I have recently started this, and it's been a huge gamechanger in addition to the other interventions on this site!
Like all things on this site, this is simply what worked for me, NOT medical advice. Always consult with your doctor before making any significant diet or lifestyle changes!
How I do it
This isn't the only way to do it, but it is what works for me. Once a month, I do a 40-hour complete fast. This means water (adding a bit of salt for electrolytes is OK), black coffee, black or green tea only. No sweeteners or creamers!
Example of a fasting schedule
Thursday night, have a nutritionally dense high-protein and carb-free dinner (I usually make a nice steak). Wrap up eating by 6pm.
As soon as I finish that dinner, I do some moderate exercise. The timing of this is intentional! A light jog or even a walk does the trick.
Friday is the only "hard day". I usually try to schedule my "hard days" for when I'm at work, the day seems to go faster that way and I'm less aware of being hungry!
Saturday at 10am is when I hit the 40 hour mark and can break my fast. I go with something that's easy to digest and yet very nutritionally dense and high-protein, usually scrambled eggs. I usually try to schedule my "break fast" days for when I am not at work, so I can sleep in until breakfast time and then have access to my whole kitchen.
Super important - You really need to be in a good nutritional state for this to work since you are briefly entering "starvation mode" physiologically. For me I strongly believe that coupling periodic fasting with a generally healthy, high-protein diet, focusing on giving my body what it needs is leading to the positive results I am seeing in my life. I wouldn't try it unless you've already established a healthy eating pattern. And don't do it if you're in the middle of a Crohn's episode, this is more for maintainence of health and not "shutting off" a flare-up. Definitely talk to your doctor before trying this.
My Results
I'll summarize the science below, but I imagine most folks will want to know what this did for me. At the time of this update, so far I have done three monthly fasts, and plan to continue monthly for the long haul. I have found that in addition to helping me continue to meet my general health and body goals (it feels amazing to have the body I want, after spending years overweight, and to know that I just have to maintain it) I simply feel amazing after I do this. Brain fog lifts, I got more done at work, I found myself able to keep up with my kids and dog more, and my mood improved. It's honestly pretty amazing.
Also, it helps with general discipline and integrity in life as well. Developing the discipline to tell myself "I am hungry, but I can master my appetites for the sake of my health" has been wonderful.
How it works: Glycogen depletion, Ketosis, Autophagy, AMPK and mTOR
As it turns out, your body can actually run on a two different fuel sources. You may recall from high school biology that glucose is the "preferred" fuel of the body, which it will readily use when available. When you get a surge of glucose, from carbohydrates, the body saves the extra glucose in a "storage form" called glycogen, which is actually a glucose polymer - a bunch of glucose molecules stuck together in a branching chain. It gets stored in your muscle tissue and your liver. This glycogen can then be broken back down into glucose to be released back into the bloodsteam for in-between meals.
The "trick" in fasting is to actually deplete your glycogen levels to basically zero, which then causes the body to run on fuel source #2, fat. When you're not getting glucose and run out of glycogen, the body will turn to burning fats, specifically triglycerides. Triglycerides are made of three fatty acid chains linked together by a molecule called glycerol. These then travel to the liver where the fatty acids undergo a chemical process that turns them into ketones. The rising levels of ketones in the blood is known as ketosis. In this state, the body is running almost exclusively on ketones, with the liver making a bit of glucose from amino acids and glycerol in a process called gluconeogenesis, just enough glucose to keep your brain functioning, as the brain is the only organ that always needs glucose to function.
That's why I do the carb-free "last meal" and exercise right after eating that meal. By not giving my body any carbs, I can't make any new glycogen at dinner, so all the time between lunch and dinner that I've been burning glycogen accelerates what happens next. Going for some light-to-moderate exercise after helps further "burn" glycogen. The goal is to get to rock bottom glycogen levels quickly. The more time your cells spend in autophagy, the more clean-up and repair they'll do, and the more benefits you'll get from a fast.
This radically alters cell physiology. A protein called AMPK (AMP-activated protein kinase, where AMP = adenosine monophosphate, a "low energy" marker in cells that results from the energy currency of the cell, adenosine triphosphate, which you may recall as ATP from high school biology, releasing it's energy. Essentially AMP is the "spent" form of ATP) activates, which in turn deactivates another protein called mTOR (Mechanistic Target of Rapamycin). This triggers a process called autophagy, which basically means that your cells "realize" that they are "running lean" and need to be as efficient as possible so they start identifying old worn-out proteins that are no longer functional, and also old worn-out organelles, including mitochondria, the famous "powerhouse of the cell." They break down the old proteins and organelles, and then recycle their chemical parts. This "spring cleaning" makes the cells function much better, including that they produce energy from food more efficiently. Autophagy literally means "self eating" because your cells essentially "eat" the worn out proteins and organelles in order to recycle them!
The coolest part of all of this for individuals with Crohns or other inflammatory conditions, is that rising AMPK and falling mTOR levels change patterns of gene expression - the way your cells literally read and use your DNA code. This transitions the cells out of inflammatory states, and chronic inflammation is at the root of Crohns. This happens because pro-inflammatory genes are downregulated when mTOR levels fall.
A simple way to think of it is, this cellular "cleaning house" helps your immune system to "chill out" and lowers inflammation body wide.
Research articles:
As always, I want to back up what I'm posting on this site with peer-reviewed science, here are a few research articles that get into this:
An important caveat: There is some evidence that extremely aggressive fasts, or fasts that are not managed well, can deplete your nutrition to the point where your gut barrier struggles to heal itself, and then the fast becomes counterproductive. That's why I'm so intentional to only do this once a month, and to have really good nutrition both right before I start the fast, and immediately during refeeding, in addition to having a healthy diet already in place so that my body is in a good nutritional state when I begin.