

This article is an excerpt from the Shortform book guide to "Fast. Feast. Repeat." by Gin Stephens. Shortform has the world's best summaries and analyses of books you should be reading.
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Are you considering adopting intermittent fasting (IF) as a lifestyle? What are some things you should keep in mind when beginning your transition to the IF feeding schedule?
Transitioning to intermittent fasting (IF) is a big step, especially if you’re used to following the standard three-meal feeding approach. According to Gin Stephens, the author of Fast. Feast. Repeat., the key to successfully transitioning to the IF lifestyle is to adapt your body to the IF feeding over a four-week period.
Keep reading to learn how to start fasting by committing to a four-week adaptation period.
Commit to a Four-Week Quickstart
If you want to know how to start fasting, Gin Stephens lays out a four-week program during which you’ll adapt to IF by depleting your glycogen stores and teaching your body to access your fat stores. While adapting, be consistent and patient. Follow through with the full four weeks once you start. If you dabble, your body will struggle to adapt and you’ll delay the benefits of IF.
Keep your fasting clean: Follow Stephens’s clean fasting guidelines to teach your body to access its fat stores. You’ll gradually deplete your stored glycogen and cue your body to search for an alternate source of fuel—your fat.
Do not change what you eat during this four-week period. Adapting to IF is a large change, and another—such as diet change—can overwhelm you and hinder the process.
Build the IF Habit While Stephens gives plenty of information about fasting, she doesn’t dive into how you can successfully change your behavior. In Tiny Habits, BJ Fogg explains that successful behavioral change occurs when you have sufficient motivation, ability, and a prompt that gets you started. He explains this as Behavior = MAP. The core of Fogg’s approach is to make the change as small as possible. He argues that motivation is unreliable, so we need to make sure our new behaviors are small enough that we won’t find excuses to avoid them. Create a “tiny habit” by finding just the first step of the behavior you want to learn—for instance, putting on your socks and shoes to start a running habit. For intermittent fasting, you might commit to one day of fasting and build momentum from there. Lastly, celebrate your success each time you perform the new behavior. According to Fogg, this reinforces the behavior in your brain and helps make it automatic. Plus, feeling good about yourself is healthy for its own sake. |
Stephens offers three approaches to begin fasting: An easy, medium, and hard approach. In each, you’ll build from shorter fasts and longer feasts to longer fasts and shorter feasts. Below, we’ve notated the fast/feast windows with the x:y ratios explained in “How to Time Your Fasts.” Pick one and follow the fasting/feasting patterns prescribed for each week.
- Approach #1: Easy—In week one, 12:12. In week two, 14:10. In week three, 16:8. In week four, 18:6.
- Approach #2: Medium—In week one, 16:8. In week two, 17:7. In week three, 18:6. In week four, 19:5.
- Approach #3: Hard—In weeks one and two, 18:6. In week three, 19:5. In week four, 20:4.
(Shortform note: In Tiny Habits, Fogg also argues that we should treat behavioral change as a scientific experiment instead of a judgment of motivation or willpower. If the approach you choose is too tough, consider that you might’ve asked yourself to make too big a change with too little motivation, ability, or no good prompt. Reflect on what didn’t work, and then use Fogg’s technique: Brainstorm several behaviors that could help you adapt to IF, and pick the “golden behavior”—the option that’s both feasible and impactful.)
Common Changes While Adapting
Once you begin, your body needs to get used to fasting instead of having the glucose from consistent eating. When that glucose isn’t available, you might experience lethargy, headaches, and fatigue. This means your body is depleting your glycogen stores. After a few weeks (usually three to four, or eight at most), you’ll experience heightened energy and ease during your fasts.
Stephens stresses that it’s fine if you get hungry during your fast—this happens to everyone. Know that hunger tends to come in waves that pass. It won’t build forever or overwhelm you. However, your appetite will likely increase since your body can’t access your fat stores very well yet. Once your body learns to access fat for energy, your appetite will settle down.
(Shortform note: Harvard Health explains that alternate-day fasting causes headaches and lethargy more often than time-restricted eating. If you experience severe pains, they recommend using TRE instead of ADF or fasting less frequently. In addition, they note that fasting often causes overeating since your appetite hormones ramp up while you aren’t eating. To avoid this, they recommend easing into IF by gradually decreasing your eating window over a period of several months—much more gradually than Stephens prescribes.)
Adjust Your Approach
If your first approach doesn’t work out, Stephens recommends you switch to another. If it was too easy, pick a harder plan. If it was too hard, pick an easier plan.

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Here's what you'll find in our full Fast. Feast. Repeat. summary :
- How intermittent fasting can help you lose weight, feel better, fight disease, and live longer
- An explanation of the cutting-edge science that supports fasting
- How to follow a four-week quickstart program to adapt to this new lifestyle