Fast-mimicking diet: results and experience

Last week we introduced the concept of the fast-mimicking diet (FMD) and looked at the potential benefits in applications ranging from longevity to improved metabolic health to cancer prevention.  As always, when we are intrigued by a new “bio-hack” we feel the best way to investigate it is to try to ourselves. And that’s exactly what we did. We embarked on a five-day FMD as outlined by Dr. Valter Longo in The Longevity Diet.  Unfortunately, despite purchasing the book and listening to several of his interviews, the exact specifics of the FMD are not available, as he is trying to get people to purchase his outrageously priced $250/box ProLon meal kits.  But you can get a pretty good idea about what is in them based on other peoples reviews and on the macro nutrient profile that he outlines in his book.  So we devised our own FMD rather than spending $500 for a week’s worth of kale crackers, dry soup mixes, and sugar-free teas…

What did we eat?

The Our Health Habit FMD is a low calorie, plant-based, dairy-free, gluten-free, sugar-free diet (Longo’s is all of these except gluten-free).  Our meals usually consisted of the following: nothing for breakfast, small salad or broth for lunch, nuts or olives/artichokes as a snack, and a small dinner comprised of salad, legumes, and broth / soup.  The Prolon meal kits are fixed in terms of calories, containing 1100 calories on day 1 and 800 calories on days 2-5.  Longo’s studies show that this deficit, albeit not complete fasting, elicits a similar metabolic response (IGF-1 levels, insulin levels, etc) as water-only fasting.

The old adage says “what’s good for the goose is good for the gander,” but we think that different size people probably need different caloric intake to stimulate this fasting response.  Other sources recommend eating 3-5 calories/pound of body weight for a more tailored FMD.  Taking this guidance into account, we both aimed for approximately 4 calories per pound of body weight. For me, this amounted to 700-800 cal/day; for Rachel it was proportionately less.  The composition, or macronutrient profile, of those calories matters too.  Longo suggests a target of one half the calories from fat and the other half from carbohydrates.  What about protein you ask?  You don’t get much on the FMD.  He recommends very low levels of less than 25 g of plant based protein daily (accounts for ~100 calories or less).  We logged all of our food using the Fitbit app and I am proud to say that we were able to stick to the diet and caloric restriction windows with relative ease. I was also shocked by how little protein I ate, which was typically less than 15 g! This is a huge departure from my prior levels, which were often closer to 100g daily (although I have been reducing protein intake lately).  Daily results from Brendan’s 5 day FMD are shown below:

As you can see I stuck with the caloric restriction window pretty well, going above one one day and below on another.  One area we strayed a bit from Longo’s protocol was with the suggested Macronutrient profile.  As you can see by the macro nutrient graph below, we ate substantially more calories from fat than recommended. This was not intentional, but just the way it worked out since we ate both macadamia nuts and avocados, both of which are rich in healthy fats. On the plus side, is this macro nutrient profile turned our FMD into more of a ketogenic diet than would be expected.  This keto variant FMD has been discussed and practiced by others, such as Peter Attia, MD, a longevity researcher and lipid specialist (who has an excellent new podcast).

What are the results?

It’s worth noting that the calorie expenditures are roughly accurate (or at least consistent), as I wore my new Fitbit Ionic 24/7 to track caloric expenditures. As we discussed in a previous post, the calorie burn estimates from Fitbits equipped with a heart rate tracker are not perfect, but they are reasonably accurate.  Not surprisingly, being in a significant caloric deficit of ~2500 calories/day, I did lose weight: exactly 7 pounds in total as you can see on the graph below.  I expect that some of this weight is “water weight” and some is “real.”  I have no easy way to track how much, if any, was lean muscle mass.  Others have shown that lean muscle mass (and even weight) can actually increase on the FMD.  Unfortunately, I didn’t have access to a DEXA scan or Bod-pod so I cant really report objectively on body composition.  I can say that even 1 week after coming off the FMD, my weight was at the same level; in other words I didn’t gain it all right back as you see in the 24-48 hrs after a water only fast where about half the weight loss is purely “water weight.”

What about those ketones?

As you can see from the table below my ketones did rise to a significant level, albeit not nearly as high as Rachel’s! By the end of day 1 her glucose-Ketone index (GKI) was less than 1, which is been reported to be associated with decreased risk for cancer development. It took me four days to reach the same! This just goes to show you that different peopel have different metabolisms and can get in and out of ketosis much quicker.  It also shows that striving to reach a number on the ketone meter may be missing the point as it only tells a small piece of the overall picture.

How did it feel?

Having done lots of intermittent fasting and even some prolonged, multi-day fasts, the FMD was surprisingly easy by comparison.  Since I cant eliminate my own bias of experience, I would have to say that it would probably be pretty darn mentally tough for most people, but certainly doable.  In fact, this is one of the key reasons Longo has developed and researched the very idea of a FMD–its waaaay easier than fasting for most people.  After about day 2, your appetite seems to readjust to the available nutrition and you just stop feeling hungry.  Ironically the same thing happens on a long water-only fast.

Not only is this easier than traditional fasting, its also a lot safer.  In Longo’s studies, they have used this regimen on people undergoing chemotherapy and it has been well tolerated (it goes without saying, but I will mention it again here: if you are contemplating fasting or even an FMD check with your physician to be sure it is safe for you).  One of the things I struggle with on long fasts is muscle cramps, likely from inadequate sodium/magnesium intake.  Even at the very end of a 5 day FMD, I had no such issues (probably because I got plenty of sodium in the vegetable broth I ate daily).

Apart from the mental challenge of eating so little, I hardly noticed any other effects.  Some people report headaches or body aches, but we did not experience these on the FMD.  Sleep was good (but not as good as it is on a water-only fast) and energy level was surprisingly normal.  I was able to perform pretty much normal workouts on days 1-3, but felt sluggish and weak on day 4 workout and I didn’t even attempt to work out day 5.

Conclusions?

In the end, the 5 day FMD was a success.  It wasn’t too difficult or disruptive to work or family life.  It was reasonably easy and had metabolic results (ketones and weight loss) similar to a water only fast.  Whether it truly does mimic the fasted state as Longo purports remains to be seen, but I feel fairly confident that it is an easier, more palatable, and safer alternative to water only fasts with most (but probably not all) of the same benefits.  I know thsi will be a part of my arsenal going forward to promote health, longevity, and disease prevention.