Richard Sprague

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My experiment with the Prolon Fasting Mimicking Diet

Created: 2019-12-04 ; Updated: 2019-12-05

Although occasional fasting is one of the best things you can do for your microbiome, it’s surprising how few people can remember ever going more than a few hours without food. Your body was not designed for a world of 24/7 food availability, and I’m convinced this is a key reason why many people suffer too long with health issues. Too many people have told me anecdotes about how they unexpectedly found themselves without food – on a trip, during a holiday when stores were closed, or for other reasons – only to find that just that one little break was enough to restart their body into a more healthy state.

For that reason I try to keep to religious fasts: the Roman Catholic tradition includes a Lenten fast, for example, although other Christians (like the Greek Orthodox) have fasts much more regularly. Islam has Ramadan, Judaism has Yom Kippur – virtually every major religion prescribes times when adherents are supposed to go without food.

But full-blown fasting is tough, and for that reason I am interested in the “fasting mimicking diet” popularized by Valter Longo in a book I read recently and now available for about $250 from Prolon. I used the official version (they generously gave it to me to try) but Quantified Bob Troia gives instructions for how to do it yourself. If you’re not already accustomed to long fasts, I don’t recommend the DIY approach on your first try. This was a long, sometimes miserable process for me and you don’t want to go through it unless you know what you’re doing.

Although the diet is describes as five days, it’s really a bit longer. My last meal was finished at 7pm on November 26, and my first non-diet meal was at 6am on December 4th, a total of 131 hours, or about 5 1/2 days. After that you’re supposed to give yourself a followup recovery day after that for “refeeding”.

On the morning of the sixth day, I measured my ketones at 1.5 mmol/L and blood glucose at 84 mg/DL, for a total GKI of 3.11. I wasn’t hungry at all, but that first cup of bone broth really tasted good!

Here’s my glucose for the full followup day:

Followup Day 6

Figure 1: Followup Day 6

And the day after that I was feeling extremely good: full of energy, well-rested (about 3 hours of deep sleep) and eager to try the diet again in the future.

Here’s the day-by-day breakdown:

Day 1

Day 2

Day 3

Day 4

Day 5


Several other people have tried the Prolon or similar fasting mimicking diets, recording their daily experiences like I did.

Quantify Fitness is a fitness center in Nashville whose 40-year-old co-founder reports much higher HRV and better workouts after the diet.

A Note on Style a short day-by-day summary. Recommends reseeding with the Ayuvedic rice porridge Kitchari.

Chris Kresser highly recommends it. The comments to his post include suggestions for refeeding.

Podcast Notes from Ronda Patrick interviews Valter Longo another summary of the benefits.