There are numerous studies that demonstrate the benefits of periodic, short-duration fasting, such as weight loss, improved insulin sensitivity and brain function, immune system regeneration, and longevity. I’ve been a fan (and practitioner) of intermittent fasting for several years, but other than an occasional 24-hour liver cleanse or protein fast, I had never done any extended fasting.

My original plan was to do a “traditional” water fast, where nothing but water is consumed for a period of 3-5 days. However, during my research I began looking for ways to get all of the benefits of a water-only fast, but in a way that was “easier” (both mentally and physically), safer, and would minimize catabolic effects (loss of muscle mass). Enter the “fasting mimicking diet”…

What is the Fasting Mimicking Diet?

The Fasting Mimicking Diet (FMD) received a lot of media attention when it was introduced a few years ago. In a study that was published in Cell Metabolism (and funded by the National Institute of Aging), researchers found that cutting daily calories in half for just four days every two weeks reduced biomarkers for aging, diabetes, heart disease and cancer with no adverse effects. FMD was tested on yeast, mice, and humans and the results remained consistent among all three groups.

The FMD describes itself as:

…a plant-based diet program designed to attain fasting-like effects while providing micronutrient nourishment (vitamins, minerals, etc.) and minimize the burden of fasting. It comprises proprietary vegetable-based soups, energy bars, energy drinks, chip snacks, chamomile flower tea, and a vegetable supplement formula tablet.

The primary researcher, Dr. Valter Longo, owns a patent on the FMD, and has since began selling a dietary product under the name ProLon.

Bonus: Free DIY FMD Spreadsheet Get a FREE copy of my FMD spreadsheet that you can use and customize! Get the DIY FMD spreadsheet

“Mimicking” the Fasting Mimicking Diet

The patent filing provided enough information to allow me to inexpensively hack together my own “close enough” version of the FMD. The nutrition rules of the FMD can be summarized as follows:

Fasting Mimicking Diet Cycle:

Day 1: Total caloric intake of 4.5 to 7 kcal/lb of body weight (10% protein, 56% fat, 34% carbs)

Total caloric intake of 4.5 to 7 kcal/lb of body weight (10% protein, 56% fat, 34% carbs) Days 2-5: Total caloric intake of 3 to 5 kcal/lb of body weight (9% protein, 44% fat, 47% carbs)

The fast is then followed by a 25-day “refeeding” period (return to regular diet). The study recommends repeating the 5-day fasting/25-day refeeding cycle 3-4 times in a row to maximize effects.

Fasting Mimicking Diet Nutrient Guidelines:

Low protein and low carbohydrate, with no or minimal animal-derived products

High micronutrient content (30-50% of recommended daily intake of vitamins, minerals, and essential fatty acids, with 50% of them coming from natural sources)

Proteins from plant-based sources

Carbohydrates ideally from complex carbohydrate plant sources

Fats primarily from “healthy” oils (coconut oil, olive oil) and nuts (almonds, walnuts)

Less than 30g of sugars on day 1, and less than 20g of sugars on days 2-5

My Fasting Mimicking Diet Meal Plan

I put together the following eating plans that got me pretty close to the target macronutrient requirements, which I would split into AM and PM “meals.” My goal was to limit ingredients and keep things as simple as possible. Again, feel free to check out my Google Spreadsheet for detailed nutrient profiles. I also came across this useful list of Fasting Mimicking Diet recipes.

My primary nutrition sources were avocados (Mexican Hass variety) and green superfood powder (to provide additional micronutrient content), along with some white rice raw cauliflower and sweet potatoes (in a bit I will explain why I took out white rice). I also had a teaspoon of sea salt each day (mixed in a glass of water) to assist with electrolytes and adrenal support during the fast.

I drank around 1.5-2 liters of spring/high alkaline water each day along with a cup or two of chamomile flower tea (which subjects in the original study were allowed to drink liberally). And each morning I allowed myself 2 cups of black coffee with some cinnamon sprinkled in (in retrospect, I would have cut my coffee consumption because according to ProLon’s FAQ, “Because coffee can interfere with the beneficial effects of the diet, it is not recommended to be consumed… however, if one must have coffee, we recommend minimizing coffee consumption to 1 cup per day.”)

Measuring out a day’s worth of FMD meals

My Fasting Mimicking Diet Results

To establish pre-FMD baselines, one week prior to starting the FMD I cycled off of all supplements. My friends at InsideTracker were kind enough to offer me complementary pre/post blood testing (I’ve included a link and discount code at the end of this post). Also, during my fast I refrained from any physical activity – no workouts or sports.

I tracked a number of markers a) prior to starting; b) during; and c) one week post FMD, and saved my experiment data in a Google Spreadsheet, which you are welcome to explore. Feel free to make a copy of it if you decide to replicate my experiment!

Glucose and Ketones

I was very pleased with the effect FMD had on my fasting glucose and blood ketones. While there were rises and dips in my PM glucose and ketone values (taken 2 hours after my evening meals), my morning fasting glucose levels steadily declined from ~80 mg/dL pre-FMD to the mid 40’s, while ketones steadily rose – after 1 day I was in “nutritional ketosis” (0.5 – 1.5 mmol/L), and after 3 days I got into what is called “optimal ketosis” (1.5 – 3 mmol/L). You can see a clear inverse relationship between glucose and ketone values.

On day 2, I consumed some white rice with my PM meal (mainly for the carbs), but as you can see on the chart, it resulted in a huge spike to my blood glucose (it’s the 111 value at around 45 hours). For the remainder of the fast, I substituted some raw cauliflower and sweet potatoes to bring down my glycemic load.

My fast happened to end on Easter Sunday (had my first meal around 11am, after taking my final measures), and once I began refeeding (think big Italian family meal!) I immediately fell back out of optimal/nutritional ketosis and glucose values returned to their pre-fast range.

So, while it was great to see the FMD’s effects on glucose/ketones, it was clear that my body wasn’t as keto-adapted as I had thought prior to starting the fast (even though I get into light nutritional ketosis for a short period each day). Apparently it can take anywhere from two weeks to 30 days for the body to fully keto-adapt.

Pro Tip When “refeeding” after a 5-day FMD, start easy! Begin with a small bowl of broth and some nuts such as cashews, followed by a light, easy to digest meal a few hours later (perhaps some eggs) and return to a regular sized meal a few hours after that. Your digestive system will thank you.

The chart above shows changes to body weight, measured first thing each morning (after urinating) using my Omron scale. I was surprised to see that I only lost around 3 lbs of body weight during the FMD (171.8 lbs to 168.8 lbs), which happened very steadily and actually stabilized after day 4, but then began to rise again approximately 3 days after completing the fast.

Typically, during a traditional water fast a person will not only lose “water weight” and body fat, but there will also be catabolic effect (loss of muscle mass). One of my theories was that a FMD-type of approach to fasting would minimize the loss of muscle. Here are my results:

Start of FMD: Body fat = 23.5% (40.37 lbs), Skeletal muscle mass = 35.2% (60.47 lbs)

Body fat = 23.5% (40.37 lbs), Skeletal muscle mass = 35.2% (60.47 lbs) End of FMD: Body fat = 21.5% (36.16 lbs), Skeletal muscle mass = 36.5% (61.39 lbs)

So, during the FMD I lost approximately 4.2 lbs of body fat, while GAINING nearly a pound of muscle! There was also a nice muscle building “rebound” effect once my fast ended and I began to exercise again. I realize that most home scales that use body impedance to measure body fat can be very unreliable, but unfortunately, for this experiment my Omron scale had to do since I didn’t have access to more accurate body composition tools such as a DEXA scan, BodPod, Inbody, etc.

Energy, Mental Clarity, and Dreaming

Overall, I maintained very good energy during the FMD. I knew from previous experience with intermittent fasting that ketosis can really provide focus and mental clarity, and by day 3 I was having some very productive days of writing, coding, and reading.

Many people claim to experience “carb-mares” (low carb-induced nightmares, or even dreaming about carbs) when switching to a ketogenic diet. In my case, I had the most frequent, vivid, and memorable dreams (not nightmares) I have experienced in a long time! Although the content of my dreams didn’t seem too profound, I could recall every little detail. Once my FMD ended, this level of dreaming and recall largely subsided.

Heart Rate Variability (HRV)

Upon waking each day I would take an HRV reading, shown in the chart below. What’s interesting to see is that there was a slight uptick in HF (sign of parasympathetic activity) each day until I broke my fast.

Another HRV measure, RMSSD, is commonly used as an index of vagally (vagus nerve) mediated cardiac control which captures respiratory sinus arrhythmia (RSA), the frequent changes in heart rate occurring in response to respiration. During inhalation, heart rate speeds up. During exhalation, heart rate slows down. RMSSD is an accepted measure of parasympathetic activity and as you can see, it correlates very well with HF of frequency domain analysis.

Blood Pressure and Body Temperature

I took my body temperature each morning upon waking, and surprisingly there was very little change during my FMD. Blood pressure trended down slightly but nothing drastic. However, I did experience what appeared to be some circulatory issues in my hands – they got rather cold by day 4 of my FMD, yet my body temperature and blood pressure were largely unchanged.

Could this be related to my blood glucose dropping so low? Or perhaps it’s the body’s attempt to maintain core body temperature so it will reduce circulation to extremities?

Blood Oxygen Saturation (SPO2)

I also tracked oxygen saturation (SPO2) each morning using an inexpensive blood oxygen sensor. I thought there would be a decrease in my SPO2 readings as the fast progressed, but my readings were consistently in the 98-99% range (sometimes even 100%).

Bloodwork

I had my post-FMD blood test from InsideTracker done 5 days after breaking my fast.

Inflammatory Markers – C Reative Protein hs-CRP

Pre-FMD: 1.5 mg/L / Post-FMD: 0.5 mg/L

InsideTracker provided me with hs-CRP results (a marker of inflammation). hs-CRP is the “high sensitivity” version of CRP measurement, and is a better indicator of inflammation in the body than the regular CRP test. Optimal hs-CRP levels appear to be an effective predictor of healthy heart, circulatory system, blood pressure, and blood glucose. My hs-CRP was slightly elevated prior to my FMD, but I’m convinced this was due to the lingering effects of a quadriceps injury suffered playing soccer the previous week (since acute muscle injuries/trauma/inflammation can throw off CRP numbers) and my creatine kinase value was also elevated prior to the FMD. The good news is that my post-FMD results showed my hs-CRP went down to 0.5. My creatine kinase returned to my “optimized zone” as well.

Immune System and Regeneration Markers – White Blood Cell Count (WBC)

Pre-FMD: 2.7 x10E3 µL / Post-FMD: 1.8 x10E3 µL

The original study showed the FMD to provide regenerative effects to immune systems, with more noticeable effects after several FMD cycles. I was especially interested to see if it would impact my WBC count, as my levels have been tremendously low over the past few years due to a suppressed immune system. Sadly, FMD didn’t have an effect on my WBC count – it actually went down. Unfortunately, the testing didn’t include IGF-1 (insulin-like growth factor 1), which was one of the key markers tracked in the original FMD study (there was a significant decrease in IGF-1 among subjects). I would have liked to have tracked lymphocytes as well.

Testosterone

Pre-FMD: 475 ng/dL / Post-FMD: 804 ng/dL

On the positive side, one of the most surprising results was an over 325 point increase in testosterone! Along with my increase in skeletal muscle mass, this shows there is definitely a “rebound”/growth factor effect after the fast is broken and refeeding begins.

Stress Markers – Cortisol and Adrenal Function

Pre-FMD: 8.9 µg/dL / Post-FMD: 11 µg/dL

My major concern with attempting a fast was the added stress it would place on my body, specifically on my adrenals. As I mentioned earlier, I made sure to consume some sea salt every day to provide adrenal support. My cortisol level remained right smack in the middle of my “optimized” zone.

Liver Function

ALT: Pre-FMD: 38 U/L / Post-FMD: 25 U/L

AST: Pre-FMD: 35 U/L / Post-FMD: 23 U/L

GGT: Pre-FMD: 12 U/L / Post-FMD: 11 U/L

Albumin: Pre-FMD: 4.5 g/dL / Post-FMD: 4.5 g/dL

I was concerned about the effects (stress) fasting would cause to my liver, but surprisingly all of my liver markers were in my “optimal zone”:

Next Steps / Future Tweaks to FMD

Based on my results and those of my friends, I plan on making a few tweaks the next time I do a FMD:

Macronutrients: I would like to consume more fats, even if that means a further reduction in carb intake. I also think that the FMD shouldn’t be so “one size fits all” based only on body weight – it should take into account things such as person’s body composition (body fat, skeletal muscle mass) and pre-existing blood markers

I would like to consume more fats, even if that means a further reduction in carb intake. I also think that the FMD shouldn’t be so “one size fits all” based only on body weight – it should take into account things such as person’s body composition (body fat, skeletal muscle mass) and pre-existing blood markers Micronutrients: While my macronutrient content matched the FMD, I found it difficult to match the micronutrient content across the board without incorporating any supplements. I wasn’t too concerned with a few of the micronutrients falling below guidelines (such as Vitamin D) since it was only over a period of 5 days

While my macronutrient content matched the FMD, I found it difficult to match the micronutrient content across the board without incorporating any supplements. I wasn’t too concerned with a few of the micronutrients falling below guidelines (such as Vitamin D) since it was only over a period of 5 days Food Choices: Generally I was happy with the eating plan I put together, but I would like to include more broths next time (both for the nutrients as well as making me feel “full”)

Generally I was happy with the eating plan I put together, but I would like to include more broths next time (both for the nutrients as well as making me feel “full”) Kidney and Liver Support: Starting on day 3, I began to experience a bit of pain in my kidneys. I believe this was due to my body detoxing. I began taking some activated charcoal, and the pain went away. I would also like to include some milk thistle to provide additional liver support.

Starting on day 3, I began to experience a bit of pain in my kidneys. I believe this was due to my body detoxing. I began taking some activated charcoal, and the pain went away. I would also like to include some milk thistle to provide additional liver support. Tracking and Testing: The next time I attempt FMD I will simplify my level of tracking by skipping those measures that didn’t add much value (no pH, blood pressure, SPO2, body temp). I would also do bloodwork 3 times (prior to, right after completion, and 5 to 10 days after completion) to get a better picture of my body’s state at the end of the fast versus after refeeding began. I would also like to add other markers such as IGF-1 and lymphocytes. And lastly, I would do more thorough body composition analysis (BodPod or InBody).

Conclusions/What’s Next?

Overall, I was very impressed with my FMD results. It was very much a learning experience and was much easier to get through than I had anticipated. The most difficult part was finding 5 days where I could limit any physical activity (soccer matches, gym, etc.). I was most happy with the FMD’s effects on glucose and ketones, as well as cognitive effects (focus, dreaming). Next time I do a FMD I want to include some cognitive testing in order to better quantify cognitive function.

Ideally, the FMD is supposed to be done over a period of several cycles (5 days of fasting followed by 25 days of regular diet). The literature suggests that the overall benefits (lowered inflammation and increased immune system function) will increase after each cycle. I plan on repeating the FMD in 45-60 days, incorporating some of the tweaks I outlined.

Please let me know if you have done some form of fasting or plan to fast in the future – would love to hear about your experience!

Bonus: Free DIY FMD Spreadsheet Get a FREE copy of my FMD spreadsheet that you can use and customize! Get the DIY FMD spreadsheet

Resources and Reading:

My Fasting Mimicking Diet Data (Google Spreadsheet) – feel free to explore my data or use it for your own FMD experiment!

“A Periodic Diet that Mimics Fasting Promotes Multi-System Regeneration, Enhanced Cognitive Performance, and Healthspan”, Cell Metabolism (original FMD paper)

“Fasting condition as dietary treatment of diabetes” (original FMD patent)

The 5-Day Fast Mimicking Diet – Damien over at Quantified Body also has a great writeup (and accompanying podcast) of his experience trying to replicate the FMD. He was the one who originally brought the FMD to my attention, so it’s great to be able to have someone else to compare notes with.

Tools, Products, and Services:

Blood Glucose and Ketones: I used a Precision Xtra blood glucose/ketone meter. You’ll also need glucose test strips and ketone test strips (note that ketone test strips can get expensive)

Blood Testing: I took InsideTracker’s “Ultimate” testing plan prior to and 5 days after completing my FMD (use my code BOBTRO11012 to save 10% off your own InsideTracker tests!)

Weight and Body Composition: I used this Omron Body Composition Monitor and Scale for weight and body composition measurements.

Heart Rate Variability: I used an HRV app called HRV4Training, which I paired with a Polar heart rate monitor. SweetBeatHRV is another HRV app I recommend.

Blood Pressure: For blood pressure readings I used this Omron blood pressure monitor

Blood Oxygen Sensor: I used an inexpensive pulse oximeter similar to this one.

Body Temperature: Kinsa smart thermometer

Breath Ketones: During my fasting mimicking diet experiment I used a device called Ketonix to check breath ketones (it works by measuring acetate in one’s breath). Unfortunately, using the device correctly takes a lot of practice (consistent breaths) and I wasn’t able to see a correlation between the breath ketone readings and my blood ketones.

Urinary pH: I used these inexpensive HealthyWiser urinary pH test strips

Activated Charcoal: I took 1-2 activated charcoal capsules to help clear any toxins from my body during my FMD. Be careful because too much activated charcoal can cause constipation!

Green Superfood Powder: I used Amazing Grass Green SuperFood powder to help with plant-based micronutrient needs

Nutritional Data: I primarily used USDA nutrient data from here and here