What is a Fasting Mimicking Diet and How to Do It

Dr. Anthony Gustin is a board-certified sports chiropractor, functional medicine practitioner, entrepreneur, podcast host, and founder of Perfect Keto.

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It’s scientific, it’s nutrient-dense, and it may have huge advantages for your long-term health. Today we’re talking about the fasting mimicking diet—an alternative to full-on fasting that more and more people are trying.

To give you an idea of what it’s all about, this article will cover what a fast mimicking diet is and how to do it, along with how it’s different from normal fasting and why you’d want to do it for yourself.

What is a Fasting Mimicking Diet?

Fast mimicking is a type of modified fasting. Instead of abstaining from food completely like a traditional fast, you still consume small amounts of food in a way that produces the therapeutic benefits of fasting.

A fast mimicking diet typically lasts about five days and follows a healthy protocol low in carbs, protein, and calories and high in fat. Calories are kept at around 40% of normal intake. This allows the body to stay nourished with nutrients and electrolytes will less stress than normal fasting—but still receiving the same benefits.

Long-term calorie restriction and long-term fasting can harmful, but fast mimicking is safer and more effective. Let’s look at how much it differs from traditional fasting:

Fast Mimicking Vs Traditional Fasting

Just like with intermittent fasting, there are a lot of myths out there about these types of modified fasts: that our muscle will waste away, that we’ll kill out metabolism, and that it’s downright unhealthy.

The above might be true with a person who is actually starving, restricting calories for weeks/months/years at a time, or doing a no-food water fast. These can lead to metabolic damage and are especially bad for people with underlying health conditions. But like we’ve said above, fast mimicking gives you all the benefits without these side effects.

Next, we’ll cover exactly what those benefits are.

Benefits of Fast Mimicking

Benefits of fast mimicking are basically the same as what you can expect from regular fasting, including:

8 Benefits of Fast Mimicking

Fast mimicking essentially “tricks” our bodies by reducing calories enough to feel as if they’re fasting so they can reap these benefits.

Now that we’ve covered what exactly fast mimicking is and why it’s worth doing, the following information will cover what you need to know to do it.

How to Do Fast Mimicking

Here are the basics of what you need to know to do fast mimicking:

Getting Started

Most research on fast mimicking shows the best results after about five days and/or when your glucose ketone index falls below 1.0, but doing it anywhere between 3-7 days is beneficial. It’s also intended to be repeated at least twice per year, or as often as once a month, to get enough benefits from it.

You might also want to measure certain biomarkers if you’re interested in tracking your fasting outcomes. This could be doing lab tests before and after the fast as well as measuring blood glucose and ketones each day and weight changes.

Also, you might want to set up your environment for the fast:

  • Tell friends and family you see during this time what you’re doing and why it’s important you have support.
  • Get rid of any snack foods at home or work that might tempt you.
  • Plan to give yourself more time for sleep, since you’ll likely be more tired than usual.
  • Plan for some physical activity each day, but only light stuff. Avoid intense exercise during this time.

Now that we have the preliminary stuff out of the way, let’s cover the basics of doing the fast.

Fast Mimicking Basics

Some people find easing into the fast helps by eating slightly higher calories the first day, around 50% of total intake. This is then reduced to around 35-40% of total caloric intake for the rest of the days. You also want the things you do consume to be easily digestible and in very small amounts.

One popular pre-packaged fast mimicking diet, called ProLon, contains all five days’ worth of food for the fast. The food is all plant-based and one day looks like tea and a nut bar for breakfast, a small amount of vegetable soup and a few kale crackers for lunch, a few olives in the afternoon, and another small portion of soup for dinner.

You don’t need a pre-packaged box to do a fast mimicking diet, though—especially if you want to keep the diet low-carb ketogenic. Just stick to the calorie percentages and considered how you’ll space them out each day. Make sure you plan out all the food for the entire fast so there’s nothing else to worry about.

Macros for traditional fast mimicking are 34% carbs, 10% protein, and 56% fat for the first day and 47% carbs, 9% protein, and 44% fat for the remaining days. This is obviously different if you’re following a ketogenic diet, which we’ll go into further below.

Black coffee and tea (a cup per day) are typically allowed, although not necessary—and make sure they don’t contain any added creams or sugars (coconut oil is okay if you factor it into macros).

Note that those with health issues should consult with their medical practitioner before doing a fast mimicking diet at home.

It’s also important to supplement the fast to ensure you receive important nutrients and electrolytes. See our ideas below.

Supportive Supplements

Taking supportive supplements helps ease into the fast, as well as provide enough nutrition throughout. Those might include:

  • Electrolytes like magnesium and salt to replenish any loss through water loss
  • Grass-fed liver tablets to provide micronutrient support
  • Branch chain amino acids (BCAAs) to help prevent any loss of lean tissue
  • Greens powder to provide micronutrients
  • Algal oil or cod liver oil for omega 3s

You can also take exogenous ketones if you’re trying to get into ketosis through a ketogenic diet. Speaking of ketones, fast mimicking is perfect for easing into ketosis before eating keto. We’ll discuss that next.

Fast Mimicking and Ketosis

Fast mimicking is a great preface to a ketogenic diet. That’s because it can help you get into ketosis faster. Plus, eating keto foods helps you stay in ketosis the whole time.

With ketogenic fast mimicking, you’ll want to keep your macros within the proper range of 5-10% carbs, 20-25% protein, and 70-80% fat. If in doubt, always choose something with more fat.

What to Consume

See this four-day ketogenic fast mimicking done by Dr. Anthony. Each day, he consumed different amounts of bone broth, coconut milk, coconut oil, BCAAs, and exogenous ketones. Avocados and grass-fed butter are other ideas. This is a great way to combine keto with fast mimicking, and it will help you get into ketosis much faster than without any fasting.

Take Home Message

To summarize everything:

Fast mimicking is a great hack to get the benefits of fasting while still providing your body with some food and nutrition.

Make sure you keep calorie counts low and use proper supplements to stay in ketosis without health issues.

Set up your environment for success on your fast mimicking diet.

Combine a ketogenic diet with fast mimicking to get into ketosis faster and get the most benefit out of both.

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Responses (18)

  1. Will someone PLEASE tell me how many calories are consumed each of the 5-days? I even have the Prolon box and it does not say. Why on earth not? If this is so scientific why no details on what is actually in all the boxes. Even American processed food needs to contain calorie count for example. Ergh. Disappointed and starting to feel I got “taken” by this Prolon kit.

  2. The first day has 1090 calories and the next 4 days have 725 calories. this would be approximately 50% of usual calories the first day and 40% of usual calories the next 4 days.

  3. I am currently on day 3 of the 5 day ProLon FMD. I feel great! No cravings for anything.

    There is a glossy sheet included with the meal boxes that provides a food label for every food item in the box. Easy peasy to figure out calories for the day. Also, lists ingredients for each food item.

    With that said, let me comment on what is working and not working for me regarding the box plan. In general, I don’t care to eat “bars”, instead, I prefer real food. The dehydrated food is good. I develop nasal congestion for about an hour or two after eating some of the products; ? potato starch, inulin, natural flavors, not sure what’s causing the congestion but I know it’s something my body does not like. This does not happen when I eat real food. Also, I feel bloated after some of the meals and that does not happen when I eat real food.

    I am disappointed that cane sugar is in some of the products as it’s unnecessary (I don’t eat sugar in any form). The crackers are amazing, the olives are amazing, teas are amazing, the choco bars are amazing but have rice syrup (again, sugar), the breakfast L bar is so-so, too sweet for my taste, wish they would leave the sugar out of this bar or make a savory option for those of us who are sensitive to any type of sugar added to our foods.

    I plan to do the recommended full three sessions of FMD (1 every 3 weeks) but will not purchase the box for my next two fasts, instead, I will eat food and create my own diet plan. Glad I did not purchase all three up front!

    Hope this reply is helpful.

  4. I would love to see ur created meal plan..I too prefer food to bars. I try desperately to limit sugar.

  5. I too wish to do this fasting with real food. Can you please tell me which real food is appropriate for the 5 day mimicking fasting please. I am vegan. Tks

  6. For further info on the FMD, you should read “The Longevity Diet” by Dr. Valter Longo, the scientist who developed the FMD from 20 years’ research as well as the Prolon pre-packaged version of it. He goes into much more detail on contra-indications for it. And of course you can google his websites.

  7. The link to Dr Anthony’s four day ketone fast mimicking is not working. Page not found. Please fix, I really want to read that. Thank you.

  8. The link for Dr. Anthony’s 4 day fast is not working nor is the second link you gave. I’m allergic to things in the ProLon plan and am looking for alternatives.

  9. Hi Josh, we’ve checked on our end and it’s working fine. You may read the blog here: MY FOUR DAY FASTING RESULTS . If it’s still not working, we recommend to clear the cache on your current browser or try using a different browser. Thank you.

  10. You will feel less hunger if you just do a water fast for five days. No kidding. Its hard to believe but try it.

  11. I just did it and am now on day 6. My diet plan consisted of diced red and green bell pepper and zucchini sauteed in olive oil + walnuts for breakfast. I had a vegetable soup made from organic, low sodium vegetable broth and 10-12 different vegetables (cheated and bought from the frozen section as I’ve read that frozen vegetables still have the good stuff in them) and almonds for lunch, and then vegetable soup and hazelnuts for dinner with some steamed carrots. I also took a one-a-day and fish oil each day. Total calorie intake was 1100 the first day and 800 the remaining days for me, but this will clearly changed based on your BMR and how active you are leading up to the FMD. If I were not very active, as a 42-year-old female, I would typically only need about 1400 calories per day, meaning I would eat 700 calories the first day and then 500 each day after that. I am an active person so chose to increase the amount eaten, just using the nutrition info to identify how much soup, how many walnuts, etc. It was hard. I felt hungry more often than not, but I stuck with it and my ulcerative colitis definitely toned it down a notch. I have hope that maybe I can stay off Imuran for the long haul with this as this was my first time. I’ve read that with autoimmunity, it takes three of these to see the full benefits. I was truly impressed and amazed with how this went!

  12. Hmmm… I’m doing one meal a day intermittent fasting. I don’t ingest any sugar at all. The only carbs I ingest are from veggies. But I noticed in the video about this, they said during the 5 day fast to greatly reduce protein, and increase fat. So, that’s what I did. Just using real food for the one meal I eat per day that’s low protein, very low carb, and high fat.

  13. Thanks Jocelyn for the info!! I am doing keto and IM after cancer x3. He has interesting research. Never knew!! Ty for sharing!!!!!

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