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.

Module 2: Fasting in Practice—It's a Learnable Skill

Okay, so the theory makes sense. But the idea of not eating can be intimidating. We're conditioned to fear hunger. The authors, however, present fasting as a learnable skill that gets easier with practice. It's about retraining your body and mind.

The first step is understanding that hunger is often a conditioned response, not a physiological need. Think of Pavlov's dogs. They salivated at the sound of a bell because they associated it with food. We are the same. We feel hungry at noon because it's "lunchtime." We crave popcorn at the movies out of habit. This is learned hunger. True physiological hunger is different. During a fast, you'll find that hunger comes in waves. It builds, peaks, and then passes, often within an hour. It is a manageable sensation. A cup of green tea or coffee is often enough to ride out the wave. With practice, you learn to distinguish between habitual urges and genuine need.

Building on that idea, a powerful way to make fasting easier is to first become "fat-adapted" through a low-carbohydrate diet. When your body is used to burning sugar for fuel, it has a very limited supply. Once you burn through your stored glycogen, your body panics, triggering intense hunger until it can switch over to burning fat. This transition can be rough. But if you're already on a low-carb, high-fat diet, your body is primed to burn fat for fuel. When you start a fast, the transition is seamless. You simply continue burning fat, but now it's coming from your body's stores instead of your plate. This dramatically reduces hunger and makes fasting feel effortless. Co-author Jimmy Moore found that once he was in a state of nutritional ketosis, he would often fast unintentionally simply because he wasn't hungry.

Next up, it’s critical to manage your electrolytes and stay hydrated. Many of the negative side effects people associate with fasting—headaches, fatigue, dizziness—are symptoms of dehydration and low salt. When you fast, insulin levels drop, which signals your kidneys to excrete excess salt and water. You need to replenish this. Drinking plenty of water is essential. Adding a pinch of sea salt to your water or drinking homemade bone broth can make a huge difference in how you feel. It's also important to avoid things that can sabotage your fast, like diet sodas. The artificial sweeteners can trigger an insulin response and stoke hunger, making the experience much harder.

Finally, remember that fasting is a flexible tool, not a rigid dogma. There are many ways to fast. You can start with a 16:8 protocol, where you fast for 16 hours and eat within an 8-hour window. This is as simple as skipping breakfast. You could try a 24-hour fast once or twice a week, eating dinner one day and then not eating again until dinner the next. Or you might experiment with a 36-hour fast. The key is to find what works for your schedule and your goals. Life happens. If you have a celebration or a special event, you feast. Then you can balance it with a period of fasting. This flexibility is what makes it a sustainable, long-term strategy.