I Wouldn't Wish ProLon's 5-Day Fast-Mimicking Diet on My Worst Enemy

Before starting the ProLon 5-Day Fasting Mimicking Diet (FMD), I called my mother. I was excited about the diet's purported benefits (which include boosted metabolism, increased energy and improved mental clarity, to name a few). Like any good daughter, I wanted my mom's blessing.

"Basically, you eat almost no food to trick your body into thinking it has eaten no food," I explained.

"It sounds like torture," she responded, concerned.

"You don't understand," I insisted. "It improves longevity."

Perhaps my explanation was oversimplified. ProLon — a trendy diet all over TikTok — is a five-day eating plan designed to deliver the benefits of fasting while allowing you to consume small amounts of food. It first rose to notoriety after being featured on Gwyneth Paltrow's Netflix series "The Goop Lab" in 2020. Over the years, ProLon has won celebrity devotees like Jennifer Aniston, Eva Longoria, and Kate Hudson.

ProLon is making the rounds on social media, where influencers tout dramatic weight loss and share tips making the challenging eating plan more palatable. (Tips like "use real dishes" to make it feel like you're eating real food and "make your soups extra watery" to make them last longer.) It's worth noting that ProLon also has a popular affiliate program where influencers can make up to 20 percent commission on kits sold, incentivizing reviewers to slant positive. (In case you were wondering: I am not affiliated with ProLon and paid for my kit.)

Here's how it went when I tried the trending diet.

Alix Turoff, RD, is a nutrition consultant and personal trainer.

Amy Burkhart, MD, is a physician and dietician specializing in gut health.

Richa Mittal, MD, is a physician and Diplomate of the American Board of Obesity Medicine and Lifestyle Medicine.

The fasting-mimicking diet was created by Valter Longo, PhD, professor of gerontology and biological sciences at the USC Leonard Davis School of Gerontology. Longo developed the FMD some 20 years ago, intending to create an eating plan that could help rejuvenate cells and organs in cancer patients. Realizing its purported anti-aging benefits had commercial appeal, Longo began selling ProLon "fasting kits" to the public in 2016.

"ProLon shifts the body into a fasting state," says nutrition consultant Alix Turoff, RD. "This encourages cellular rejuvenation, autophagy, and metabolic changes similar to those seen in water-only fasting without requiring complete food abstinence."

Does "eating while fasting" sound too good to be true? There's science behind ProLon's claims, according to physician and dietician Amy Burkhart, MD. The ProLon fasting-mimicking diet has undergone clinical trials and obtained a patent for age reversal, Dr. Burkhart says. "As more research emerges, I believe we are only beginning to see its full potential."

Preliminary research into fasting-mimicking diets like ProLon looks promising, agrees Richa Mittal, MD. In one recent study, participants who followed three cycles of the FMD saw their biological age decrease by an average of 2.5 years. "Research shows people following ProLon once a month for three months experienced improvements in blood pressure, blood sugar, cholesterol, and markers of inflammation," Mittal says.

Turoff, a registered dietician and nutrition consultant, is more reserved about singing ProLon's praises. "Many of the benefits are also seen with general caloric restriction," Turoff says. "More research is needed to determine whether FMDs offer unique benefits beyond just eating in a controlled calorie deficit."

ProLon tricks your body into fasting mode with a 5-day eating kit that consists of prepackaged foods and nutrients designed to keep calories low enough to mimic a fasting state, explains Dr. Burkhart. Doing so triggers autophagy, one of the diet's most important benefits, according to Dr. Burkhart. "Autophagy is the body's natural process of clearing out damaged cells and cellular waste," she says. "It plays a vital role in slowing aging, preventing disease, and slowing disease progression."

Other potential benefits include improved metabolism and weight loss, adds Turoff.

ProLon is not for everyone, Mittal says. "Even though ProLon is available to the general public, ProLon is a medical intervention," she stresses. Certain groups should avoid ProLon, including kids and people who are underweight, pregnant, or breastfeeding, or have a history of eating disorders, Mittal says.

The diet is very restrictive, Turoff notes. Day one of the ProLon contains around 1,100 calories. Days two through five contain around 720 calories per day. On average, adult women need around 1,800 to 2,400 calories per day to maintain weight. Adult males need 2,400 to 3,000 calories per day. The sustained calorie deficit will likely result in muscle loss, Turoff says. As with any restrictive diet, it's best to consult a healthcare provider before trying ProLon.

ProLon also comes with a high price tag. At full price, the five-day fasting kit comes in at a steep $229. "ProLon is expensive and some of the benefits — including weight loss and metabolic improvements — could likely be achieved through a well-structured, less restrictive dietary approach," Turoff says.

Call me crazy, but I was excited to start the ProLon diet. I was also very confident I could succeed. As someone who writes about health and wellness for a living, I've long practiced intermittent fasting and have tried every health hack and wellness trend under the sun — from going keto to infusing my water with okra.

I purchased the ProLon 5-Day Program kit ($185, originally $215) when it was on sale. During checkout, I was successfully upsold on an additional box of "Fasting Bars" ($35) for an additional charge. These bars would help me transition back to normal eating post-fast, the site claimed. (Go big or go home, I reasoned as I added them to my cart.)

My order arrived two days later. The five-day eating plan came neatly packaged in a fancy white box. Inside the box were five numbered boxes, each containing the meals for that day. Per the instructions, I was to consume only water and the items included in the box if I wanted to optimize results. However, if I absolutely had to, I was allowed to add up to one teaspoon of dried herbs and up to one teaspoon of sea salt to my meals per day. Sparkling water with natural flavors was permissible, if I must. Plain coffee with up to 140 mg of caffeine (around eight ounces) per day was also allowed but discouraged.

Learning that I could only have a small amount of caffeine alarmed me. Eight ounces is a lot less than my traditional coffee allowance (unlimited). I was also bummed to discover I wasn't supposed to exercise during the diet. What was I supposed to do for fun, if not eat and work out?

After waiting a few days for a cold to clear up (ProLon wisely recommends not doing the diet if you're sick), I was off. On Day 1, allegedly one of the easier days, I am allowed around 1,000 calories. These 1,000 calories consist of dehydrated instant soups, Fasting Bars, a Fasting Shake, olives, kale crackers, tea, and supplements.

I kick off Day 1 with a Fasting Bar, which turns out to be made of mixed nuts and about the size of a leather bracelet. I devour it in five seconds. Pretty yummy, I think. What's next? I check the menu. The only thing left for breakfast is two large capsules containing "algae oil."

I am still hungry and start to get concerned. Maybe ProLon isn't the walk in the park I expected.

I want to quit. I'm hungry, tired, achy, and experiencing some severe brain fog. On top of my physical symptoms, my mood has taken a dive: I feel anxious and even a little depressed. To make matters worse, today's menu includes even less food. Day 2 totals around 700 measly calories, which includes a similar lineup of Fasting Bars, instant soup, and supplements. "I HATE PROLON," I write in my journal as I polish off my eight ounces of coffee.

I receive an email check-in from the company that informs me my unpleasant symptoms mean I'm transitioning into ketosis. "As of today, your body has likely used up its glucose stores and is beginning to rely entirely on your internal fat stores to produce ketones, an internal process called ketosis," the email reads.

This initial transition into ketosis might also bring symptoms such as brain fog, fatigue, or body aches. "This will balance in the coming days — stay strong!" the email adds. Resentful, I trash the email.

I'm seriously considering quitting. I promised my editor an article about completing the ProLon fast, but surely "I Tried the ProLon Fast and Failed" or "I Made a Mature Decision Not to Finish the ProLon Fast" or even "Screw the ProLon Fast I Want a Sandwich" are all acceptable angles?

Day 3 includes the usual sad menu of power soups, nut bars, and supplements. I drop a single olive off my patio and run down to get it. I eat it off the ground and am not ashamed.

Out of nowhere, my energy spikes. I feel invincible and unusually chatty. The difference is palpable — almost as though I'm on some kind of energy-boosting drug. Food? Who needs it? I send out a flurry of texts.

"I feel amazing!" I text my boyfriend. "The science is real!"

The daily check-in email from ProLon confirms my experience is common for Day 4. "By Day 4, autophagy is exponentially in motion. Some may begin to experience what's often called 'keto-euphoria,'" the email reads. "This is when you start to feel the positive changes happening within. You may begin to experience increased mental clarity, focus, energy, and vitality."

On Day 5, my energy and mood plummet back down. I feel foggy and grumpy again, in addition to being disappointed by how short-lived my "keto-phoria" was. I feel lousy enough that I consider canceling my afternoon improv class. Luckily, I don't: It turns out to be a welcome distraction from my growling stomach and misery.

A few of my friends go out to dinner together after the class. I eat a Fasting Bar in my car. It is the saddest car dinner I have ever eaten.

The nightmare is over. I can eat real food! Not so fast, says my daily ProLon email check-in. "Remember when introducing food back into your system, you want to keep it light and mostly consume plant-based foods," it reads.

The email informs me I might find I'm not as hungry as I thought I would be on day 6. Wrong. Nevertheless, I dutifully stick to a healthy, plant-based diet all day. Tomatoes have never tasted so delicious.

By nightfall, I'm still ravenous. The ProLon email tells me I probably won't want snacks (again, wrong), but if I do, I should eat one of their Fasting Bars (which, conveniently, they'd already upsold me on). Good thing I bought these, I think as I chew. When I finish the first one, I unwrap a second. Uh oh, I can't stop. I gobble down a second and then a third bar. I tell my boyfriend to hide the box to stop myself from scarfing all 12 bars.

Expecting transformative mental clarity and superhuman energy, I eagerly monitor my symptoms in the days that follow. I am disappointed. Life after ProLon is more or less unremarkable. Really, the best post-fast symptom was the intense gratitude I feel to no longer be living off of bland soups.

I lost four pounds, but in the two weeks that follow, I regain most of them. My main reason for doing the diet had been to improve longevity — a difficult metric to measure. If I did slightly lengthen my life span by doing the fast, it came at the cost of four days of "torture," as my mother sagely predicted.

So, would I do the diet again? For best results, ProLon recommends doing the diet once a month for three consecutive months, followed by cycles every three to six months for maintenance. My reaction to that: Hell no. "Those who repeat the diet — whether monthly or every few months as recommended — tend to find it gets significantly easier each cycle," Dr. Burkhart says.

If you do try again, go easy on yourself, Mittal recommends. "Make sure you get enough sleep and stay hydrated," she says. "If you feel miserable and go off-plan on day three, it's not the end of the world — you're still getting benefits."

But Turoff recommends trying a different, more sustainable, and enjoyable strategy to enhance longevity. "If it's the longevity benefits that you're after, a good goal might be to add an omega-3 supplement or eat one to two servings of fatty fish each week," she suggests.

I love me some mackerel, so this is a strategy I can get behind.

If you are a person who struggles to eat healthily and wants an extreme, no-nonsense reset, I see how the diet could appeal. But I wouldn't recommend the diet to my worst enemy.

On the other hand, the MDs I interviewed are more encouraging. Both Burkhart and Mittal say they recommend ProLon often to patients. Mittal has done FMD twice herself. "I have seen it bring real improvements to the health of many of my patients," Burkhart says. "It may be something to consider if someone is looking to improve health and longevity, reduce inflammation, and support weight loss."

Turoff, on the other hand, was slightly more hesitant to recommend the diet, concerned that it could contribute to disordered eating. "Much of ProLon's advertising — especially its influencer marketing — frames it as a quick weight loss fix, which could contribute to unhealthy dieting behaviors," she says.

ProLon can be a useful tool for promoting metabolic and longevity benefits. However, it's not a magic bullet, she says. "Long-term health and weight management come down to consistent, sustainable habits rather than periodic extreme interventions," Turoff says.

María Cristina Lalonde is a freelance journalist and aspiring kickboxing champion. She has written about sexual health, fitness, and wellness for over a decade. Her articles have appeared in, among other publications, the Houston Chronicle, Everyday Health, Giddy, and U.S. News & World Report.