(Please note: this post contains links to ProLon, if you make a purchase from one of these links, I may receive a small commission. Learn more here)

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:

  • Supporting the body’s natural processes of cellular clean-up and cell renewal
  • Supporting overall metabolic health and maintaining healthy systolic blood pressure*
  • Inducing fat loss without decreasing lean body mass*

*(Results were based on 3 consecutive monthly cycles)

That all sounded really good to me, so I ordered one to check it out. At $250 per box, it isn’t cheap, but there was only 1 way for me to find out if it was worthwhile (side note – get 15% off via this ProLon coupon code)

I’ve broken the review into 2 sections;

  1. My Experience – Meal Pictures, Pro’s & Cons, etc
  2. The Geeky Stuff – Blood glucose & ketone measurements, etc

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 food 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 taste palate).


There are 5 types of soups; minestrone with vegetable, minestrone with quinoa, mushroom, tomato and vegetable. For sure my favourite was the minestrone with vegetable (below). I think it’s the most filling, and feels like you’ve had a semi-decent meal by the time you finish.

Mushroom – doesn’t look too exciting! But tastes reasonable:

And then my least favourite, the tomato soup (below).

There’s also a variation of the minestrone with quinoa – which gives it some extra substance:

Then a vegetable soup:


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

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

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) overly sweet. That said, a friend of mine who has done ProLon a few times says these are her favourite (!) so appreciate I’m in the minority on this.

The nut based bar:

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:

Soup Variety 2

Since I did the ProLon fast, and reviewed it here, ProLon have released some new soup options for the meals. You’ll see it as “Soup Variety 2” on prolonfast.com when you go to make an order:

Soup variety 2 swaps out:

  • Mushroom
  • Minestrone
  • Minestrone with Quinoa
  • Vegetable Soup

And adds in:

  • Butternut Squash
  • Butternut Squash with Quinoa
  • Spinach with White Bean
  • Black Bean

I ordered the set this month, and am sharing them below, with the hope that it helps in choosing which option to go for. If you’d like to see the new soup’s ingredients list, look here:

Butternut Squash Soup

This soup has a thick texture (owing to the rice flour they added) which makes it feel more substantial as a meal. If you’re familiar with butternut squash – then it tastes just like that – but with a slightly tart (sour) taste to it.

Butternut Squash with Quinoa Soup

This was my favourite of the two butternut squash soups. Whilst I liked the thickness of the “butternut only” version (this one is more watery) I think overall the flavour is slightly nicer.

Spinach with White Bean Soup

In contrast to the sweeter tasting butternut squash soups – this one (unsurprisingly) tastes more of spinach leaves. It’s perhaps a bit bland, but probably quite healthy!

Black Bean

The black bean soup is pleasant, but not packed with flavor! I wouldn’t even say it tastes particularly strongly of black beans (at least not the way I’m used to them being prepared). But again, it’s tasty enough, and gets you through the day.

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, so day 4 was a bit of a struggle!

Note: next fast I may try Peter Attia’s sleep cocktail on nights 3, 4 or 5. Particularly interested to see if the phosphatidylserine reduces the cortisol increase enough for restful onset of sleep.

When I got through days 4 and 5, and realized whoops, 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.

Refeeding & Recovering After

This is the part I always seem to mess up on when water-fasting, and this time 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”  how it might affect autophagy, systolic blood pressure etc – not without measuring them.

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

I’d like to say at the end I was brimming with energy, but I wasn’t. Instead I was hungry and excited to get stuck in to some food!

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 more comfortable with ProLon.

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 ProLon 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.

Pros & Cons

Pros Cons
✅ The packaged meals make adhering to the diet very simple. The boundaries are clear, and there’s no accidentally giving yourself bigger portions

✅ Given that the food is pre-made, and needs to have a long shelf life, it’s actually quite tasty. And I’ve never seen powdered soups with such a healthy ingredients list!

✅ It’s very well organized, from the daily food boxes, to the guides that lay out the meal plan

❌ The food for the meals is fixed, and can’t be customized. For those people with specific eating restrictions, whether for medical reasons, or simply preference, this can be a hindrance. Fortunately, it’s possible to customize the soups using very low-calorie ingredients, to improve the flavor. See the section below for more info.

❌ Because most of the potential health benefits take place under the skin, you can’t see them. Thus there’s a certain amount of trust that all your self-discipline will be worth it! I found myself reading through research papers (linked further down) mid-fast, to help with motivation.

❌ Blood ketones do not rise as high when doing a water only fast

❌ This is a small niggle, but there’s quite a lot of disposable waste in the ProLon box. The wrapper of every product, and even a big plastic water bottle they provide. It would be great if they can reduce this, and/or make it more recyclable

Can I Add Herbs & Spices to the Soups?

If you’d like to “spice up” the soups, then the good news is you can. I’ve written a guide here, that provides (tried and tested) recipes for each of the soups.

ProLon’s official guidance on this is 1 teaspoon of herbs and spices (maximum) per day. In line with this, the recipe’s each use up to 1 teaspoon, which means you can only “spice up” 1 of your 2 soups each day.

See ProLon Soup Tips – How to add some flavor and spice to the soups for all the recipes.

The Geeky Stuff

Blood Glucose & Ketone Readings

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.

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.

Why I Became Interested in Fasting – An Evolutionary Perspective

Taking a step back for a minute, I’d like to cover what got me interested in fasting in the first place. 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, possibly regularly, gone for extended 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 at cellular processes developed as a result. These processes only ramp up when we go without food. That means, if the cell doesn’t experience prolonged nutrient deprivation, the process doesn’t kick in. For example, we know that a cellular cleanup called “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.

ProLon vs DIY FMD

It is possible to create your own “fasting mimicking diet”. However it requires very strict and diligent about adhering to calorie, macronutrient and micronutrient amounts.

Too much protein… and you negate the impact on IGF-1 hormone & the mTOR growth pathway. Both of which should decrease during a fast.

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, and used his research to formulate a fasting mimicking diet, without over-doing it in terms of calories or macronutrient ratios, then boxed them up as plant-based meals.

They contain:

  • Soups
  • Nut bars
  • Crackers & Olives

Whilst there isn’t anything wrong in doing a DIY fasting mimicking diet, there can be a real benefit for many people, to ProLon, which saves time, and reduces effort and complexity.

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.


Ok, so you’ve got to the end of my review on the ProLon diet – hopefully it has been helpful! If you’ve got any further questions, please leave them below in the comments and I’ll do my best to answer. If not, all the best in your fasting journey 🙏

If you’re interested in saving 15% on a box of ProLon, see this coupon code.

Research Paper Links

As I mentioned in the Pros & Cons above, something that helped with my motivation during the fast was reading through the great clinical results that had been achieved using a fast mimicking diet (of which ProLon is one), which you can read more about here:

Posted by John Alexander

Hi, I'm John, a researcher and writer.

With a keen interest in health and longevity.

Note: not an MD or PhD.

Hope you enjoy the site. If you've suggestions for content you'd like to see - let me know.

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Self Worth Sam
Self Worth Sam
3 days ago

Great article. Thx.

9 months ago

John, what about the impact on women’s hormones. The hormones regulating key functions like ovulation are incredibly sensitive to energy intake so I’m curious if any studies have been conducted on this diet (or other calorie fasting diets) on hormone levels.

1 year ago

I take a lot of high-dose vitamins and supplements which I consider important to my health. Although I could drop a few for the 5 days, there are some which I do not want to skip. Would I be able to take the essential ones and still have the benefits? Please don’t tell me the program contains all essential nutrients. I know it does. But I approach it from an orthomolecular perspective.

Reply to  John Alexander
1 year ago

Methy B9 and B12, Vit C, D, K2, ubiquinol, fish oil, magnesium, lysine, proline, aged garlic extract. I don’t really get the issue with taking supplements as they don’t have calories except maybe the fish oil. Not sure why it would mess with the fast.

1 year ago

Hi. Can a person who is on the blood thinner Coumadin do this fast? Vitamin K content of supplements? Kale chips? I know consistent amount of Vit K is fine when on Coumadin however curious if this contains an excessive amount of Vitamin K. Thanks ! Deb

Last edited 1 year ago by Deb
Wolfgang Menzel
Wolfgang Menzel
1 year ago

Hi John,

thank you for your great article. I have only done the 5:2 water diet, so I am a little nervous to do the 5 days. Do you have any advise in general? Also, do you eat anything especial before you do the 5 day diet?

Thank you, Wolfgang

2 years ago

Hi John,

Did you drink coffee while on the fast?

I have been doing intermittent fasting on and off and typically drink black coffee while fasting.


Sarah Sandoval
Sarah Sandoval
2 years ago

This is an excellent review- just what i was looking for i have done several 3-4 day fasts but often cap it at 4 days because of energy and sleep disturbances. While i understand that FMD and fasting has many reasons for Being effective beyond calorie restriction. I still can’t wrap my ahead around how these pre-packaged meal plans can be standardized for all BMI/weights- so a 250# man eats the same as a 125# female???

Sophia Grace
Sophia Grace
2 years ago

After reading about your experience, I ordered the Prolon FMD, and was surprised how well it went. I’ve done intermittent fasting on and off for a couple of years, so I thought it was definitely doable. I lost 8.8 lbs (so far only about .6 lbs regained). They have a new partner, Nutrition for Longevity (plant based 1600/2000 calorie meals), so I ordered the 3 day trial to transition slowly back into normal eating. I got the pescatarian version of their food plan. I still have more weight to lose, so I signed up to receive another fast in January.… Read more »

2 years ago

John, you say “Valter and colleagues have calculated down to the last calorie and macronutrient how much is optimal.” How does that work given the huge differences in weight of potential customers? A 250lb man will find it much more difficult than a 95lb woman if they’re both receiving identical 5 day portions.

2 years ago

Hi there,

Do you know or have seen if this FMD has shown to help colitis? It is something I struggle with and dont know if this is a good option to help “reset” my system.

2 years ago

Can you still continue to exercise while on the FMD fast?

2 years ago

Can it be prescribed and then covered by Medicare

Eric Stein
Eric Stein
2 years ago

Hi John, thanks for putting up pictures of the new soups. I was interested in trying a different box this time, and wanted to know a bit more about what would be included – so this helps.

Shannon W
Shannon W
2 years ago

Hey John, thanks for the info!

What do you think of this diet for losing weight? I’ve a wedding coming up and I’d like to lose a few pounds.

Dr. Charles M. Paolino Sr
Dr. Charles M. Paolino Sr
3 years ago

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?