The Complete Guide to Fasting

In 1977, the United States published its first official dietary guidelines, launching a massive, decades-long public health experiment. The core advice was to reduce fat intake and increase carbohydrate consumption. Over the next 40 years, total calorie consumption rose by an average of 400 calories per person per day, with carbohydrates accounting for nearly all of that increase. Simultaneously, national obesity rates tripled, climbing from around 15% to over 42%. The incidence of type 2 diabetes followed a near-identical trajectory, increasing more than seven-fold from 1980 to 2018. This was the predictable outcome of a flawed nutritional model.

This exact paradox—eating less fat and exercising more, yet getting sicker and heavier—is what nephrologist Dr. Jason Fung witnessed in his practice for years. He specialized in treating kidney disease, a condition often caused by severe type 2 diabetes. He prescribed the standard treatments, including insulin, yet his patients consistently worsened. He realized that the conventional advice was failing them at every turn. Dr. Fung began an exhaustive search of medical literature, uncovering centuries of evidence for a powerful, forgotten therapeutic tool: fasting. He partnered with veteran health podcaster and author Jimmy Moore, who had personally struggled with and overcome obesity, to combine the deep scientific evidence with practical, real-world application. They created this guide as a clear explanation of the science and a structured approach to reclaiming metabolic health.

Module 1: The Hormonal Truth of Weight Gain

Let's start by dismantling a core belief. For decades, we've been told that weight management is a simple math problem. Calories in versus calories out. If you're gaining weight, you just need more discipline. Eat less, move more. But the authors argue this is a dangerously flawed model. The real driver is hormones.

This leads to the first major insight: Obesity is a hormonal imbalance, not a caloric one. The key player is insulin, the body's primary fat-storage hormone. When you eat, especially carbohydrates, your insulin levels rise. This signals your body to store energy. Some of this energy goes into your liver as glycogen, which is like a refrigerator—easy to access but with limited space. Once that's full, the excess energy is converted to fat and stored in your fat cells. This is your body's long-term energy reserve, like a basement freezer. High insulin levels essentially lock the door to that freezer. Your body is in storage mode, not burning mode.

So what does that mean in practice? When someone with high insulin levels tries to cut calories, their body can't access its fat stores for energy. It's locked away. The body's only response is to slow down its metabolism to conserve energy. You feel tired, cold, and hungry. Your body is fighting you every step of the way to regain the weight. This explains why studies of shows like The Biggest Loser reveal that nearly all contestants regain the weight. Their metabolisms had slowed so dramatically that maintaining their new weight became almost impossible.

This brings us to a crucial distinction. The body has two separate energy compartments: sugar and fat. Think of it this way. Glycogen, the sugar, is your spending cash. It's readily available but you don't have much of it. Body fat is your savings account. There's plenty in there, but you need the right key to access it. That key is low insulin. When insulin is low, your body can easily switch from burning sugar to burning its vast reserves of stored fat.

The authors argue that modern eating habits keep us in a constant state of high insulin. A typical day might involve breakfast, a morning snack, lunch, an afternoon snack, dinner, and a late-night snack. This pattern means insulin is almost always elevated. Your body never gets the signal to unlock the fat stores. It's always in storage mode.

So, the solution isn't just to eat less. The solution is to create periods of low insulin. This is where fasting comes in. By abstaining from food, you give your body the time it needs for insulin levels to fall. This drop signals your body to start burning its stored energy. First, it uses up the glycogen in the liver. Then, it seamlessly switches to burning body fat. Fasting fundamentally changes the hormonal signals that control your metabolism. It gives your body the key to unlock the freezer.