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 (referred to as autophagy) around days 3-5
• Lowered IGF-1 (due to the reduction in protein intake)
• Help maintaining healthy blood glucose and healthy blood pressure levels

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 $25 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!

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.

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).

Soups

So there are 3 main 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.

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:

Snacks

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

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.

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:

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 sleep.

When 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.

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!

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 it was noticeable even 2 weeks post-fast.

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.

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 ✅ Looking in the mirror after the fast and seeing clearer skin + visible fat loss is a nice reminder that all that effort you went through with the fast actually made a difference

❌ 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 hinderance. ❌ 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 the 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

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.

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.

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…

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!

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?

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 will 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, 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.

Whilst there isn’t anything wrong in doing a DIY fasting mimicking diet, there is a real benefit for many people, to ProLon, which takes away lots of the complexity.

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 fast mimicking diets (of which ProLon is one), so I’ve linked to two of the main studies below:

Initial research (in mice): A Periodic Diet that Mimics Fasting Promotes Multi-System Regeneration, Enhanced Cognitive Performance, and Healthspan

Clinical study in humans: Fasting-mimicking diet and markers/risk factors for aging,diabetes, cancer, and cardiovascular disease