How to Start Fasting: A Practical Guide for Beginners Based on Dr. Jason Fung's Work
When I first came across Dr. Jason Fung's book The Complete Guide to Fasting, I had the same hesitation most people do. Fasting sounded extreme. It sounded like something reserved for people with iron willpower or specific medical conditions, not something a regular person builds into an ordinary week.
What changed my mind was the research behind it. Dr. Fung is a nephrologist who spent years treating patients with type 2 diabetes and obesity, and what he found consistently was that the timing of eating matters just as much as what you eat. Fasting, in his framework, is not a punishment or a deprivation strategy. It is a way of giving your body the time it needs to do what it already knows how to do: regulate insulin, burn stored fat, and repair at the cellular level.
I have had my own experience with fasting and seen what it does when you approach it thoughtfully. This guide covers the essentials so you can start with a clear understanding of what you are doing and why.
What Fasting Actually Does in the Body
Before getting into the how, it helps to understand the why.
Every time you eat, your body releases insulin to manage the glucose entering your bloodstream. Insulin's job is to move that glucose into cells for energy, and to store what is not immediately used as fat. The problem with eating frequently throughout the day, which is what most modern dietary advice has encouraged, is that insulin stays elevated for most of your waking hours. When insulin is consistently high, your body stays in storage mode. It does not have the opportunity to access and burn the fat it has already stored.
Fasting changes that. When you stop eating for an extended period, insulin levels fall. Once they fall low enough, your body shifts from burning glucose to burning stored fat for fuel. This metabolic shift is at the heart of why fasting works for weight management, insulin sensitivity, and metabolic health. Dr. Fung documents this extensively in his research with patients who had tried conventional low-calorie dieting without lasting results.
Starting with Intermittent Fasting
The most accessible entry point into fasting is intermittent fasting, often called IF. Rather than going days without eating, intermittent fasting works by compressing your eating into a defined window each day and fasting during the remaining hours.
The most commonly used approach is the 16:8 method, which means fasting for 16 hours and eating within an 8-hour window. In practice, this is simpler than it sounds. If you finish dinner at 8 p.m. and do not eat again until noon the next day, you have completed a 16-hour fast. Most of those hours happen while you sleep, which makes the fasting window feel much shorter in real time.
For many people, skipping breakfast is the easiest way to implement this. Your body is already in a fasted state when you wake up, and extending that state by a few hours requires relatively little adjustment. Black coffee or unsweetened tea in the morning makes this easier because neither raises insulin and neither breaks your fast.
If 16 hours feels like too much initially, starting with 12 hours is completely reasonable. The goal in the beginning is simply to establish the habit and let your body adjust to going longer between meals. Once that feels comfortable, you can gradually extend the fasting window.
Other Fasting Schedules Worth Knowing
Intermittent fasting is not the only format, and Dr. Fung outlines several approaches depending on your goals and how your body responds.
Alternate-day fasting involves fasting every other day, either with a complete fast or by eating a small, nutrient-focused meal of around 500 calories on fasting days. This approach tends to produce more significant metabolic changes than daily 16:8 fasting, and it works well for people who find it easier to commit fully on some days rather than partially every day.
24-hour fasts involve stopping eating after dinner one evening and not eating again until dinner the following day. This creates a full day without food while still allowing you to eat every day. Dr. Fung often recommends this format two to three times per week for people working on metabolic health or weight loss. It sounds demanding at first, but most people find it much more manageable than they expected once they try it.
Extended fasting refers to fasts lasting 48 hours or longer. This is not a starting point, but for people who have established a solid fasting practice and want to go further, extended fasts produce deeper effects on insulin sensitivity, cellular repair, and inflammation. If you consider going beyond 48 hours, doing so with medical guidance is a sensible precaution, particularly if you take any medications or have existing health conditions.
What You Eat When You Break Your Fast Matters
Fasting and food quality are not separate conversations. What you eat when you break your fast directly determines how effectively your body uses the metabolic window fasting has opened.
The goal when you end a fast is to keep insulin from spiking sharply. A meal high in refined carbohydrates or sugar immediately after a fasting period undermines much of what the fast was building toward. Your insulin, which had been low and allowing fat burning, surges in response to a high-glucose meal, and the metabolic shift reverses quickly.
I focus on low-carb, nutrient-dense whole foods when I break a fast. Eggs are one of the most practical choices, high in protein and fat, deeply satisfying, and easy to prepare in any form. Avocado provides healthy fat that sustains energy without triggering an insulin response. Fatty cuts of meat, fish, non-starchy vegetables cooked in good quality fat, and full-fat dairy if you tolerate it well are all foods that work with your fasted physiology rather than against it.
Processed foods, grains, and sugar have no place in a meal that follows a fast. They undo the work and train your body to expect the glucose rollercoaster rather than stable, fat-based energy.
Hydration During a Fast
Staying well hydrated during a fasting window is one of the simplest things you can do to make the experience more comfortable.
Water is the foundation. Drink it consistently throughout the fasting hours rather than waiting until you feel thirsty. Black coffee is well-suited to fasting because it does not raise insulin and, for many people, it genuinely reduces hunger during the morning hours. Plain teas, including green tea and herbal varieties, work the same way.
If you are extending into longer fasts, electrolyte balance becomes more important. When insulin is low, your kidneys excrete more sodium, and sodium loss pulls other electrolytes with it. Adding a small amount of high-quality salt to your water, or choosing a clean electrolyte supplement without sugar or artificial ingredients, helps prevent the headaches, lightheadedness, and fatigue that people sometimes associate with fasting and that are almost always a hydration issue rather than a fasting issue.
How to Extend Your Fasting Window Gradually
One of the most common mistakes people make with fasting is trying to go from three meals a day to a 24-hour fast in a single week. The adjustment period matters, and rushing it tends to create a difficult experience that makes people give up before the body has had time to adapt.
The transition to fat burning as a primary fuel source takes time. In the early days of fasting, if your body has been accustomed to running on glucose, it may not yet be efficient at accessing fat stores. That inefficiency shows up as hunger, irritability, or low energy during the fasting window. These are not signs that fasting is wrong for you. They are signs that adaptation is underway.
Starting with 12 to 14 hours and living comfortably at that level for one to two weeks before extending further is a reasonable way to build. Moving to 16 hours, then experimenting with 18, then introducing a 24-hour fast occasionally as your schedule allows, gives your physiology time to shift without the experience feeling punishing.
What Your Body Is Telling You
Fasting works best when you stay connected to how you actually feel rather than following a rigid schedule regardless of signals.
Hunger during a fasting window is normal, and for most people it comes in waves rather than as a constant state. A wave of hunger that feels urgent often passes within 20 to 30 minutes if you stay occupied and drink water or black coffee. Riding through those waves gets easier with practice, and most people are surprised by how manageable real hunger is once they experience it.
What is worth paying attention to is the difference between hunger and something else. If you feel genuinely weak, dizzy, or unwell during a fast, ending the fast and eating is the right call. Fasting is a tool, not a test of endurance. If your body is sending clear signals that something is off, trust that.
Keeping Fasting Flexible
Dr. Fung is clear on this point: fasting should serve your life, not complicate it.
A social dinner, a family occasion, or a week when your sleep and stress are already strained are all reasonable times to step back from a fasting protocol without guilt. The value of fasting comes from what you do consistently over time, not from perfect adherence on any given day. Missing a fast is not a failure. Adjusting when you eat to fit a real and full life is exactly what a sustainable practice looks like.
What tends to erode results is the habit of abandoning fasting entirely at the first inconvenience. Flexibility means adapting, not stopping. If you know a week is going to be demanding, eating an earlier dinner and a later breakfast, even if it is only a 12-hour window, keeps the practice alive without requiring anything extreme.
The Mental Shift That Makes Fasting Sustainable
The most significant change fasting produces for many people is not physical, at least not at first. It is a shift in the relationship with food.
Most of us have been conditioned to eat on a schedule, to snack to prevent hunger, and to interpret any hunger at all as something that requires immediate resolution. Fasting gently challenges all of that. When you discover that hunger passes, that you are capable of going hours without eating and feeling completely functional, something changes in how much control food has over your day.
That shift tends to be quiet but lasting. You stop organizing your time around meals the way you once did. You make better food choices when you do eat because you are eating from a place of intention rather than impulse. The mental freedom that comes with that is something many people describe as one of the most unexpected benefits of fasting.
If you are considering trying this, start simply. Begin with 12 to 14 hours, pay attention to what you eat when you break the fast, drink enough water, and give yourself two weeks before making any judgments about how it feels. Your body often needs more time than we expect to show you what it is capable of.
References
Fung, J., & Moore, J. (2016). The Complete Guide to Fasting: Heal Your Body Through Intermittent, Alternate-Day, and Extended Fasting. Victory Belt Publishing.
Fung, J. (2016). The Obesity Code: Unlocking the Secrets of Weight Loss. Greystone Books.
Anton, S. D., et al. (2018). Flipping the metabolic switch: Understanding and applying the health benefits of fasting. Obesity, 26(2), 254-268.
Patterson, R. E., & Sears, D. D. (2017). Metabolic effects of intermittent fasting. Annual Review of Nutrition, 37, 371-393.
Cahill, G. F. (2006). Fuel metabolism in starvation. Annual Review of Nutrition, 26, 1-22.
This article is for informational purposes only and does not constitute medical advice. Please consult a qualified healthcare provider before beginning any fasting protocol, particularly if you take medications or have existing health conditions.
