and making me feel bad. Gee, wonder why I wasn’t losing weight?)

[Paula · Rating 2 out of 5]

If you have read any of Dr. Fung's books on obesity or diabetes you will find this book very repetitive. He is still a strong supporter of not only intermittent fasting but also longer fasting periods of 5-7 days over and over. While I can buy into intermittent fasting to help with insulin levels in the body, I am not a fan of his multi day fasting advice - it looks too much like an eating disorder to me - restrict and then binge.

[༺ Jason ༻ · Rating 4 out of 5]

This is a companion to his exceptional book The Obesity Code. That book was a game changer and highly recommended. This book he goes into more detail about what things that help control our hunger. His logic has been sound and a simple solution well just works. Although I did enjoy it and recommend you read both, but if you’re just going to read one that you want a complete picture go with The Obesity Code. For that book alone I can’t thank Dr. Fung enough. This book was an excellent follow up.

[January · Rating 5 out of 5]

The Hunger Code: Resetting Your Body's Fat Thermostat in the Age of Ultra-Processed Food by Jason Fung, MD (2026)
xviii+270-page Kindle Ebook story pages ix-235

Genre: Health > Nutrition, Medicine; Nonfiction, Science Self Help, Hormones, Food and Drink > Food, Diets; Psychology > Eating

Featuring: Contents, Preface, Overview, The Quick Start Guide, Diagrams, Answer Key, Homeostatic Hunger, Debunking the Calorie Delusion, Regulating the Body Fat Thermostat, Eating a Low-Insulin Diet, How Hormones Sustain Hunger, Managing Hunger, Not Calories; Hedonic Hunger, Getting Hooked On Ultra-Processed Foods Chapter, Understanding Food Addiction, Managing Emotional Eating, Conditioned Hunger, Living in an Obesogenic Environment, Recognizing Eating as a Habit, How to Manage Hunger, Making Weight Loss Automatic Chapter, Unlocking the Secrets of Success—Your Mindset and Your Habits Chapter, Putting the Golden Rules of Weight Loss Into Action, Weight-Loss Tips, Notes, Index, Partial Bibliography for Dr. Jason Fung With Covers - The Obesity Code, The Diabetes Code, The Obesity Code Cookbook, The Diabetes Code Journal, The Diabetes Code Cookbook

Rating as a movie: PG

My rating: 🌟🌟🌟🌟🌟😋🧠🍛🥘🚫🥫

My thoughts:🔖 Page 64 of 270 [Chapter] 4 How Hormones Sustain Hunger - This is good. I'm going to take a break so I can absorb the information in the form of notes.
🔖209 [10 pages into Chapter] 12 Unlocking the Secrets of Success—Your Mindset and Your Habits - My eyes gave up on this page. I went back and skimmed to make sure I didn't miss anything before I clocked out and restarted this chapter.

This book contains much of the same information as his previous books, but it is presented with significantly more detail and clear instructions. I first encountered Jason Fung's work in 2022 when his book, The Diabetes Code, was instrumental in helping my mother discontinue insulin and completely reverse her type 2 diabetes. Since then, I have read most of his books, and I can attest to his brilliance. I am eager to follow the advice provided in this book. He has conveyed the information with utmost clarity.

Recommend to others?: Absolutely! This is a must read and a must buy for reference.

Books and Authors mentioned: The Obesity Code: Unlocking the Secrets of Weight Loss (Why Intermittent Fasting is the Key to Controlling Your Weight) by Dr. Jason Fung; Handbook of Obesity: Two-Volume Set by George A. Bray and Claude Bouchard, The Principles and Practice of Medicine by Sir William Osler, The Blob by by Theodore Simonson and Kate Phillips, The Odyssey by Homer, Food Rules: An Eater's Manual by Michael Pollan, Hunger: A Memoir of (My) Body by Roxane Gay, Yojokun: Life Lessons from a Samurai by Kaibara Ekken, The Blue Zones Secrets for Living Longer: Lessons From the Healthiest Places on Earth by Dan Buettner, The End of Overeating: Taking Control of the Insatiable American Appetite by David A. Kessler MD; Atomic Habits: An Easy & Proven Way to Build Good Habits & Break Bad Ones by James Clear, The Complete Guide to Fasting: Heal Your Body Through Intermittent, Alternate-Day, and Extended Fasting by Jason Fung, MD, with Jimmy Moore; Antifragile: Things That Gain from Disorder (Incerto 3) by Nassim Nicholas Taleb, Skin in the Game: Hidden Asymmetries in Daily Life (Incerto 5 ) by Nassim Nicholas Taleb, A Midsummer Night's Dream by William Shakespeare, The Sound of Music by Richard Rodgers Oscar Hammerstein II [based on] The Story of the Trapp Family Singers by Maria Augusta von Trapp;

Memorable Quotes: This is Not a Diet Book The second step to solving a problem is to follow good advice. We have a magic formula for everything. For example, when putting together a boy band, you need the tough guy, the fun guy, the sensitive guy, the nice guy, and the jock. Mix in some rudimentary dance steps and a catchy pop tune, and magic happens. Diet books also follow a winning formula. I will reveal the secrets, right here, right now, for free! ​1.​ The Promise: Plain old, boring weight loss isn’t enough anymore. You must promise more, more, more! “Effortless” weight loss. Without sacrifice. Without hunger. While still eating cookies. “Rapid” weight loss. It doesn’t really matter whether these promises are true or not, as long as they sound good and are not completely impossible. You can also throw in other promises. Your skin will clear up. Your asthma will go away. Your wrinkles will disappear. Dogs will love you. You’ll be a Jeopardy! master. You’ll live to 100 or your money back! Anything you can think of, promise that your diet will do it. ​2.​ The Hook: Pick any foundation principle or ingredient: low carbs, low calories, sugar-free, blood type, acid-alkali balance, Glycemic Index, Zone macronutrients, hormones, exercise, high-intensity exercise, meal replacements, grapefruit, green juice, cabbage soup. Make sure it is unexpected, unusual, and not completely implausible. Claim that if you do or eat this (one thing), you’ll lose weight effortlessly and keep it off! ​3.​ The Secret: Explain the diet trick that has remained undiscovered for the last 5000 years of human history, the one that magically targets fat cells. For example, humans may have been growing and drinking coffee for many thousands of years, but what if they had crushed the green beans and eaten them instead! What if people had thought to juice their celery! Do Zone 2 exercise! Make cabbage soup! You can also claim that somebody is actively suppressing this secret. Do these headlines sound familiar? “The Fat Burning Secret They Don’t Want You to Know!” “Belly-Busting Secrets of the Rich and Famous!” ​4.​ The Science: Now, establish some science, or at least some pseudoscience. Meat is bad for you. Plants are bad for you. Fat is bad for you. Carbs are bad for you. Protein is bad for you. The sun is bad for you. Ancient aliens are behind the obesity epidemic. ​5.​ The Success Stories: People love a good story, so pair a few before-and-after photos with one or more stories of transformation. For example: “I used to weigh 400 pounds and my hair was awful! Then I started juicing baby kale and, ba-bam, the weight melted right off! Plus, check out my soft, silky hair.” ​6.​ The Plan—Phases I, II, and III with recipes: Finally, provide the super-secret plan in three phases: the induction phase, the continuation phase, and the maintenance phase. Begin with some kind of severe restriction, then slowly reduce it over time. Add a few recipes to make the plan easy to follow. Lamb’s brain stew? Tree bark jerky? There you have it: the winning formula you need to write a bestselling diet book. So how does The Hunger Code stack up? ​1.​ The promise? No. The Hunger Code promises nothing but a deep scientific discussion about the causes of weight gain and weight loss. ​2.​ The hook? No. ​3.​ The secret? No. ​4.​ The science? Yay. The whole book is about science, after all. ​5.​ The success stories? Lots, but none of the my-life-transformed-in-twenty-four-hours variety. ​6.​ The plan? No. ​7.​ Recipes? That’s a different book. The final tally is 1/7, or a final grade of F. The Hunger Code is not a diet book. That’s good. I didn’t intend to write a diet book. Instead, I give dietary advice that is generations old: eat fewer ultra-processed foods, fast intermittently, and create the social bonds that will ensure success. That’s not the latest and greatest advice. It’s the tried and true.

If you remember nothing else from reading this book, remember the three Golden Rules of weight loss: ​1.​ Avoid ultra-processed foods. ​2.​ Don’t eat all the time. Fast regularly. ​3.​ Commit to a health mindset and healthy social habits.

The standard concept is that weight gain is a simple problem of Calories In, Calories Out. Eating too many calories causes you to gain body fat, and therefore weight loss is simply a matter of eating fewer calories, as this diagram appears to illustrate. This simplistic perspective is known to be incomplete and mostly wrong, confirmed by decades of scientific evidence. The corollary to Calories In, Calories Out is Eat Less, Move More. Again, this advice is based on a largely incorrect understanding of human physiology and is therefore largely ineffective for losing weight. The more accurate, physiological model recognizes that how much body fat we carry is dictated, like almost everything else about the human body, by changes in hormones. Some hormones increase body fat storage and some hormones decrease it. The body regulates the balance of these hormones using a homeostatic mechanism that’s much like a body fat thermostat. The foods that we eat influence which hormones our body produces. Food contains both ​1.​ energy (calories) and ​2.​ information (the body’s hormonal response to the food). Both are important. When we eat different foods, our body responds by releasing different hormones. For example, eating 200 calories of cookies releases the hormone insulin, which tells our body to store the incoming calories as fat. Little of the hormone that tells our body to feel full—glucagon-like peptide 1 (GLP-1)—is released. If we eat 200 calories of eggs, we don’t release much insulin, so most of the calories are available as energy for the body. We release more GLP-1, which tells us to stop eating when we’re full. The difference between these two is not the calories but the body’s hormonal response, and that matters a lot.

We have different types of hunger. The physical hunger that we experience as our body tries to get nutrients is homeostatic hunger. However, we eat for reasons other than nutrition. We also eat for pleasure or comfort, and that’s hedonic hunger. And we eat out of habit, which is conditioned hunger. The three main types of hunger to manage are: ​1.​ Homeostatic hunger (physical) ​2.​ Hedonic hunger (emotional) ​3.​ Conditioned hunger (environmental and social)

“Experts” blame the victim, which leads to fat shaming. However, the inconvenient truth stares us in the face. If you are one of the 99.63 percent of people who have tried calorie restriction and failed to lose weight, remember this: you were expected to fail. The doctors, the scientists, the dieticians, the “experts” all knew you would fail. The advice was bad. Here’s the thing. It doesn’t matter if scientists think that calorie restriction should work. Test it, and if it doesn’t work, then accept that it doesn’t work. Move forward. That’s the way of science.

Or consider McDonald’s french fries. Once upon a time, they consisted of potatoes deep-fried in a natural fat—beef tallow—and salt. On its website in 2024, McDonald’s listed the following ingredients for its fries: Potatoes, Vegetable Oil (Canola Oil, Corn Oil, Soybean Oil, Hydrogenated Soybean Oil, Natural Beef Flavor [Wheat and Milk Derivatives]*), Dextrose, Sodium Acid Pyrophosphate (maintain Color), Salt. *Natural Beef Flavor Contains Hydrolyzed Wheat and Hydrolyzed Milk as Starting Ingredients. Contains: Wheat, Milk. Seriously. McDonald’s french fries contained wheat and milk. The potatoes, colored with sodium acid pyrophosphate to look like french fries, are really only there to hold the “beef flavor.” And why would “natural” beef flavor start with hydrolyzed wheat and milk? The answer is money, money, money. Why use expensive real ingredients when you can use cheap, industrial chemicals? The UPF will still taste pretty much the same as the natural food. Why use real vanilla for several hundred dollars per pound when you can use vanillin, a synthetic chemical that tastes like vanilla for about ten dollars per pound? Vanillin is one of the most common chemical additives in UPFs. Why use real strawberry when you can use ethyl methylphenylglycidate with some modified starches and other chemicals to mimic the texture, flavor, and aroma of strawberry?

[Ivana Amidzic · Rating 5 out of 5]

I love Dr. Fung's common sense practical advice and timeless wisdom, and have read all of his books so far (my favorite one so far is The Cancer Code). I enjoyed this book I mostly read for my research on psychology and food as well as for personal knowledge. Especially given that I currently do live and work in obesogenic environment in North America (one of top 3 most obese provinces in Canada) where I moved to from totally opposite situation. And indeed, just trying to move, eat well and maintain basic health is like "trying to stay afloat in a tsunami". Moving around and living on different continents and in different countries in the world really gave me first hand insight into how important environment and culture are when it comes to health and food.

Looking forward to soon seeing warning labels on UPFs, similar to those on cigarettes.

[S. · Rating 3 out of 5]

Some useful insights into hunger and hormones, although there is no new information in this book — the author is just repeating what he's been writing and saying for years.
Anyway, it's a good motivational book.

As a scientist he fails to acknowledge the work and research of plant-based doctors (The China Study, for example) and simply states that there has been no research on the benefits of low-fat, plant-based diets. This simply shows author's ignorance and lack of intelligence.
Also, he completely ignores that the most successful and healthy populations in the world have foods like rice, corn, sweet potatoes, etc. as their staples with a very small percent of calories coming from animal sources.

As a human being he fails to understand that killing and abusing animals for one's nutritional needs is completely unnecessary and inhumane, which simply shows his lack of compassion.

The author's message about avoiding ultra-processed foods, fasting and eating in a more traditional way makes it an interesting and useful read, but don't expect scientific or intelligent nutritional advice.

[Cheryl · Rating 5 out of 5]

This is the second book I’ve read by this author, and I learned a lot from both. The first was “The Obesity Code” which I read about seven years ago. I’ve always enjoyed learning about endocrinology and this book provides a lot of interesting information about the interaction of various hormones that control our body’s hunger and metabolism. The biggest fallacy in weight loss is that weight loss is all about calories in vs calories out. Weight loss is SO much more than that.

There are several hormones which interact to set our metabolic thermostat. Some of our metabolic thermostat is genetic but a lot of it is also behavior and eating habits. The author explains the interactions of insulin, cortisol, the vagus nerve, incretins (like GLP-1), adrenalin, noradrenalin, thyroid hormones, dopamine and serotonin as well as sex hormones. The author explains how these hormones impact our ability to lose weight.

Dr. Fung’s three golden rules for managing weight:

Avoid ultra-processed food.
Ultra processed food is designed to make you want to overeat. It causes a massive spike in insulin. Chemicals are added to UPF to make you want to eat more of it. When smoking cigarettes became passee, Big Tobacco companies looked for other ways to increase their revenue as revenue from cigarette sales dropped. The author writes “In 1985 R.J. Reynolds Tobacco company bought food giant Nabisco. The same year tobacco giant Phillip Morris (now Altria) bought processed food giant General Foods and then added Kraft in 1988. It was time to dust off the Big Tobacco ‘How to Sell an Addiction’ playbook.” Big food companies have been using many of the same advertising strategies they used to sell cigarettes to market ultra processed foods.

Don’t eat all the time (grazing all day) but rather fast regularly.
This allows insulin levels to fall and fasting switches our bodies from fat-storing mode to fat-burning mode. Fasting (perhaps 12-14 hours a day) helps reduce hunger (long term) because of its effects on ghrelin and the sympathetic nervous system as well as breaking food addictions and emotional eating.

Commit to a health mindset and healthy social habits.
Create a lasting social system that supports you in your quest to lose weight and keep it off. This is essential for dealing with emotions, addictions and breaking bad habits.

In addition to these three “Golden Rules” the author includes a list of 50 weight loss tips at the end of the book that are very helpful. I’ve been using one of them since I read his first book. Wherever possible, eat the protein on your plate first and the carbohydrate last. Doing this reduces the level of blood sugar spike and insulin spike that results from eating the carbohydrates first. Protein in your stomach slows the digestion and absorption of the carbohydrates.

If you would like to understand weight loss, this is an excellent book.

[Anna Catherman · Rating 1 out of 5]

Look, so i tried to like this book. Honestly, the first fifty pages or so were enjoyable. Avoiding high insulin spikes as a way to manage weight is something that's definitely not talked about enough, and it convicted me to lay off the cookies a bit, and plan such carb-heavy components as a small part of a balanced meal.
But as the book went on and the advice grew more and more repetitive, unrealistic, and judgemental, I grew frustrated. Eating a low glycemic index diet? Great. Avoiding ultra-processed foods? Also great. Intermittent fasting? Quite possibly good, although I don't feel like he provides enough scientific backing for it (or anything in the book, to be completely accurate) to make a strong case.
Along the way, he chastises other diets for proclaiming one-size-fits-all approaches to weight loss and highlights the importance of hormones in controlling hunger and, in turn, weight. But rather than advise actually going to get treatment for other problems (he mentions thyroid issues, perimenopause, stress, etc.) first before tackling weight loss, he instead launches into saying we need to guilt ourselves into not eating ultra-processed food! OH and the solution to stress? Not lifestyle changes or therapy or deep breathing. "Meditation". That's it. I guess I should be meditating more and then a variety of unfulfilling at best and toxic at worst environments in my life would just stop affecting me? /jk
And then at the very end, he proclaims a very simplistic approach that steamrolls any sort of nuance and proclaims that everyone ought to be following his diet which feels dangerously close to an eating disorder: he goes so far as to advise ("after consult with your medical doctor") fasting for as long as five to seven DAYS on a regular basis.
It's not all bad. In fact, I can see a version of this with a little more science and a little less shame, plus smaller, more actionable steps, to be successful. But a book that ostensibly centers around *PREVENTING* hunger, then proceeds to say "just fast til you start ignoring your hunger" is deceptive. Add in the fact he repeatedly says to change your mindset to think of processed or sugary foods as "poison" but then tells you "you can still eat it, just be mindful and know what you're doing!"?
When growing up, my uncle had this infamous roll of metallic wrapping paper that was impossible to rip open, and combined it with equally indestructible tape. Every Christmas, it was sort of pretty, but it was insanely difficult to get to the goodies inside. I don't remember a single present given in that wrapper; I only remember the horrible, horrible wrapping paper.
This book is like that Christmas present: good stuff inside, but covered in so many layers of unpleasantness that all that's remembered is the poor presentation, and not actually the solid advice within.

[Rodney · Rating 5 out of 5]

I just finished The Hunger Code: Resetting Your Body’s Fat Thermostat in the Age of Ultra-Processed Food by Dr. Jason Fung, and it is another masterful addition to his groundbreaking series on obesity and metabolic health. If you’ve read The Obesity Code, The Diabetes Code, or The Cancer Code, you’ll recognize the familiar clarity, evidence-based approach, and no-nonsense style that makes Fung’s work so powerful. Yet this book feels especially urgent and timely.

Fung cuts straight to the heart of why America (and much of the modern world) has become so overweight: it’s not simply about calories or personal willpower. He masterfully explains how hormonal imbalances, particularly chronic high insulin driven by ultra-processed foods, have hijacked our body’s natural “fat thermostat” and created relentless hunger signals that override common sense. The science is presented in a way that’s accessible without dumbing it down — full of eye-opening studies, clear diagrams, and practical insights into why traditional “eat less, move more” advice keeps failing on a population level.

What sets this book apart is its unflinching look at the bigger picture. Despite the overwhelming statistics on rising obesity rates, the practical application of this knowledge remains frustratingly limited. Fung makes a compelling case that real, lasting change will require systemic action. He argues convincingly that Congress must finally hold the FDA accountable to do its job and rein in the flood of ultra-processed foods engineered to exploit our biology. At the same time, he doesn’t shy away from acknowledging the rampant corruption and conflicts of interest that plague both political parties, making meaningful reform an uphill battle — though he leaves room for cautious optimism that change is still possible if enough pressure is applied.

This isn’t just another diet book. It’s a wake-up call that blends rigorous science with a sharp societal critique. Whether you’re struggling with your own weight, work in healthcare, or simply care about public health policy, The Hunger Code delivers the tools to understand the problem at its root and the motivation to push for solutions beyond individual effort.

Highly recommended — easily one of the most important books I’ve read on the modern obesity epidemic. If you’re ready to move past blame and toward real understanding, start here. Five stars without hesitation.

[Aaron Beyk · Rating 5 out of 5]

Seek the cause(s), find the solution

A more complete version of the obesity code that factors in more of the effects causing the hormonal imbalances driving obesity. Factors in the importance of UPFs in the diet and how the environment we live in today is obesogenic, not in an environmental pollutant manner, but in a sociological and economic manner. I hear lots of allusions to the “trapped fat” hypothesis— what came first, the chicken or the egg—is it the behavior of overeating calories or the hyperinsulinemia? I also hear lots of points made by other authors like Lustig and Moss— this is important as the ideas of IPF’s and leptin resistance are not only coming from Fung, validating his points. Insulin’s action to cause the preferential storage of nutrients away from the blood and into cells, causing overeating is arguably the main physiological cause of the obesity epidemic, but Fung compiles the environmental and emotional factors that would cause insulin-ogenic feeding to begin with. By recognizing the multifactorial causes of the neuroendocrine imbalances seen in obesity, Fung presents practical ways to modify the real causes of the physiological changes seen in obesity. I do believe that by recognizing our lever to pull in this conversation as insulin effects from the diet, we can make great strides in our overall health (ie, having more nutrients available in the blood to be able to do hobbies that allow us to cope well with the stressors that would cause hedonic eating). I don’t know which lever is best to pull here, but maybe by attacking the issue from many sides (a method that could be detracted from this read— perhaps obesity is caused in a particular person because of their life stressors causing hedonic hunger and homeostatic hunger from the UPFs causing insulin resistance in the VMH, blocking leptin signaling), we can attack the multifactorial cause of the most prominent epidemic in human history.

[Lynnette · Rating 5 out of 5]

Dr. Fung reinforces timeless dietary advice: reduce intake of ultra-processed foods, avoid constant eating, and build social connections that support healthy habits. He explores the three types of hunger—homeostatic, hedonic, and conditional—explaining their underlying causes and offering solutions to address them. Understanding these root causes can help guide effective solutions. Dr. Fung demonstrates how factors like food structure, digestion, absorption, and hormones influence weight.

In part four of the book, How to Manage Hunger, readers will find the most practical suggestions in three chapters: Making Weight Loss Automatic, Unlocking the Secrets of Success—Your Mindset and Your Habits, and Putting the Golden Rules of Weight Loss into Action.

The book is easy to understand and includes 50 tips boxed throughout the text and compiled at the end. It uses good analogies to explain concepts. Key information is highlighted in bold to catch the reader's eye.

While Dr. Fung is recognized for his fasting protocols, this isn't the focus of the book. His humor and choice of words might put off some readers.

I would recommend this book for all readers.

[Kaylin · Rating 4 out of 5]

Dr. Fung's perspective on obesity, hunger, diabetes, PCOS—in short—metabolic dysfunction, was a new take. And I've enjoyed learning about his perspective.

I've spent nearly 15 years believing "calories in, calories out". I've payed hundreds of dollars for a PCOS coach and thousands for a health coach with various levels of success.

Naturally, I have started to wonder if our paradigm is wrong. Dr. Bikman, Dr. Fung, and Dr. Boz have been some of the biggest voices I've found that talk about a different paradigm (endocrine focused as opposed to calorie focused).

Time will tell if this paradigm will give us answers for the nationwide obesity epidemic. But I'm trying it out, along with knowledge from Dr. Bulsiewicz (the plant guy haha). And the understanding that humans aren't machines and sometimes we need a more gentle approach to nutrition (Intuitive Eating, anyone?)

What I didn't appreciate was his *small* detours about the 70's being an amazing time in America. Specifically the one line about parents sending their kids to bed without dinner being emotionally fine? How about we stay in our lane, my dude. Leave the child development to the experts and we'll leave you the nephrology.

[DR S · Rating 5 out of 5]

the ideal “diet” book for those who follow the science, rather than the fads and hype

As with The Obesity Code, Dr. Fung has shared true evidence-based information in an accessible way that should not intimidate the non-scientist reader. As a physician, I appreciated the reminder of all of the nutrition science I learned, and the synthesis and succinct presentation of the stuff I never really learned, or that was yet-to-be discovered when I trained. His matter of fact, and often humorous tone makes for an enjoyable and refreshing read. I also appreciated the detailed literature citations, which allowed me to further explore the research in the areas of particular interest to me. I used The Obesity Code many years ago to lose 20 pounds, and managed to maintain that weight loss for years, until menopause and career/life/health stresses took their toll. I can’t wait to start again, armed with this new knowledge from The Hunger Code, and lose 20 pounds again. I have absolute faith that it will work again….and I know that I will enjoy getting back into the fasting habit because I really felt great doing it. Thanks Dr. Fung!

[Sonny Fertile · Rating 5 out of 5]

This is the sixth book I have read that this author has either written himself or co-written. The first five I read back to back when I first discovered them. For the most part, I already adhere to all of the advice I gleaned from all of them. Not 100% of the time as I once did mind you. Probably around 90%. I still engage in both intermittent and prolonged fasting up to 5 days regularly. I bought this book as a refresher to get back up to 100% when it comes to avoiding all processed foods though that is still only a very occasional occurrence for me. My mindset has been rebooted and I'm back to zero tolerance. Ok 99%. And with the information provided in this book it's easier to do that than not to do that. And I also bought this book because I can trust the author's advice. I ended a 20-year relationship with insulin and lost the most stubborn of the last 20 lbs of fat in the final six weeks of my 30% body weight reduction campaign by following the methods outlined in those books. Those books changed my life.

[Nina · Rating 5 out of 5]

This book is great. I’m a disciple of this Canadian doctor after I read his first book, The Obesity Code, last year. This book is a follow-up to his Obesity Code which he says is The Obesity Code Part 2. He goes more into Ultra-Processed foods than he did in the Obesity Code, but less into autophagy, mentioning it only once in passing.

Really hammers into it that grazing or eating 6 small meals a day is not the way to lower your insulin as popular thinking would have us believe or believing that being in a calorie deficit is the way to lose weight.

I highlighted the shit out of this book. I tried to listen to the audiobook but found it difficult to follow. The book has a lot of headings and subheadings and graphs and he likes to number and bullet point thoughts, so I would go with the physical book for this over the audiobook.

A must read for anyone looking to lower their AC1 (to avoid pre diabetes or diabetes) or lose weight.

[Reader · Rating 5 out of 5]

This is the second-most important book I’ve read in years. My sister turned me on to Dr. Fung’s first book, “The Obesity Code”, and his follow-up “The Hunger Code” rises to that same level of importance to me.

Fasting & restricted eating are the best tools I have for significant weight loss and reversing my type 2 diabetes. That is what “The Obesity Code” is all about - giving you the knowledge needed to reach your goal weight and thereby improve your health.

The Hunger Code answers the “what now?” question, once you’ve lost the weight … how do I keep it from creeping back?

Anyone who read and was helped by the first book, needs the second book. And if you lost some weight and then stalled, plateaued, got stuck at a lower weight but not low enough, this book helps you with what to do in that case too.

[Michael Jones · Rating 5 out of 5]

I'm an overweight middle aged man who thought there was very little chance of me ever doing some pounds... I don't think very deeply about food, so calorie counting is unlikely to happen for me. According to this book, luckily for me that doesn't matter! It gave me a lot to think about, and some things I can try even not being a very food-aware person (for example: becoming a little more food-aware, but not too try to count calories!) I have heard that Dr. Fung's work is sometimes controversial, but while I was reading this book I talked about it with my son, who is studying biomedical science in college, and there really isn't anything that he said that contradicts basic science. I may not drop 40 pounds in the next six months, but if I can fennel incrementally better habits, maybe I can make slow progress. It doesn't hurt to try!

[Vicky Miles · Rating 3 out of 5]

This book contains a lot of useful information. Fung explores hunger from multiple angles and offers practical strategies for managing different types of hunger. The tips provided at the end are especially helpful.

That said, I often found his tone condescending. For example, the suggestion that most overweight people are simply bored and spend their time watching TV, while slim individuals are more productive and therefore avoid overeating, feels outdated and reductive. I expected more nuance from Fung.

The book is also highly repetitive. Much of the content could have been condensed, and the same insights delivered in half the length without losing value.

[Barb · Rating 5 out of 5]

This is a must read for everyone. I'm old enough to remember when fast food was not on every corner. Gas stations only sold gas, drug stores sold drug and personal items. Did have soda counters and a soda jerk, but only sold drinks while you waited for your prescriptions. Restaurants sold home cooked food with only a few options. No candy at the checkout lines at the grocery stores. If you wanted candy you had to go to a department stores candy counter and candy was weighted and charged accordingly. And guess what, practically no obesity. To see a fat person was shocking. Now when we see a normal weighted person we think skinny/ must be ill.

[Leah Shelton · Rating 5 out of 5]

The Hunger Code is an excellent, approachable, and more nuanced sequel to The Obesity Code. Although there is no “one size fits all” approach to nutrition, Fungs “golden rules” are broad enough to be applied to any individual. As a physician, I particularly appreciate how Fung explains complex scientific topics in multiple ways, first using medical jargon, then using lay-person terms, and then often using metaphors to solidify understanding.

I would highly recommend this book to anyone who wants to learn more about nutrition and obesity for themselves, their patients, or their community.

[Wanda · Rating 3 out of 5]

The Hunger Code is a continuation of Dr. Jason Fung’s earlier book about healthy eating, The Obesity Code. It restates the message that controlling insulin is the key to weight management and addresses the issue of hunger. While there is some very useful information in this book, overall I thought it was repetitive and longer than it needed to be. Also, I felt that Dr. Fung promoting his online fasting support community without mentioning the membership fee was questionable. I’ll still read anything he adds to his Code series of wellness books because The Cancer Code and The Obesity Code are both exceptional, but I have to say this one just doesn't measure up.

[Amber Faustino · Rating 5 out of 5]

Wonderful information

I have a lot of education on fasting and have lost 80 lbs before with it but sadly gained it all back. I'm back to fasting now and learning about HUNGER is a huge game changer. I really appreciate this book and the knowledge from Dr. Fung. My mother was a type 1 and my father is a type 2. You have helped me prevent becoming a type 2 and saved my life. Truly.

[Ursula S · Rating 5 out of 5]

This book is chock full of information as was The Obesity Code, but with more hands on application and guidance on what to do to counter modern food challenges.

I listened on Audible as well as purchased the book The Audible narrator is funny and easy to listen to. I also took advantage of Dr. Fung's Masterclass related to this book. I've learned a lot. I will likely be referring to this book frequently as it will be a reference and guide for years to come.

[Sage · Rating 5 out of 5]

4.5 stars for me. This was an easy read. Engaging and simple to understand. I found it concise which meant I didn't get lost and lose the point of what he was trying to say. Quite a few practical ideas that I've taken note of and will give a try. Not necessarily new information (which really is his point anyway) but good to have it all in one book in such an easy, engaging and accessible way.

[Josh Katz · Rating 2 out of 5]

So far a very interesting book full of misinformation. If you look at the sources he cites and compare to what he said, he's either a moron or intentionally misleading people to make his case. He seems to be a master of the art of cherry picking. Still, I will finish reading it, but would encourage others to take nothing he says as fact without double checking it. redpenreviews.org critically looks at some of his other books.

[Suzanne · Rating 3 out of 5]

I am a regular reader of Jason Fung’s books and have learned a lot from them. THE HUNGER CODE is far less focused than his previous work and suffers from a desire to be all things to all people. I had hoped to learn more about GLP-1s but the addition of them to the book was minor; they were not well addressed. Much of this book can be found in previous work. New readers will benefit from the information, as I have. There were also sections devoted to behavioral mod techniques to manage eating. Again, this will be most helpful to new readers of his work. I received my copy from the publisher through edelweiss.

[Martin · Rating 4 out of 5]

Some great insights here, and a good follow-up to the Obesity Code.

Downsides are the book was not edited very well, I noticed many spelling/grammar mistakes. There were parts that were quite repetitive, by the end of the book I felt I'd read the same sentence several times before.

Overall, Jason Fung has changed my life, so will always read anything he publishes.

[Christine Dimino · Rating 5 out of 5]

Another great read by Dr Fung! If you want to understand what is behind the obesity crisis in America or what is making it seem impossible for you to lose weight…this is the read! Simple concepts with lots of science to back it up. Very accessible and at times downright charming writing. TEN STARS!