In a world flooded with diets, workout plans, and weight loss supplements, The Obesity Code by Dr. Jason Fung offers a radically different perspective—one grounded in medical science, historical evidence, and clinical observation. Fung, a Canadian nephrologist, dives deep into the root causes of obesity and challenges many of the most common (and mistaken) beliefs about weight gain and weight loss.
Rather than focusing solely on calorie counting or exercise, Dr. Fung brings a metabolic lens to the conversation, arguing that hormones—particularly insulin—are the true drivers of fat storage and obesity. The result is a thought-provoking book that rewires how we think about body weight and reveals why so many modern diet strategies fail.
The Central Premise: It’s Not About Calories
Dr. Fung begins with a bold claim: “Obesity is not a problem of caloric excess. It is a hormonal imbalance.”
For decades, the dominant model—known as the Calories In, Calories Out (CICO) model—has told us that weight gain is simply the result of eating more than we burn. Fung calls this view overly simplistic and misleading. He argues that it ignores how our hormonal environment, especially insulin, governs when and how we store fat.
Think of insulin as the “fat-storage hormone.” Every time we eat (especially carbohydrates), insulin levels rise. When insulin is high, the body stores energy. When insulin is low, the body burns stored fat for fuel. So, if we’re always eating—especially refined carbs—insulin stays elevated, and fat-burning gets shut off.
What Really Causes Obesity?
Fung identifies chronic high insulin levels (also known as hyperinsulinemia) as the real culprit behind most cases of obesity. He backs this up with evidence from:
- Historical trends: Obesity rates have exploded since the late 1970s—coinciding with the introduction of low-fat dietary guidelines and a rise in carbohydrate intake.
- Hormonal research: Insulin not only stores fat, but it also blocks its release. Over time, persistently high insulin levels make fat loss nearly impossible, even if you reduce calories.
- Clinical experience: As a physician treating patients with type 2 diabetes, Fung observed that patients often gained weight as their insulin dosage increased.
Debunking the Myths
Dr. Fung takes aim at several major myths perpetuated by the diet industry:
1. A Calorie Is a Calorie
False. The body processes calories differently based on their source. 100 calories of sugar spikes insulin much more than 100 calories of fat. The hormonal response—not just the energy content—matters.
2. Eat Less, Move More
This advice has failed millions. While it sounds logical, the body fights back against calorie restriction by slowing metabolism and increasing hunger. That’s why most people regain weight after dieting.
3. Fat Makes You Fat
Another myth. Fat does not raise insulin nearly as much as carbs do. Low-fat diets often lead to increased carb consumption, which elevates insulin and can lead to weight gain.
The Real Solution: Control Insulin, Unlock Fat Loss
Dr. Fung proposes a radically different approach based on insulin control rather than calorie restriction. His recommended strategies include:
1. Reduce Refined Carbohydrates
White bread, pasta, rice, and sugar spike insulin dramatically. Replacing these with whole foods (especially healthy fats and proteins) can stabilize insulin and promote fat loss.
2. Avoid Snacking
Every time you eat—even small snacks—you trigger an insulin response. Fung recommends returning to fewer meals per day to allow insulin levels to drop between meals.
3. Implement Intermittent Fasting
Fung is one of the strongest advocates for intermittent fasting (IF). By extending the time between meals (e.g., 16, 24, or even 36 hours), insulin has time to fall, and the body begins burning stored fat for energy.
Contrary to popular belief, fasting doesn’t wreck metabolism—in fact, it often improves energy, mental clarity, and fat-burning capacity. Fung emphasizes that fasting is not about starving—it’s about giving the body time to switch from storing to burning mode.
Understanding Set Weight
A key idea in the book is the “set weight” theory: your body has a weight it tries to maintain, based on hormonal signals. Repeated dieting and frequent eating (especially of high-insulin foods) push this set weight higher. To lose weight and keep it off, you must reset your body’s internal weight thermostat, and the only sustainable way to do this is by reducing insulin over time.
Food Quality and Timing
Fung offers broad guidelines for what and when to eat:
- Eat real, whole foods. Focus on vegetables, healthy fats (like olive oil and avocado), moderate protein, and whole carbs like sweet potatoes.
- Avoid processed and sugary foods. These spike insulin and create cravings.
- Use fasting strategically. Start with 12- to 16-hour fasts, and increase gradually. Include longer fasts occasionally if desired.
He also reassures readers that fat is not the enemy. In fact, healthy fats (like nuts, seeds, butter, and coconut oil) are often more satiating and have a minimal impact on insulin levels.
Final Thoughts: A Paradigm Shift
The Obesity Code doesn’t just give readers a list of foods to eat or avoid—it reshapes the entire way we think about weight loss. By reframing obesity as a hormonal, not caloric, issue, Dr. Fung opens the door to more effective, sustainable strategies for achieving a healthy weight.
This is not just about looking better. It’s about reversing type 2 diabetes, improving metabolic health, and breaking free from the cycle of crash diets and frustration.
If you’ve tried counting calories and failed…
If you’ve eaten “low fat” and felt hungry and deprived…
If you’ve exercised for hours but made little progress…
Then this book might be the breakthrough you’ve been waiting for.
Bottom Line:
The Obesity Code teaches that insulin control—not calorie restriction—is the key to weight loss. By changing what and when we eat, we can work with our bodies instead of against them to unlock sustainable fat loss and better health.
