Although I believe 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. I’ve heard too many anecdotes from people who 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 recommend religious fasts: the Roman Catholic tradition includes a Lenten fast, for example; other Christians (like the Greek Orthodox) fast even 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 in an easy-to-use kit for about $250 from Prolon. Longo’s book (and diet) are based on

I used the official version (they generously gave it to me to try), which comes with everything you need, along with detailed instructions. There’s nothing special or even proprietary about the kit ingredients: Quantified Bob Troia gives instructions for how to do it yourself if you’d like and (Aging Advice even gives recipes). But unless you’re 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.

Bottom line: I’m glad I did the fast, and I highly recommend it, not just for people trying to lose weight, but even for people who are generally healthy (like me). It’s not easy, at least not for me the first time, but I will certainly repeat it again [update: in fact, I did, sort of].

Although the diet is described 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”. If you are in a social situation that requires eating, you’ll need to plan it accordingly. This is not the kind of diet where you can “cheat” – you must be prepared to follow the instructions perfectly.

I chose to begin the day after Thanksgiving, which worked well for me because I was able to enjoy the holiday with my family, yet take advantage of a long weekend to minimize the impact on my work.

I carefully tracked my blood ketone and glucose levels, as you’ll see in the details below. My final measurement, on the morning of the sixth day, showed 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:

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