Fasting is challenging, there’s no denying that. 

There are ways to make fasting a little easier, like drinking your water and taking your electrolytes. But what if there was a shortcut? A way to get all the benefits of fasting without actually fasting? How would that work? Too good to be true? Let’s talk about it. 

What is the Fasting Mimicking Diet?

The Fasting Mimicking Diet (FMD) was developed by longevity researcher Valter Longo, PhD. It was designed to produce some of the body’s responses to fasting while still allowing you to eat. Sounds nice, right?

The best-known commercial version is ProLon, a five-day program that provides carefully measured amounts of calories, carbohydrates, protein, and fat. Current versions provide about 1,100 calories on day 1 and 700-800 calories on days 2-5. 

But the obvious question is this: if you’re eating food, how can it be fasting? 

FMD vs. water fasting

A traditional water fast means consuming no calories for a set period of time. Once food stops coming in, insulin and blood glucose generally fall, stored glycogen is used, and then the body starts to turn to stored fat and ketones for energy. 

A Fasting Mimicking Diet takes a different approach. Instead of eliminating food, it dramatically reduces calories and limits certain nutrients, particularly protein and sugars, while providing mostly plant-based foods and higher amounts of unsaturated fat. 

The idea here isn’t that the body can’t tell the difference between eating and not eating. It clearly can. 

Instead, the goal is to mimic some of the metabolic signals associated with fasting while still providing enough nutrition to make a five-day program easier to tolerate. 

That distinction is important. A Fasting Mimicking Diet is not the same thing as a true water fast, and it never claims to be. 

Why include carbohydrates?

This is probably the part that makes people who enjoy fasting or Keto/Carnivore diets scratch their heads. 

The FMD contains carbohydrates. In fact, the original research formulation provided roughly 43% of its calories from carbohydrates on Day 1 and about 47% on Days 2-5. 

So why not simply remove the carbs?

Because the FMD isn’t trying to copy a Ketogenic diet. It is trying to manipulate several nutrient-sensing pathways at the same time. 

In other words, the goal isn’t necessarily to make carbohydrate intake as close to zero as possible. It’s to create a specific combination of low calories, relatively low protein and sugar, and higher unsaturated fat. 

That may sound strange if your idea of fasting is simply “don’t eat.” But again, that’s why the word mimicking is there. 

Does it actually work?

The answer to that probably depends on what you mean by “work.” There is legitimate human research behind the concept. 

A 2024 analysis published in Nature Communications found that three FMD cycles were associated with improvements in insulin resistance, prediabetes-related markers, and liver fat. The researchers also reported a reduction in a biological-age measure of about 2.5 years. However, this was a secondary and exploratory analysis, so not exactly proof that the diet actually extends lifespan. 

And a 2025 systematic review and meta-analysis of 11 randomised controlled trials found that FMD was associated with reductions in HbA1c, IGF-1, and blood pressure. However, the researchers did not find consistent significant effects on body weight, BMI, or waist circumference across the studies. 

Is the FMD too good to be true?

It’s iffy. The science behind FMD is interesting, but some of the claims that surround fasting mimicking diets go further than human research can currently prove. 

Animal studies have produced impressive findings involving longevity, cancer, immune function and cellular regeneration. But humans aren’t mice, and showing that a dietary intervention changes a biological marker is not the same as proving it prevents disease or extends human lifespan. 

Another point to take into consideration here is that some of the major FMD research involves researchers who have financial or intellectual-property relationships with L-Nutra, the company behind ProLon. 

That doesn’t mean the research is wrong. It simply means independent replication is especially valuable. 

Why try FMD? 

Obviously, the idea of being able to eat daily seems easier than going long amounts of time without food. But if that’s the reason that you’re drawn to the FMD, why not choose Intermittent Fasting instead? With most Intermittent Fasting schedules, you have a daily eating window and you get many of the same benefits as Extended Fasting. 

And there’s this: a consistent fasting lifestyle tends to settle hunger and food noise, whereas restrictive diets like FMD can make hunger worse, especially if you’re including carbs in that diet. If easier plus benefits is what you’re after, Intermittent Fasting is your best bet. 

Not the same

The bottom line is that a Fasting Mimicking Diet is not the same as a true water fast. You’re still consuming hundreds of calories each day, including carbohydrates. And as we know, some food is not the same as no food. You’re still spiking insulin throughout the day. 

There is evidence that a FMD can produce measurable metabolic changes, and the concept is scientifically plausible. But it is important not to confuse promising results with proof of every health claim made about it. 

The most reasonable way to look at FMD right now may be that it’s an interesting strategy with some promising research behind it, but it’s not a magic shortcut to longevity. And when something sounds too good to be true, a little healthy scepticism is a good idea. 

Citations: 

Brandhorst, S., Levine, M.E., Wei, M. et al. Fasting-mimicking diet causes hepatic and blood markers changes indicating reduced biological age and disease risk. Nat Commun 15, 1309 (2024). https://doi.org/10.1038/s41467-024-45260-9

Mohammadzadeh M, Amirpour M, Ahmadirad H, Abdi F, Khalesi S, Naghshi N, Bahrami A, Hejazi E. Impact of Fasting Mimicking Diet (FMD) on cardiovascular risk factors: a systematic review and meta-analysis of randomized control trials. Diabetol Metab Syndr. 2025 Apr 26;17(1):137. doi: 10.1186/s13098-025-01709-5. PMID: 40287774; PMCID: PMC12032729.