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Fast Mimicking Diet – 5 Day Fast

Fast Mimicking Diet – 5 Day Fast

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This article is going to discuss the use of a technique referred to as the “Fast Mimicking Diet”. I am going to discuss with you my experience with a recent 3-day fast, which then got extended to 5 days.

The intention was to use the “Tim Ferriss method” (described here). Essentially accelerating the shift from glucose burning to fat burning as quickly as possible, avoiding the negative physical feelings associated with the transition period. (60-second video primer on ketones if you’re not familiar).

However, in the process of researching more about the measurement of blood glucose and blood ketones, I realized it’s possible to experiment a bit.

There has been some excellent research done by Valter Longo and his colleagues at the University of Southern California on a way to do fasts, without completely stopping eating.

Essentially, they were aware of all the health benefits associated with fasting but wanted to tackle the problem of compliance. For most people these days, the thought of not eating for 5 days is too much to handle.

They have come up with a 5-day diet that is low enough in calories and carbohydrates, that the user gets the vast majority of the benefits from fasting.

Actually, they have patented their specific diet as – and are selling it to hospitals to prescribe to their patients. Your first thought is probably…huh? But its more about “playing the game”, and finding ways to get fasting into hospitals, than it is about money. I believe Valter has even gone as far as to pledge his profits from the endeavor to charity. For more on Valter Longo and the FMD diet – check out Rhonda Patrick’s podcast with him.

Image of Prolon Fast Mimicking Diet food, vitamins and minerals.

What patients would find when they use the prescription Prolon diet

Luckily for us, we can use Valter’s research into fasting, without getting a prescription for his diet. In the paper that they published to, they describe the details of the diet:

The human FMD diet consists of a 5-day regimen:
– Day 1 of the diet supplies 1,090 kcal (10% protein, 56% fat, 34% carbohydrate)
– Days 2–5 are identical in the formulation and provide 725 kcal (9% protein, 44% fat, 47% carbohydrate).

Damien, of Quantified Body, figured out that this is similar to the macronutrient composition of avocados. So for example, you could have 2 medium-sized avocados per day on each of the 5 days.

It doesn’t have to be avocados though, it could be anything that fits the above macros.

The other interesting piece of the puzzle, that contributed to this fast, was a paper by Dr. Seyfried on treating brain cancer.

They found that if they could keep patients in what they termed “nutritional ketosis”, then they could achieve remission in some patients. Nutritional ketosis differs from simply measuring blood ketone levels, instead, it measures the ratio between blood glucose and blood ketones.

Results of Fast Mimicking Diet on glucose and ketones.

The sweet spot where blood glucose drops and blood ketones rise. This graph comes from Seyfried’s experiment to halt tumor growth.

My 5 Day Fast Mimicking Diet Details

The crux of my experiment is that adding in some calories/nutrition makes the whole process MUCH easier. And you can do it in a calculated way.

Here’s the rough breakdown of what I consumed:

  • First Day (after 24h) – 2 tablespoons of MCT oil (C8)
  • Second Day – 2 tablespoons of MCT oil (C8) + 1 tablespoon of coconut oil
  • Third-Day – 1 avocado (with Himalayan sea salt + apple cider vinegar) + 1 tablespoon of coconut oil
  • Fourth Day – 2 avocados (again with Himalayan sea and apple cider vinegar) + 1 tablespoon of coconut oil + 1 tablespoon of MCT oil (C8)
  • Fifth Day – Nothing until I broke the fast in the evening

You can see I didn’t adhere strictly to the fast mimicking diet and was generally under the prescribed amount of calories. But, despite the lack of full compliance with the fast mimicking diet, I was still far in excess of a simple water fast.

From the end of day 1, my ketones were elevated (no doubt helped by the MCT oil). The highest they reached was about 4.4mmol/L, with blood glucose simultaneously at 2.8mmol/L.

Blood glucose reading during fast mimicking diet.

2.8 mmol/L blood glucose reading (about 39 mg/dL)


Ketone reading during fast mimicking diet.

4.4 mmol/L blood ketone

The measurement of blood ketones and blood glucose come in useful for 3 main things:

  1. Tracking your transition into ketosis – and correlating how you feel with where you are in ketosis
  2. Giving you a measurable goal to aim for and maintain! (this is huge)
  3. You can use the blood measurements to test how different foods/ketone sources affect them

Regarding #3 – when I ate the second avocado on day 3, my blood actually showed I came out of nutritional ketosis. Even though my ketones were at 2.5 mmol/L, my blood glucose was at 4.3 mmol/L. Which is a ratio of 1.72 – rather than the desired ratio of 1.0 or lower.

After Thoughts

All in all this experiment was a success.

It’s becoming increasingly more obvious that fasting is an important tool in humanities arsenal against disease. Once upon a time, as hunters and gatherers, these fasted states were forced on us by the environment. Now in today’s abundant first world, we have to artificially create them. As with any “medicine”, one of the big hurdles is compliance.

The last 5 day fast I did was back in November, and it was taxing, to say the least. I certainly didn’t feel in a rush to do another one. However, this time around, I felt much more functional throughout the fast – and can see a way to make this a more regular thing. Regular being something in the order of twice per year.

Exiting The Fast + Ketosis

Probably the worst part of the fast this time was the exit. I think it comes down to the harsh way I went from nutritional ketosis to consuming reasonably high levels of carbohydrates. In retrospect, I’d probably prepare a meal with some fatty meat + vegetables. That way it fulfills the need I have to *eat everything*, but won’t cause such a harsh crash after.

I’ve never been a strong believer in living in ketosis, and I’m still not. But there’s something about seeing the effect on your body when it goes from ketosis to glucose burning, that makes you question if you’re doing it a favor!

Further Questions from Fast Mimicking Diet

There are a few points from this experiment I’d like to learn more about:

  • On the evening of day 3 I was still producing ketones, but out of nutritional ketosis. Morning of day 4 I was back in nutritional ketosis. What effect does coming out of nutritional ketosis have on the overall efficacy of the fast?
  • If we were to compare a complete water-based fast, to the fast mimicking diet, what are we losing out on?
  • If we are losing out on something, could this be mitigated by going to 6 or 7 days?

Anyway, that’s a rather large brain dump. Hopefully, if you’ve got this far you’ve been able to extract some nuggets to help you in your journey. If you’ve got any questions or comments, please leave them below. As for any blogger, feedback is wonderful.

Emory Neely

Wednesday 28th of February 2018

Great article! I'm curious, have you found any more info about what you're losing out on by not doing a water only fast?


Friday 26th of January 2018

Great article! I like all the links to papers and your scientific look at this. I just watched Fasting on Amazon last Saturday and am entering Day 6 ! I appreciate the mention of avocados also since I just ate one and it seemed like the right thing to do. I feel much better now. Thank you for sharing!!


Monday 8th of January 2018

So 2 questions 1- How do you officially start the fast? Do you eat normally one day and the next you begin? 2- Are 2 avocados per day (and greens powder) as efficient as prolon for days 2-5? If so, what should I eat on day 1 assuming I'm doing the avocados for the rest? Thanks a lot!!


Monday 8th of January 2018

You should check out this step by step guide here -

Hopefully that answers most of your questions.


Friday 8th of September 2017

Thanks - I think I remember Longo saying that the ideal is a 4 day water fast - but to compensate for the nutrition in the FMD - it needed to be 5 days to capture the same effects. So possibly, not losing anything?


Friday 8th of September 2017

Hey, thanks for the input. Ah interesting, didn't know Longo had made that specific differentiation. Yeah, in that case quite possible one isn't losing out on much. Would like to look at some clinical data to feel more confident. Ideally a paper that discusses blood test resuts at different fasting durations, if such a paper exists. Will try and dig up some next time I'm fasting. Rather than fall down the research hole right this moment, hehe. Also will look again at the papers Longo has published.