Why I Tried the ProLon Fasting Mimicking Diet: A Functional Medicine Doctor's 5-Day Fast Experience with her colleagues

Let me tell you about the week my colleagues and I decided to eat like monks, well, sort of.

As functional medicine clinicians, we at Parsley Health are always exploring cutting-edge approaches to help people live longer, healthier lives. One of these approaches was the Prolon fasting mimicking diet. So we decided as a company to embark on this collective journey to try this 5-day fast.

There's something about shared suffering (and success!) that makes these experiments more bearable, and more enlightening.

What exactly is a fasting mimicking diet?

Here's the genius of it: the fasting mimicking diet (FMD) lets you get the benefits of fasting without actually starving yourself. For five days, you eat a carefully designed menu of plant-based foods, nutrient bars, soups, olives, crackers, and herbal teas, totaling between 800 to 1,100 calories daily.

The Prolon fasting mimicking diet, developed by longevity researcher Dr. Valter Longo, "tricks" your body into thinking it's fasting by restricting protein and carbs while providing healthy fats. This triggers something remarkable: your cells start cleaning house through a process called autophagy, essentially taking out the cellular trash and making way for fresh, healthy cells.

Fasting for longevity: why does this matter?

Think of fasting as a reset button for your body. Research on fasting for longevity shows that cycles of fasting mimicking diets can:

Turn back the biological clock by promoting cellular renewal and activating dormant stem cells, like energizing your body’s repair crew to do its job efficiently.

Cool down inflammation, which is at the root of most chronic diseases. Less inflammation means your body can focus on healing rather than firefighting.

Boost your metabolism by improving how your body responds to insulin and reducing stubborn belly fat. Your body becomes more efficient at using fuel. Your body has to shift from being carbohydrate dependent to becoming flexible and utilizing fats to create energy.

Preserve your muscle while burning fat. Unlike crash diets, this approach protects your lean tissue, which is crucial for staying strong and functional as you age.

Sharpen your mind. Many people report crystal-clear thinking and unexpected bursts of mental energy during and after the fast.

Studies suggest fasting for longevity may lower cardiovascular risks, strengthen immune function, and potentially help manage autoimmune conditions and certain cancers.

My 5-Day fast experience: the real talk version

Day 1: The Honeymoon Phase

Surprisingly easy! Some of us had smartly tapered our calories beforehand, which helped. One colleague texted the group: "It was good, just eating my bar now. The bars are sweet, and the L drink is helpful later, though I might add tea for flavor." Another chimed in: "I put the L drink into plenty of water to dilute the taste and really like the bars. This fast reminded me I don't need to eat as much regularly, best to always be a bit hungry."

A third colleague, who'd tried ketogenic fasting before, noted that the ProLon fasting mimicking diet felt smoother, with no headaches that she experienced with the keto diet, and less lonely. Having the team made all the difference.

Day 2: Reality Check

This was the rough one. "Food noise", that constant mental chatter about eating, kicked in hard for most of us. Some struggled with sleep. We leaned on hydration and group texts to power through. Pro tip: sparkling water and adding the hibiscus tea to the L drink and water became our best friend.

Day 3: The Turning Point

Brief moments of weakness gave way to surprising energy as the day went on. Our bodies were adapting, shifting into true fasting mode. We could feel the metabolic switch flipping.

Day 4: The Sweet Spot

Hunger? What hunger? Mental clarity and focus took over. Some of us actually forgot it was mealtime. The finish line was in sight, and we felt genuinely good.

Day 5: Victory Lap

We made it! Sleep quality improved dramatically, and nobody felt desperate to break the fast early. The consensus: staying well-hydrated was absolutely key to success.

The good, the bad, and the hangry

The Wins: Weight shifted in the right direction, mental fog lifted, cravings diminished, and we gained respect for how little food we actually need. The structured plan prevented the worst fasting side effects, like migraines or overwhelming hunger.

The Challenges: Managing initial hunger pangs, resisting psychological food cues (those coffee shop smells!), and functioning on lower energy during busy workdays required real commitment. One of us was also cooking for the family, which takes discipline. In other words, it impacts the control we have over our minds.

Functional medicine fasting: before you jump in

This isn't a casual undertaking. Success with any fasting mimicking diet requires planning, commitment, and lots of water. Tapering your calories a few days before starting makes the transition much smoother. Understanding what to expect, from initial hunger to the metabolic shift, helps you stay the course.

Most importantly, consider your baseline health and chat with your healthcare provider first. This is a tool, not a miracle cure, and it works best as part of a broader health strategy.

When fasting isn't your friend

The ProLon fasting mimicking diet isn't suitable for everyone. Skip it if you're pregnant, breastfeeding, have a history of eating disorders, or manage certain chronic conditions without medical supervision. In functional medicine, we believe in personalization; what works brilliantly for one person might not be right for another.

The functional medicine fasting difference

This is where functional medicine fasting shines. Rather than a one-size-fits-all approach, we tailor fasting protocols based on your unique metabolic health, hormonal balance, nutrient status, and gut function. We're not just handing you a box of food and saying "good luck"; we're optimizing the approach specifically for you.

At practices like Parsley Health, a functional medicine clinic nationwide, clinicians are integrating fasting mimicking diets into comprehensive wellness plans that address root causes of disease while supporting longevity goals.

Bottom line for patients

The ProLon fasting mimicking diet is a scientifically backed, practical way to trigger cellular renewal, metabolic balance, and longevity support without the extreme challenge of complete fasting. My 5-day fast experience taught me that doing it with others made the process more manageable and, honestly, more fun.

The keys to success with any fasting for longevity protocol? Proper hydration, preparation, and professional guidance from a functional medicine practitioner who understands your unique needs.

If you're looking to optimize your health and tap into your body's natural longevity mechanisms, the fasting mimicking diet could be a powerful option. The science supports it, the results speak for themselves, and you don't have to do it alone.

Join us if you are ready to hit your RESET button.

FAQs about the Prolon 5-day Fasting Mimicking Diet:

What exactly is a fasting mimicking diet (FMD)?

A fasting mimicking diet is a scientifically formulated 5-day, plant-based eating program that provides approximately 800-1,100 calories daily through specific nutrient combinations designed to "trick" the body into entering a fasting-like metabolic state. Unlike water-only fasting, the FMD includes carefully balanced foods, soups, energy bars, herbal teas, healthy snacks like olives and kale crackers, plus targeted supplements that avoid triggering the body's nutrient-sensing pathways while still providing essential nutrients

Why does functional medicine include FMD? 

Functional medicine embraces FMD because it represents a powerful, evidence-based intervention that addresses root causes of dysfunction rather than merely managing symptoms. The scientific foundation is robust, with research demonstrating that periodic FMD cycles can reduce biological age by 2.5 years, activate autophagy (cellular housekeeping), regenerate stem cells across multiple organ systems, and reset immune function.

Additionally, functional medicine practitioners appreciate that FMD can be personalized based on individual biochemistry, genetics, and health status, making it a versatile tool that can be integrated with other therapeutic approaches.

How safe is FMD? 

FMD has demonstrated excellent safety profiles in multiple clinical trials when used appropriately. Current research indicates that consuming FMD for 5 days monthly is safe and effective for most healthy adults, with only mild, transient side effects reported.

Can FMD help with hormonal / thyroid / metabolic issues (e.g., PCOS, insulin resistance)?

Yes, FMD shows remarkable efficacy for metabolic and hormonal dysfunction. The diet's primary mechanism involves restoring metabolic flexibility, the body's ability to efficiently switch between glucose and fat burning, which becomes impaired in conditions like insulin resistance, PCOS, and metabolic syndrome.

For insulin resistance and diabetes, multiple studies demonstrate that FMD cycles significantly improve glucose metabolism, with reductions in fasting glucose (up to 11.3%), HbA1c levels. In diabetic patients, 57% were able to reduce anti-diabetic medications after FMD cycles, with 16% completely discontinuing medications. The diet appears particularly beneficial for individuals with higher BMI or multiple metabolic syndrome components.

Regarding PCOS, emerging research suggests FMD may help by reducing insulin levels and inflammation, two key drivers of PCOS symptoms. The diet's ability to decrease adiposity and improve insulin sensitivity could theoretically improve hormonal balance in PCOS patients, though more specific research is needed.

For thyroid and adrenal function, the relationship is more complex. While direct studies on FMD's effects on thyroid hormones are limited, research shows intricate connections between thyroid function and metabolic flexibility. Hypothyroidism can impair adrenal function and metabolic flexibility, suggesting that interventions improving metabolic health might indirectly support thyroid and adrenal function. However, individuals with significant thyroid dysfunction should undergo FMD only under medical supervision, as fasting can influence thyroid hormone regulation.

Is there an optimum age to do FMD?

The optimal age range for FMD appears to be 18-70 years, based on current safety data and research populations. Most clinical trials have focused on adults in this age range, demonstrating both safety and efficacy.

Middle-aged adults (30-60) represent the ideal demographic for FMD, as this is when metabolic dysfunction, insulin resistance, and chronic inflammation typically begin to manifest. The diet's ability to reduce biological age by 2.5 years is particularly relevant for this population seeking to optimize healthspan and prevent age-related diseases.

For older adults (60-70), FMD can be highly beneficial under medical supervision, especially for those with metabolic dysfunction or early signs of cognitive decline. Research shows FMD can improve cognitive performance and promote hippocampal neurogenesis in aging mice.

How do I know if I should do FMD?

Consider FMD if you're experiencing metabolic dysfunction, chronic inflammation, or age-related health concerns that haven't responded adequately to traditional approaches. FMD serves as a gentle yet powerful "reset" tool that can break through metabolic resistance patterns.

The Fasting Mimicking Diet or FMD is not a quick fix. It is one of the tools to support the body’s innate ability to heal and become metabolically healthy.

References:

https://pmc.ncbi.nlm.nih.gov/articles/PMC11943686/

https://pmc.ncbi.nlm.nih.gov/articles/PMC10589984/

https://pmc.ncbi.nlm.nih.gov/articles/PMC4509734/