The fasting mimicking diet is meant to evoke the physiological effects of intermittent fasting, but unlike IF, it calls for following (and paying for) a specific meal plan.

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Intermittent fasting (IF) has become increasingly popular in recent years. As a dietitian and health coach, it's one of the topics I get asked about the most. Is it good or bad? Will it help me lose weight? My doctor said I should try IF — does this count? etc.

The answer is (probably not surprisingly) more complex than a simple yes or no. And if you do decide it's right for you, there are multiple variations of intermittent fasting to choose from — including a specific, patented and research-backed plan called the ProLon Fasting Mimicking Diet. This five-day pre-packaged meal kit claims to support your mind and body, "with delicious food for energy, to fight cravings, and protect critical lean body mass while rejuvenating you from the inside out." But does it actually come with the same perks as intermittent fasting, is it worth the $$$, and should you try it?

First, Some Basics About Intermittent Fasting

Intermittent fasting is, essentially, an eating pattern where you designate hours or days with no (or minimal) intake of food, while still making sure to meet your essential nutrient needs. As mentioned, there are a variety of regimens, but some of the most commonly studied ones are alternate-day fasting, 5:2 intermittent fasting (fasting two days each week), and daily time-restricted feeding (where eating is limited to a specific window of time).

Most of the research done (thus far) on IF is in animals, but a growing body of research in humans has delved further into the potential benefits of IF and interventions that mimic fasting. Some of the conditions intermittent fasting has been shown to improve are diabetes, obesity, cardiovascular disease, and some cancers and neurological disorders, according to the National Institute on Aging. While animal studies have shown mixed results (but promise) in regard to longevity, human trials have mostly involved short-term interventions, so it's unclear yet whether intermittent fasting has long-term effects on health or lifespan in humans. (See: What to Know About IF and Weight Loss)

Researchers believe that these benefits can be attributed to metabolic switching, a mechanism in which the fasting triggers the body to switch to ketones stored in fat rather than glucose as its energy source. This may sound familiar if you're at all familiar with the keto diet, but in this case, rather than focusing on the specific ratio of macronutrients someone eats, it's looking at what happens in the body on a cellular level during periods of fasting, when it doesn't have fuel from food coming in. Yes, both approaches lead to ketosis, but the practices themselves are different (unless, of course, someone following a keto diet is also restricting the window of time during which they eat). With fasting, what researchers have been highlighting is that this ketogenesis then leads to changes in cell and organ function that can carry over into non-fasting periods and have an impact on health — for example, improved blood sugar management, anti-inflammatory effects, resilience against stress, and improved mental and physical performance over time. (More on that here: Why the Potential Intermittent Fasting Benefits Might Not Be Worth the Risks)

What Is a "Fasting Mimicking" Diet?

A "fasting mimicking" diet (FMD) is a specific intermittent fasting plan that restricts calories for a set period of time, but during which you don't stop eating entirely. Think of it as tricking your cells into thinking they're not being fed. This five-day weight loss program developed by Valter Longo, Ph.D., an Italian biologist and researcher, is meant to mimic fasting effects, physiologically. The program is called ProLon Fasting Mimicking Diet by L-Nutra, a nutrition technology company that Longo started, which creates meal and snack products that are to be eaten during this program.

"The program, which is based on 20 years of clinical research sponsored by the National Institutes of Health and conducted at USC, has been patented for its healthy aging effects," though many people turn to it for weight loss, explains dietitian Lauren Harris-Pincus, M.S., R.D.N., founder of NutritionStarringYOU and author of The Protein-Packed Breakfast Club.

How does it work, exactly? You follow the diet five days per month, then you resume your usual eating pattern the rest of the month. When you sign up, you receive five boxes of food (one for each day) with pre-packaged foods that are meant to be eaten in a very specific order. During those five days, you consume only what's included in the meal kit: soups, bars, drinks, olives, crackers, and supplements. Participants are encouraged to hydrate with water and decaffeinated tea. Substitutions aren't allowed, and the company also has an extensive FAQ page with info on things like coffee and considerations for allergies and intolerances.

The plant-based, whole food-derived products that make up this calorie-restricted meal plan are low in carbs and protein and high in healthy fats. Day one provides just shy of 1,100 calories, and 750 calories for the remaining days. If you're reading this and thinking, "holy crap, that's really low in calories" you're right, it's really low, and not something that would be particularly safe or sustainable for the long term — remember, this is a five-day plan that includes supplements to help participants cover their nutritional bases. The nutrient profile is meant to mimic the physiological response to traditional fasting approaches shown in studies by restricting calories and providing a carefully calculated ratio of protein, fat, and carbs over the course of those five days. The L-nutra website describes it as "stressing the body enough to go beyond fat burning associated with reduced calorie intake to cellular regeneration and rejuvenation." That's why L-nutra also recommends doing this diet under the supervision of a licensed healthcare professional, and discourages strenuous exercise during the five days, as it "may impact the outcome of the fast and increase potential complications." However, more gentle movement like "slow pace walking, stretching, mobility exercises and light yoga" are encouraged. (Related: What Fit Women Need to Know About Intermittent Fasting)

This plan is not intended to be a one-time diet; rather, it's recommend you follow it every one to six months for best results. People who are interested in fasting but don't want to do it consistently may find that level of time commitment more appealing when compared to a more traditional IF protocol.

There's also the money piece of the picture. The program costs $249 per box, or $225 when you purchase three or more boxes. People who are used to spending that much on takeout or dining out in a similar timeframe or those who can comfortably make that financial investment in a diet plan may not freak out about the cost. However, for many people, that's above and beyond a week's worth of groceries. Given that there are specific products involved in this diet, it would be very difficult to create a DIY fasting mimicking diet.

Is the Fasting Mimicking Diet Effective?

Maybe. There haven't been many studies on the ProLon Fasting Mimicking diet, specifically, though Longo did lead a small study on people who completed three cycles of the diet. When compared to a control group, they showed an average loss of 6lbs and a reduction in body fat, blood glucose, and blood pressure — all risk factors and markers associated with aging and age-related diseases. A study in mice reflected this as well. More research, especially in humans, is needed to determine whether it's any more effective than — or even as effective as — other IF approaches, and what the long term effects may be. It's also TBD how FMD and other fasting approaches may impact disease prevention and treatment for specific conditions.

While the science is intriguing, the "on/off" mentality that the five-day approach may foster could lead to it being used in a similar way to crash diets masquerading as healthy cleanses. While a lot of my fellow dietitians have been quick to write it off — or on the flip side, to jump enthusiastically onto the bandwagon — many want to learn more before taking a side, and to proceed with caution, especially when making recommendations for individuals for whom restricting timing, calories, and/or macronutrient ratios could be mentally or physically risky. (Read more: How Intermittent Fasting Can Impact Your Mind, According to Experts)

Harris-Pincus, who's tried the plan twice, says that, "while you do lose weight because the calories are low, that is an additional effect that comes along with [the reported] metabolic benefits and not the primary purpose. I lost five pounds both times, though some does come back when you go back to regular food." She says that she felt hungry on the plan and found it key to stay hydrated, but she did notice a reduction in cravings for sweet foods. "I'm interested in the metabolic benefits and will probably do it again," she says. (Related: Can You Really Speed Up Your Metabolism?)

Registered dietitian Andrea Chernus, R.D., was also intrigued and wanted to learn about the program firsthand. "It wasn't as difficult as I imagined it would be," she says. "During the fast I didn't feel tired except for one day when I exercised... Afterward though, I felt really fatigued. I did a body composition test before and after. I lost four pounds and most of it was water. Some was actually muscle and only a very little was fat loss. I didn't maintain the weight loss and that was probably because of the water loss." She also shares that she didn't find the food "particularly appetizing" and adds that even though she doesn't like olives, she ate them because the diet guidelines state not to make substitutions, something people with specific restrictions or preferences may find limiting.

Is the Fasting Mimicking Diet Safe?

There are a few safety concerns. Obviously, don't use these products if you're allergic to or intolerant of the ingredients. This plan is also not recommended for pregnant and breastfeeding women, since there's currently a lack of evidence on its safety in pregnant and lactating women, and it's also vital to focus on nourishment during those stages, when a mother's nutrient needs are higher.

The ProLon website also warns not to attempt to treat type 1 or type 2 diabetes with a fasting mimicking diet, even with the help of a doctor, as studies show that FMD combined with insulin can cause serious complications. (The company also provides a full list of conditions for which this program is contraindicated.)

From a mental health standpoint, if you have an active or past eating disorder, fasting is not recommended, as it may be triggering. "If you have tendencies or a history of disordered eating, I never recommend restrictive eating, even if only for a few days," says Harris-Pincus.

So...Is it a Good Idea?

That depends. From a research standpoint, there's still a lot to learn about IF and FMD. It's important to consider that real life and a controlled research setting can look very different — how might these principles be applied to everyday living so someone could achieve the desired effect in a safe and sustainable way?

It's also important to call out that this is a branded, product-driven diet, which, even though it's based in research, doesn't provide much education about how to eat off of the plan, which could make it difficult for someone to maintain results over time. Being dependent on these products for the long run may not feel sustainable, and subsisting for five days a month on packaged foods — and thus skipping out on fresh foods — may not be a good fit mentally or physically for a lot of people. For example, if you're prone to emotional eating or bingeing when you feel deprived, definitely consider how a prepackaged diet could trigger you. And if you have digestive issues that are aggravated by greatly changing up what you eat, you may need to think through the pros and cons and have a conversation with your doctor.

When deciding whether to try FMD (or any new eating plan, for that matter), think about your "why" and what you hope to get out of the experience — and when in doubt, talk to a registered dietitian about whether it's an appropriate approach for you.