I Tried the Fasting Mimicking Diet and Here's What Happened
Insights from 5 days of fasting with Prolon, including thoughts on the cognitive boost
I’ve been meaning to try this for years—the Fasting Mimicking Diet—in part because I wondered what the cognitive benefits might be.
The FMD was developed for people who are about to get chemo. Someone noticed (in a clinical setting at USC) that chemo was more effective if you fasted for a few days beforehand (because by doing so, you could starve the cancer cells before you zapped them, a double whammy for them, whilst the other cells in your body can get by on ketones instead of glucose).
The problem was, none of the patients could manage to do the fast, because eating nothing for days at a time can be extremely challenging and painful.
So to boost compliance with fasting, they developed this program whereby people could eat a little something, just enough to make the intense cravings for food go away (mostly), but not enough to put one’s brain and body out of its beneficial fasted state.
On this FMD, you eat less than 700 calories per day. It isn’t pleasant.
If you use Prolon’s products, you get this astronaut food, dehydrated soups, and revivify them with hot water. It’s just barely palatable.
There’s also a few crackers, pouches of bland olives, dry nutrition bars.
Oh, and some delicious bottles of glycerol that you dilute with water and drink throughout the day, along with a bunch of goddamned herbal teas. (After drinking this much herbal tea over 5 days, you earn the right to day ‘goddamned’ herbal teas.)
The only reason I finally decided to take the leap and try the FMD was that a friend challenged me to do it together as a team. And with friends like that…
One reason I had resisted doing this was my concern that it might degrade my gut microbiome. But then I found a study done with mice where the microbiome was seen to actually improve on a Fasting Mimicking Diet. How relieved those mice must have been to go back to their decadent food pellets.
The last straw that convinced me was hearing Dr. Peter Attia describe his experience with it—FMD is a tool that he relies on with his own patients. Some of his patients do it multiple times a year depending on their health status.
It’s a 5-day process.
Day 1
I started each day by placing the packets of dehydrated food powders I was allowed to eat (that day) onto a nice serving platter, as if I was in some hotel where this was being presented to me. How lovely.
It’s a bit daunting to know this is the sum total of what you can eat that day. Nothing else. That’s it, those few little packets right there.
I quickly realised this would be a psychological challenge, really.
At some point during this first evening, all I had left was one nutrition square (something nominally chocolate flavoured, meant to be your dessert). I stared at this thing and decided I would not eat this bar.
Choosing to reject this thing completely changed my orientation. I went from mildly trepidatious about becoming hungry later, to totally resolved. I wondered if having rich parents was like that—allowing you to feel generally emboldened about your journey rather than fearful that you might starve. Attitude is everything, as they say.
I told my son about my decision to refuse this last loathsome bar, and he did an impression of Edith Cranwinkle from Bob’s Burgers. ‘He’d rather die than drink your water!’ Indeed.
Day 2
This was the only day I had hunger pains, which happened in the evening when I went on a quick errand without bringing any snacks from my FMD box, and by the time I got home, my intestines were cramping pretty bad.
From then on, whenever I went somewhere, I’d bring something from the kit just in case. It was a comfort.
Day 3
This was the day I really crashed, energy-wise. I noticed I was barely moving.
In the diet and nutrition science world, there’s something called Non-Exercise Activity Thermogenesis, or NEAT. Everyone has their own innate level of NEAT, and it’s a huge factor in weight maintenance. People with high NEAT levels burn hundreds of calories more per day than people with low NEAT levels.
I’m guessing my usual NEAT level is on the high end. But on this FMD, it felt like my brain and body shut everything down to preserve energy, including body movements linked to NEAT.
Day 4
Bounce back time. Not physically. Mentally. This was the day when I really began to notice a cognitive boost. And this was likely because my brain was now running on ketones.
I still wasn’t very active or moving a lot (I was in some kind of energy conservation mode), but my brain’s processing speed was clocked up a few ticks. I spoke faster. I didn’t search for words.
And I could do something conversationally that I haven’t done much of in recent years, which is: I could speak several sentences driving toward one idea, then mentally table that whilst interjecting another related idea, and then (and this is the cognitively challenging bit), return to the original line of thinking, having braided the two trains of thought into a cohesive whole. I could even form multiple braids.
Day 5
On this night, I opened the very last pouch of food, dehydrated black bean soup that I’d eaten like 3 or 4 times already (they give you the same thing over and over, very thoughtful). I poured it into a bowl. I poured the hot water in. I stirred it. I stared at it. And I just couldn’t eat that goddamned bowl of soup. I felt nauseous looking at it. Every fibre of my being rejected it.
So I looked on the back of the pouch, saw that it was 120 calories, and ate a soft-boiled egg instead. My fast was done.
Post-Fast Questions and Next Steps
The cognitive boost I experienced during the fast lingered over the next few days, seeming to recede and return to varying degrees as I began to eat a larger daily volume of food.
Still, some level of that boost seems to have remained. It would seem that I am in a better state now than I was before the fast. This might also be a function of my renewed focus on intermittent fasting every day (something I’ve been doing off and on for a couple of years now).
I watched a few videos about the cognitive boost people report when they are in ketosis on a ketogenic diet.
I also watched an interview with Dr. Chris Palmer (on the Tim Ferriss podcast) about the role of diet in treating brain dysfunctions. Dr. Palmer was also interviewed by Dr. Huberman, which was just posted today, and I haven’t listened to that yet.
For me, the big question is… did the cognitive boost I experienced result solely from running on ketones, or was some measure of it—any measure of it—attributable to other factors that I might now look at adjusting or improving? For example, gut permeability, brain permeability, microbiome improvements, or not eating specific foods that might be triggering subtle autoimmune reactions.
So that’s the next phase, that investigation, and I’ve already taken several steps in that direction, and I will write about those soon.
Would I do the Prolon FMD again? At the moment, it’s hard to imagine eating that stuff one more time, let alone for 5 days. But the experience has made me more resolved to be disciplined about the intermittent fasting I do on an almost daily basis. And it’s motivated me to investigate some new avenues.
Would I recommend it to anyone else? Probably. With the appropriate caveats, which I’ve attempted to lay out here.
I guess I should add that, in the immediate aftermath of a concussion, I personally would not look at doing an FMD at that point in time (as I understand it, the brain’s metabolic needs are very different during that time). As with so many things with our health, timing is everything, especially when it comes to effective therapeutic windows. I am of course compelled to remind you that I am not a medical professional of any kind. Prolon will tell you a million times over, online and in print, to consult a relevant medical professional before starting FMD. Definitely do that.
Brainwave is an informational resource for people whose symptoms haven’t resolved after a concussion or mTBI. I aim to present this information in a clear and concise way, spelling out what’s backed by science and what remains unknown. Nothing here is meant as a substitute for professional medical advice, diagnosis, or treatment. I am not a physician or a healthcare practitioner of any kind; I’ve had a lot of sports-related concussions and had to learn this stuff the hard way. If you found this information helpful or know someone who might benefit from it, please share and subscribe to Brainwave.