What Is a Fasting-Mimicking Diet?

Fasting has become a hot topic in the world of science and nutrition.

A growing body of research conducted has suggested that fasting — or abstaining from eating food — for certain periods of time might pay significant health dividends, such as improved weight management, lowered risk of developing Type 2 diabetes and a reduced risk of cancer.

Fasting Diet Overview

So, how do you take advantage of this growing practice? Let’s walk through everything you need to know, including:

— What a five-day fasting-mimicking diet is.

— How it works.

— How a fasting-mimicking diet compares with intermittent fasting.

— Whether you can lose weight on a fasting-mimicking diet.

— How much it costs.

— How to follow a five-day fasting-mimicking diet.

— 5-day meal-plan examples.

— Which foods and snacks should be on your shopping list.

— Health benefits.

— Health risks.

What Is a 5-Day Fasting-Mimicking Diet?

There are several different ways to approach fasting, including the five-day fasting-mimicking diet. Also referred to as FMD, fasting-mimicking is “a low-calorie diet designed to mimic fasting without fasting,” explains Dana Ellis Hunnes, a senior clinical dietitian at UCLA Medical Center and assistant professor at the UCLA Fielding School of Public Health in Los Angeles.

The approach effectively “tricks” your body into thinking you’re fasting when you’re actually still taking in some food. The eating period lasts five days, a time span that’s been associated with the benefits of fasting.

[Read: Intermittent Fasting: Foods to Eat and Avoid.]

How Does the Fasting-Mimicking Diet Work?

The fasting-mimicking diet relies on a specific macro- and micronutrient breakdown:

Day 1: You consume 1,100 calories. Of those calories, 11% are from plant-based protein sources, 46% from mono- and polyunsaturated fats and 43% from complex carbohydrates that are rich in fiber.

Days 2 to 5: You’ll consume just 725 calories per day, with a macronutrient breakdown of 9% protein, 44% fat and 47% carbohydrates.

During each of the five days, it’s recommended that you consume a minimum of 70 ounces of water. This fast period should be repeated once per month for the first three consecutive months to achieve optimal results, says Kristine Dilley, a registered dietitian at the Ohio State University Wexner Medical Center in Columbus.

[See: 11 Healthy, Low-Calorie Snacks.]

Intermittent Fasting vs. Fasting-Mimicking Diet

The fasting-mimicking diet is a little different from the popular intermittent fasting, or IF, approach, which can take a number of different forms. The 16:8 method, for instance, consists of a daily 8-hour window when you can eat, followed by a 16-hour rest period when you don’t consume anything. Another approach to IF includes fasting for one or two nonconsecutive days per week.

Regardless of the approach, intermittent fasting dictates when you can and can’t eat, but it doesn’t typically prescribe what you can and can’t eat.

With FMD on the other hand, you’re allowed a limited intake of calories from specific food sources at certain times, so you won’t be completely fasting. This is how the two diets differ — one includes some food, while the other removes all of it for certain lengths of time.

[Read: Calorie Reduction vs Fasting.]

Can You Lose Weight?

Because you’ll be strictly limiting calories for a period of five days, it’s likely you will lose weight on the fasting-mimicking diet.

One study on humans indicated that three cycles of a five-day fasting-mimicking diet per month were effective in reducing body weight, waist circumference and BMI, systolic blood pressure and IGF-1, a marker associated with aging,” Dilley explains.

These effects were found to be sustainable for up to three months after completing at least three monthly cycles, she adds.

But this frequency can be onerous and expensive, and it may not be suitable for all.

“You need to assess whether you could comply with a restrictive diet on an ongoing basis and whether the diet is financially feasible,” Dilley says.

Doing a single round of the fasting-mimicking diet is also unlikely to make a difference.

“One five-day plan in combination with the rest of your month or year eating ultra-processed foods and not following a health-promoting lifestyle will not do anything,” says Megan Wroe, a wellness manager with Providence St. Jude Medical Center in Southern California. “Lifestyle changes are required in order for this program to do any good, whether you try it monthly or just a couple times per year.”

A fasting-mimicking diet is not intended to be a daily regimen but to support and enhance a healthful lifestyle, she adds.

How Much Does the Fasting-Mimicking Diet Cost?

Anytime you change your diet, you may be looking at changes to your food shopping budget. Below, we’ve set out the differences between paying for the Prolon diet — a commercially available form of the diet — and a DIY approach that follows similar rules.

Prolon diet

The Prolon Fasting Mimicking Diet was pioneered by Dr. Valter Longo, a biogerontologist and cell biologist at the University of Southern California. Prolon has undergone 32 clinical trials with 18 universities around the globe, including USC’s Longevity Institute, and it has more than 130 issued patents.

Prolon features a meal program you can buy and easily follow. It includes all the foods you need in specifically formulated quantities and ratios for five days.

“(The) Prolon prepackaged meal kit consists of plant-based whole foods and does not include the use of any meat or dairy foods, gluten, GMOs or processed foods,” Dilley explains.

It also contains a supplemental energy drink made from vegetable glycerine as well as a plant-based omega-3 supplement from algal oil.

Currently, Prolon offers several product subscriptions and bundles. The basic five-day program currently sells for $175 when set up as an every-four-months subscription or $195 when ordered just once.

“This is slightly higher than many other diet plans, but it is also not intended for long-term use,” Wroe notes, adding that “the ingredients are incredibly high quality, which is something that cannot be said of most other premade diet foods.”

Prolon recommends users follow the plan once per month for anywhere from one to six consecutive months, depending on your health goals and current health status. After this initial onboarding period, the company claims you can maintain the cellular rejuvenation and healthy aging benefits (more on that below) by following the program just three times a year thereafter.

DIY approach

Because the Prolon program can be a financial commitment, some are trying a do-it-yourself approach to the fasting-mimicking diet. Theoretically, this is possible, but Melanie G. Murphy Richter, a registered dietitian nutritionist and director of communications for L-Nutra, the company that owns Prolon, warns it’s not a good idea.

“As much as I’d love to say it’s possible to DIY, there is no way to ensure that a person’s composition (of nutrients) is precise,” Richter says. “Our FMD is based on 25 years of diligent research and specially formulated ingredients. It’s not something that can be perfectly replicated, and I think it’s important to call that out.”

Wroe also cautions that on your own, there’s a greater risk of failing to meet your nutrient needs. It may also be more difficult to safely enter ketosis — when the body burns fat for fuel — because if your ratios of macronutrients aren’t exactly right, the diet may not be as effective as if they were precise. These two potential problems are less likely to develop when using the tested and prepackaged program, she says.

How to Follow the Fasting-Mimicking Diet

Despite those warnings, if you’re still determined to try a fasting-mimicking diet on your own, the rules are fairly straightforward. You’ll be tricking your body into thinking you’re fasting by removing most of the calories you’d typically eat each day.

The rules

The general idea on the fasting-mimicking diet is to consume fewer calories on fasting days and to adhere to a ratio of macronutrients that, as mentioned, are roughly 10% protein, 45% fat and 45% carbohydrates.

In general, you’ll be eating:

— Clear broths and vegetable soups.

— Small amounts of nuts and olive oil for good fats.

— Vegetables, including leafy greens, broccoli, cucumbers and carrots.

— Small servings of berries.

— Limited amounts of beans, lentils and legumes, such as chickpeas or hummus.

You’ll also need to restrict high-glycemic ingredients, Wroe says. High-glycemic foods are those that rapidly elevate blood sugar. Examples include:

— White rice, pasta, bread and other grains that have been stripped of their fiber and nutrients.

— Pastries, cookies, cakes and other baked goods.

— Foods with added sugar and sugar-sweetened beverages, such as soda and coffee drinks.

— Instant oatmeal, cornflakes and other prepared breakfast foods.

— White potatoes.


Keep in mind, as well, that the quality and types of proteins, carbs and fats matter, Richter says.

“The magic of Prolon and the FMD is that it has figured out not just the ratio, but the types of nutrients that will not trigger the body’s nutrient-sensing pathways to keep you in a fasted state,” she explains.

NSPs are your body’s way of keeping tabs on how much food you’re taking in and whether it needs to slow down metabolism if it senses you don’t have enough calories or nutrients coming in.

For instance, Richter points out that eating jerky, a processed meat stick product that clocks in at 90 calories and provides less than 10% of daily protein, offers the same nutrient profile as a legume, but “they don’t get digested the same. One will, in fact, trigger our NSPs even if the ratio is within range.”

5-day Meal Plan

Use the following meal plan to help guide your way on a DIY fasting-mimicking diet:

Day 1

Breakfast: 1 cup of herbal tea or black, decaf coffee (unsweetened) and a half cup of berries.

Lunch: Mixed salad with various leafy greens and cucumbers with a tablespoon of a light vinaigrette dressing.

Snack: 10 almonds.

Dinner: A cup of steamed or lightly sautéed vegetables, such as broccoli, cauliflower and carrots, with a tablespoon of olive oil.

Day 2

Breakfast: Herbal tea or black, decaf coffee (unsweetened) and a small piece of fruit, such as an apple or pear.

Lunch: A cup of clear vegetable broth with added diced vegetables.

Snack: Celery sticks with ¼ cup of hummus.

Dinner: A cup of low-salt, broth-based lentil or vegetable soup.

Day 3

Breakfast: Herbal tea or black, decaf coffee (unsweetened) and a half cup of berries.

Lunch: Half a cup of cooked quinoa or brown rice with a cup of steamed vegetables.

Snack: A piece of fruit, such as a small orange or a plum.

Dinner: 4 ounces of baked or grilled fish with a cup of mixed greens and 10 olives.

Day 4

Breakfast: Herbal tea or black, decaf coffee (unsweetened) and a small piece of fruit.

Lunch: Leafy green salad with a cup of assorted colorful vegetables and a tablespoon of a light vinaigrette dressing.

Snack: 10 almonds.

Dinner: A cup of stir-fried or roasted vegetables, such as broccoli, bell peppers or zucchini, with ¼ of a block of tofu.

Day 5

Breakfast: Herbal tea or black, decaf coffee (unsweetened) with a half cup of berries.

Lunch: A cup of clear vegetable broth with a cup of fresh, diced vegetables.

Snack: A medium cucumber, sliced, and a medium carrot sliced into sticks with ¼ cup of hummus.

Dinner: ½ cup of cooked black beans or kidney beans with a cup of sautéed spinach and a tablespoon of olive oil.

Sample 5-Day Fasting-Mimicking Diet Food List

Here is a handy food list to take to the grocery store with you when shopping for five days’ worth of FMD meals.

— Tofu or tempeh.

— Vegetable broth.

— Leafy greens (lettuce, spinach, kale, arugula).

— Carrots.

— Celery.

— Zucchini.

— Cucumber.


— Broccoli.


— Strawberries.

— Raspberries.


— Apples.

— Oranges.

— Plums.

— Almonds.

— Walnuts.

Olive oil.


— Black or kidney beans.

— Hummus.

Desserts and snacks

Traditional desserts aren’t really an option on the fasting-mimicking diet because of the diet’s calorie restrictions. But you can certainly use berries and other fruit as a kind of dessert while staying within the parameters of the plan.

You can also enjoy a daily snack, such as:

— Almonds.

— Walnuts.

— Pecans.

— Macadamia nuts.

— Hummus.

— Carrots or celery sticks.

— Kale crackers.

— Specially formulated nutrition bars that conform to the prescribed macronutrient profile.

Health Benefits of the Fasting-Mimicking Diet

The fasting-mimicking diet purports to carry many of the same health benefits as other fasting regimens.

These benefits include:

— Weight loss.

— Reduced belly fat.

— Decreased cholesterol levels.

— Decreased blood sugar levels.

— Decreased inflammation in the body.

The fasting-mimicking diet also claims to slow the aging process through improved cellular repair — a process called autophagy — and cellular regeneration, Dilley says.

“Autophagy is a process in which old, damaged cells are recycled to produce new, healthier ones,” she explains. “Intermittent fasting has been shown to optimize autophagy, which may protect against mental decline and slow cellular aging.”

Wroe adds that while the research isn’t conclusive, there is a theory that the diet can help the body better use chemotherapy, at least when it’s used ahead of time.

“It primes the immune system to work efficiently,” she says.

The Prolon diet also has health benefits, Richter adds: It can help target fat loss while protecting lean muscle. There have also been documented benefits related to cardiometabolic health and biological age score reduction associated with this plan.

Health Risks of the Fasting-Mimicking Diet

While there can be health benefits from the diet, it’s not entirely a risk-free endeavor.

For example, Hunnes notes that because the fasting-mimicking period only lasts five days, results will not be sustainable.

“For diet results to be sustainable, you need to be following a lifestyle diet or change for several months, if not years,” she says. “This is setting you up for yo-yo dieting and weight loss/regain.”

Dehydration is also one area of concern because you’ll be limiting the amount of food you take in, which can be a source of fluid. It’s important to follow the recommended fluid intake and be sure you’re drinking enough water, Dilley advises.

“There’s also a risk for possible adverse effects on individuals taking medications that may be affected by food and fluid or specific nutrient intakes,” she adds.

When you’re fasting or fasting-mimicking, you’re also depriving your body of calories. As mentioned, your nutrients not being in exactly the right balance could trigger your NSPs. That process could then slow your metabolic rate and lead to you “feeling starved,” Hunnes points out. This starvation effect could also impact mental health. As such, it’s always best to check with your doctor or a registered dietitian before making any major changes to your diet.

Who should use (or avoid) a fasting-mimicking diet?

There are some people who should not adopt a fasting-mimicking diet. These include:

Women who are pregnant or breastfeeding. Women need to take in more calories, not fewer, when pregnant or breastfeeding because these activities gobble up a lot of energy.

People with nutritional deficiencies or those who are underweight. Cutting out so much food can worsen existing nutritional deficiencies.

People with a history of eating disorders. Restricting your food intake can be triggering for people who have or have had an eating disorder and could lead to unhealthy behaviors. A 2022 study conducted among adolescents and young adults in Canada noted that intermittent fasting was associated with disordered eating behaviors.

Dilley adds that people with a preexisting medical condition, such as hypertension, diabetes, liver disease or another chronic condition, should consult a physician prior to starting this or any other new diet.

Bottom Line

While some research has suggested that a fasting-mimicking diet could offer health benefits, more studies are needed.

As mentioned, before undertaking a fasting-mimicking diet, it’s imperative that you talk with your health care provider or a registered dietitian to ensure it’s safe for you to follow this approach. Restricting your feeding periods could be problematic for many people, and fasting isn’t a good idea for people with certain medical conditions.

What’s more, the fasting-mimicking diet may be difficult to take on, Dilley says.

“All diets are only effective if they’re reasonable for you to follow and maintain,” she explains. “Some patients may benefit from a program that’s this structured and takes the guesswork out of planning their diet, while others may not be able to comply with the strict limitations of what’s allowed on the diet.”

FMD does allow for unrestricted eating during three weeks of the month, with just the single five-day fasting-mimicking period, which may be easier for some people to adhere to.

“However, the hefty price tag may be prohibitive for many to be able to stick with the diet long enough to show benefits,” Dilley says.

Hunnes agrees: “It seems to be another fad diet that most likely isn’t worth its high price tag.”

More from U.S. News

Healthy Weight Loss Snacks

Healthy Meals for Weight Loss

How to Lose Weight by Shifting Your Mindset

What Is a Fasting-Mimicking Diet? originally appeared on usnews.com

Update 04/25/24: This story was previously published at an earlier date and has been updated with new information.

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