I first came across the man behind the ProLon 5-day fast; Valter Longo – via one of Rhonda Patrick’s podcasts. He talked about many of the potential health benefits their fast can bring, including:

  • Decrease in visceral fat (the “bad” fat around the organs)
  • Increase in cellular cleanup (autophagy) around days 3-5
  • Lowered IGF-1 (due to the reduction in protein intake)
  • Help maintaining healthy blood glucose and blood pressure levels

That all sounded really good to me.

Fasting from an Evolutionary Perspective

But what really got me was this idea of humans evolving fasting dependent processes. The idea goes that before we had refrigerators, supermarkets and fast food delivery, before we had agricultural farming and means of mass production, our ancestors would have occasionally gone for periods of time without food.

Unlike today where we panic if haven’t eaten all day.

From hunter gatherer to fast-food gatherer

Not only did our ancestors survive these periods of fasting, but it appears they developed cellular processes dependent on fasting. If the cell doesn’t experience prolonged nutrient deprivation, the process doesn’t kick in. For example, we know that a cellular cleanup process known as “autophagy” only really kicks into gear when we go without protein and substantial calories for a long period of time.

So what if we’re actually doing ourselves damage by not allowing these fasting dependent processes to take place?

We know our bodies were not evolved to be staring into phones and computer screens all day – is it possible we weren’t evolved to have access to food 24/7 365 days a year?

Image via Gizmodo

It was this line of thinking that got me into the idea of fasting as a therapeutic tool for good health. It may take decades to unravel all the complexities of fasting from a biological stand-point. But replicating the (unintentional) good-practice of our ancestors seems simple enough.

Could “fast mimicking” be better than water-only?

Prior to this I had experimented with water-only fasts, having gradually built up from 2-day to 5-day water-only fasts.

In that process I came to understand how difficult (and debilitating) water fasts can be.

It’s not just the 5 or so days you’re fasting where your ability to do things you take for granted (hit the gym, handle stressful situations) decreases, there’s also a recovery period of ~3 days post-fast to take into account.

Trying to plan out multiple days of water-only fasting, when you know you’ll have a busy work schedule and potential lack of sleep isn’t easy.

Fasting is a stressor, so ideally you don’t want to pile-on additional stressors.

So when I heard that Valter and his colleagues were investigating their “fast mimicking” diet as a means of getting many of the benefits of a fast, without avoiding food completely, I was excited.

Would this mean I can live “normally” whilst fasting?

Enter ProLon…

The ProLon Box

The issue with creating a “fast mimicking diet”, where participants make the meals, is they have to be very strict and diligent about adhering to calorie, macronutrient and micronutrient amounts.

Too much protein… and you negate the impact on IGF-1 (and likely mTOR too).

Too much calories… and you run the risk of not being in a fasted state at all.

So to make it easier for people to follow, Valter formed a company called L-Nutra, who came up with the above boxed formula of fast mimicking foods. Containing:

  • Soups
  • Nut bars
  • Crackers & Olives

Valter and colleagues have calculated down to the last calorie and macronutrient how much is optimal, then boxed it up as plant-based meals.

Each of those little boxes labelled 1 to 5 contains all the food you’ll be eating for that day – making them easy to slip into a bag and bring with you.

My Experience – Days 1 to 5

Kicking off day 1 I was pretty excited about the whole thing. When I opened the box to find what I would have in store to eat, it seemed like a good amount!

Day 1’s haul… looked pretty size-able

Psychologically, and practically, day 1 is super easy. You get around ~1,100 calories – whereas days 2-5 you get around ~800 calories.

As you can see below, this drop in calories after day 1 is the equivalent of 2 less bars, which is in fact very notice-able.

Day 5 box contents

Partitioning the food

The diet is set up to allow you to eat 3 separate “meals”, conforming to typical eating routines. However, these meals can literally be as much as a nut bar and a cup of tea!

My personal eating habits are weighted towards the evening – I like to have a good meal not long before I go to bed.

So to make things work for me, I’d partition the meals into two and have one around midday, and then one at the end of the day. That way, however hungry I felt throughout the day, I always knew there would be a “big” meal waiting for me when I got to the end.

At least it’s flexible in that way, and you can make that part work for you.

How were the meals?

Fortunately, I’m not a fussy eater. So for me the foods were all edible and enjoyable. That being said, I’ll often choose savory over sweet – and there was a lot of sweet food (for my palate).

Nut Bars

All the nut bars in the box are sweet, and thus if I had the choice I’d swap them out for savory options. In particular I found the “Choco Crisp” bars (below) super sweet. If I eat chocolate, normally, it’s 85% or 90% cacao content. Which this bar is nowhere near. Appreciate I’m in the minority on this.


So there are 3 different flavours of soups; tomato, mushroom and minestrone. For sure my favourite was the minestrone (below). I think it’s the most filling, and feels like you’ve had a semi-decent meal by the time you finish.

And then my least favourite, the tomato soup (below). I forgot to take a picture of the mushroom soup:

Other Bits

One fun addition, for those like myself who enjoy crunchy textures, was the kale crackers:

Each pack has 3 or 4 crackers stacked on top of each other (hard to see that in the picture)

There’s also a daily multivitamin and occasional omega-3 supplement:

Lastly, there was something called the “L-Drink” – it’s a flavored glycerol concentrate that you measure out into the provided water bottle, and then drink throughout the day. It makes the water taste mildly sweet – like drinking flavoured water. At least part of the reason for its inclusion is for hydration and energy:

Was it Difficult?

So, having done multiple day water-only fasts in the past – I didn’t think this would be too hard.

Well, I was wrong, it’s definitely hard – even with those small meals each day.

What I didn’t appreciate is how little food ~800 calories is. In retrospect I’m eating 3x that at least each day.

That being said, it’s definitely a lot easier than water-only – so helps with fasting compliance. People who wouldn’t consider a long water-only fast, might be able to do ProLon.

Blood Glucose & Ketones

When you water-only fast you end up switching from fueling your energy primarily via glucose, to primarily via ketones. The latter (ketones) being a form of energy currency derived from fat stores.

Thomas Seyfried, a professor at Boston college, coined a term “therapeautic ketosis”, where your blood glucose drops, and your blood ketone increase to equivalent or greater than your blood glucose.

He actually uses this “measurement” for tackling glioblastomas (aggressive brain tumors) – and likes patients to be at this “therapeutic” level of ketosis for some time.

Previously I’ve hit this level of ketosis around day 3 of my water fasts. So out of curiosity, I wanted to measure my blood glucose and ketones on the FMD, to see what level of ketosis I achieve.

Day 3 – before eating first meal

As you can see – by day 3 there was some ketones being produced. However I wasn’t into the “deep ketosis” you might see via water-only.

I was curious if by day 5 I was any deeper into ketosis…

Day 5 – before eating first meal

And… the answer was no! For whatever reason blood glucose was higher and ketones were lower on day 5, compared to day 3.

But at least it was evident I was still producing ketones.

I would have liked to check the morning of day 6 before refeeding – but unfortunately I had run out of ketone sticks! And my Amazon Prime delivery of them had not yet arrived.

Ending The Fast

The only day I really struggled was day 4. I always find that cortisol rises when fasting (which makes sense, fasting is a stressor), and that affects my sleep. So the night of day 3, I didn’t sleep well, which made day 4 a bit of a struggle from an energy perspective.

So then I got through days 4 and 5, and realized shit, I can’t actually eat normally again until day 6. Intuitively I had thought, ok, a 5-day fast, I’ll be able to eat again normally on day 5. But actually with the ProLon you have to wait until day 6 before you start eating again. It’s a bit different with water fasts, where you can count the hours from last meal – so 5 days = 120 hours. Meaning a fast that begins Sunday night, ends Friday night.

So it was annoying to have to sleep through Friday, and start eating again on Saturday. Annoying… but manageable.


This was the part I always seem to mess up on when water-fasting, and this was no exception. What you absolutely want to avoid are:

  • Eating heavy meals that put a burden on your digestive system – before its had a chance to adapt back to normal. You want to keep the foods quite simple.
  • Eating carbohydrate heavy meals – which will spike your blood glucose and insulin, causing a nasty crash in mood and energy levels.

Typically foods people recommend for refeeding:

  • Soups
  • Bone broths
  • Fruit

I think I probably overdid it in terms food quantity on my refeed, causing stomach cramps, and requiring close proximity to the toilet for a couple of hours. That being said, once that was over I was fine.

Image via Shutterstock

One likely scenario is that on the ProLon diet, the quantities of food hadn’t been high enough to “move things along” – so as soon as a decent quantity of food was consumed, the body had to make way.

As awkward a topic conversation as it is, it’s worth mentioning because I’d imagine I won’t be the only one.

How Did I Feel After ProLon?

Many of the benefits of ProLon are hard to see visually. You can’t “see” improved blood glucose, blood pressure, lowered IGF-1, or increased autophagy – not without measuring them.

So in terms of quantifying the results, it’s not easy without extensive testing.

That being said – there were 2 noticeable changes:

  1. A few days after the fast I noticed the skin on my face looked clearer and better. I don’t have any skin issues (acne etc), and yet this was still noticeable. I think it looked fresher and younger (subjective). That lasted for maybe 2 weeks, something like that
  2. I looked leaner. This makes sense, no doubt I’d lost a bit of weight on the fast. And this wasn’t just water weight either, as a couple weeks it had persisted.

In terms of actual “feeling” – I’d say I felt the same as before. I wasn’t suddenly brimming with energy or anything like that.

Would I do it again?

Yes! Whilst the fast was harder than I anticipated, it was still significantly easier than water-only fasting. That for me is a huge win. One of the problems with water-only fasts is I find them hard work, and thus haven’t managed to make them a routine thing.

I could see myself fitting in an FMD in situations where I wouldn’t be able to fit in a water-only fast. Which would help with the goal of doing 5-day fasts 2x per year – for the health benefits.

If you’ve made it this far, thanks for reading. L-Nutra, the company behind ProLon have been kind enough to share a 10% discount on ProLon boxes for readers of the blog. So if you’d like to try a ProLon fast and haven’t yet taken the plunge – now is a good time to get one at a discount.

A selfie I sent on Day 1 – telling a friend I’m about to start the diet


Posted by Alex


  1. Avatar
    Dr. Charles M. Paolino Sr June 10, 2019 at 11:42 pm

    I have viewed the video senario of your 5 day program. I am a physician, and my son, Dr. Charles Paolino, and daughter in law.\, Dr. Pam Yee, are starting your diet for the next 5 days. I will hear of their results. Next I will try such, based upon their experience. If as stated, we will be happy to recommend, and personally use for our patients. What is the professional price of this diet?


    1. Alex

      Hi Charles. Hope the fast went well! If you’re asking whether or not it’s possible to “sell” the diet to patients, it may well be. However you’ll need to contact L-Nutra directly (the company who make ProLon) to ask those types of questions. This post is a review of their product, but I am not a part of the L-Nutra company.


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