Fasting Mimicking Diet: What happened when I tried It for five days

A few weeks ago, I signed up for a five-day fasting mimicking diet by ProLon. I was curious about the research and, as a Nutritional Therapist, wanted to know what it actually felt like before recommending it to clients.

There was another, less scientific reason too. By the end of August, despite my best intentions, I'd fallen into a pattern of mindless eating. I wanted to break the leftover-fish-fingers habit and become a bit more conscious of when and why I was eating.

What is a fasting mimicking diet?

I wasn't actually going without food for five days. I followed a fasting-mimicking diet, which provides a small amount of food while keeping intake low enough to try to reproduce some of the metabolic changes associated with fasting.

You get a box for each day, with everything you're allowed to eat divided into little packets. Breakfast was an incredibly dense nut and honey bar. It was ok. Not particularly sweet and felt like the sort of thing you might survive on if you were lost in the jungle.

The rest of the day's food consisted of different soups, sometimes kale crackers and occasionally olives, which were probably the highlight. Caffeine was allowed but milk wasn't. I decided that if I was going to get through the week, my morning coffee was keeping its splash of milk.

And that was essentially what I ate for five days.

Was I hungry?

Interestingly, hunger wasn't really the problem I had expected it to be. Day one was relatively easy. There was slightly more food and everything was a novelty.

Days two and three were much harder. I'm used to a diet rich in fresh, whole foods and I found several days of packet soups, bars and crackers quite a stretch. My energy was low and I just didn't feel particularly good.

By day four, things changed. My energy picked up, I could see the end in sight and by then I was determined to finish.

What was happening in my body?

When we dramatically reduce our energy intake, the body gradually changes where it gets its fuel. As readily available glucose and stored glycogen fall, we rely increasingly on stored fat and produce more ketones.

Fasting is also associated with autophagy, a word you'll hear a lot in discussions about fasting. It literally means “self-eating”, although it's better thought of as cellular housekeeping: a normal process through which cells break down and recycle damaged or unnecessary components. Fasting appears to stimulate it, although much of what we know comes from laboratory and animal research and we still don't know exactly how much fasting is needed to increase autophagy in humans. A small 2025 randomised trial of 30 healthy adults produced some encouraging results, but larger studies are needed.

What does the research say?

The ProLon programme recommends repeating the five-day fast once a month for three months, and there is some interesting research behind this protocol.

In one human trial, three monthly cycles were associated with reductions in body weight, body fat, blood pressure and IGF-1, with some of the greatest improvements in people who had less favourable health markers to begin with.

A 2024 analysis also reported improvements in insulin resistance and pre-diabetes markers. Intriguingly, median estimated biological age fell by nearly 2.5 years after three cycles. Before we all rush to knock 2.5 years off our age, however, this was a calculation based on biomarkers. The study didn't show that people actually lived longer or developed fewer age-related diseases.

Some of the research has involved researchers connected with the development of the diet or received funding from L-Nutra, the company behind ProLon. That's worth knowing when interpreting the evidence, and I'd like to see more independent research as this field develops.

What about weight loss?

Several people doing the programme alongside me reported losing five pounds. I lost a rather more modest two, but what interested me was that, according to my smart scales, the weight I lost came from fat while my muscle mass stayed stable. Home smart scales can give a useful indication of changes in body composition, although they are less precise than clinical methods such as a DEXA scan and changes in hydration during fasting can also affect the readings. It was reassuring nonetheless.

Preserving muscle becomes increasingly important as we get older, and rapid weight loss can include some loss of lean mass as well as fat. Interestingly, research into fasting-mimicking diets has also reported preservation of muscle alongside reductions in body fat. My bathroom scales certainly can't prove that this happened to me, but it was an encouraging observation.

The benefit I hadn't expected

By the end of August I'd slipped into grazing. I wasn't necessarily hungry when I ate; food was simply there and I was picking at it without paying much attention. Five days in which every morsel of food was prescribed certainly put a stop to that.

When I started eating normally again, I noticed my food much more. I looked forward to meals, took time to taste them and naturally left proper gaps between eating rather than wandering into the kitchen whenever I passed it.

That was the outcome I'd hoped for and, for me, it worked. Not because I believe I'd magically “reset” my metabolism, but because I'd interrupted a pattern of eating that wasn't serving me particularly well.

Fasting isn't for everyone

Fasting isn't appropriate for everybody. Pregnancy and breastfeeding, a history of disordered eating, being underweight, certain medical conditions and some medications can make prolonged fasting inappropriate or mean it should only be considered with medical supervision. Anyone taking glucose-lowering medication in particular shouldn't embark on a prolonged fast without discussing it with their healthcare professional.

Would I do it again?

Probably not immediately. Although I found the research fascinating and there were aspects of the experience I really valued, I didn't enjoy how depleted I felt. For now, I'd rather use shorter periods without food that fit more easily into normal life and don't affect my energy in the same way.

I wouldn’t rule out doing it again though. And for the right person, I can see why a clearly structured five-day programme might be easier than trying to fast without any food at all.

And yes, that first proper meal on day six was glorious.

If you’re curious about whether fasting might be right for you, or would like support with any other aspect of your nutrition and health, get in touch to arrange a free introductory call.