What is an FMD (Fasting Mimicking Diet)?
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What is an FMD (Fasting Mimicking Diet)?

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What is an FMD (Fasting Mimicking Diet)?

Posted 3 years ago

Luke Jones

Luke Jones


Tony O’Neill, PT, DPT, MSc, RDN

Tony O’Neill, PT, DPT, MSc, RDN

Author and Scientific Reviewer

Expert Approved

The Fasting Mimicking Diet (FMD for short, not to be confused with the Fast Metabolism Diet)  is the brainchild of Dr. Valter Longo, an Italian-American Cell Biologist and Biogerontologist.

Longo is known for his widespread research on the role of fasting on cellular protection, aging, and diseases. He proposes that longevity is regulated by similar genes and mechanisms across all animals, plants, and fungi and that fasting can potentially play a role in supporting a long, healthy lifespan.

With this in mind, the FMD is a 5-day dietary protocol that supplies the body with essential nutrients but is low in total calories, protein, and sugar. It’s thought that through controlled caloric restriction, typically 34-54% of normal caloric intake [*], the body enters a somewhat fasted state. 

It’s proposed that the FMD may provide some of the benefits of extended water-only fasting, but without the associated risks. 

We’ll touch on these in more detail later in the article, but fasting has been linked with:

  • Weight loss
  • Cardiovascular health
  • Improved blood sugar 
  • Supporting cancer treatment

Calories on a Fasting Mimicking Diet 

The typical recommended caloric intake throughout the 5-day period is as follows:

  • Day 1: 1,090 kcal (10% protein, 56% fat, 34% carbs)
  • Days 2-5: 725 kcal (9% protein, 44% fat, 47% carbs).

It's important to note that the FMD is not designed to be a long-term diet plan. 

It’s meant to be performed anywhere from once a month for obese patients looking to lose weight for health reasons (or those looking to manage chronic health conditions), and 2-3 times per year for health-conscious individuals looking to reap the potential longevity benefits. 

If you are interested in experimenting with the FMD, it’s important you consult a medical practitioner to determine if it is suitable for you.

What Do You Eat on an FMD?

The FMD is based around simple, whole foods that are low in protein and sugars.

Although you can perform a DIY fasting mimicking diet at home (more on this later), Dr. Longo founded a company called ProLon that sells the specific FMD protocol.[*]

Through ProLon, you can order a pre-packaged kit containing all the plant-based meals and snacks you’ll need over the course of the 5-day protocol.

A typical day of eating on the FMD looks like:

  • Breakfast: Nut butter bar & herbal tea
  • Lunch: Small serving of soup with kale crackers
  • Snack: A few olives
  • Dinner: Another small bowl of soup
  • Supplements: Glycerin drink with multivitamins + DHA supplement

Who is the FMD Suitable For?

The Fasting Mimicking Diet is designed for:

  • Obese and overweight individuals looking to lose body fat. 
  • People looking to stay on top of their health and potentially enhance healthspan.
  • Those with certain chronic health conditions (although approval from a medical professional is needed)

The FMD is not suitable for: 

  • Pregnant or lactating women.
  • Children.
  • Anyone with a history of eating disorders.
  • Those with certain health conditions (unless cleared by a medical professional) - particularly blood pressure issues, diabetes, cardiovascular disease, and certain forms of cancer.  

What Occurs During a 5 Day Fasting Mimicking Diet?

According to the ProLon website [*], the body goes through the following stages during a 5-day FMD:

  • Day 1 - The body begins to transition into a fasting state, upregulating fat burning and preparing to initiate cellular clean-up. 
  • Day 2 - Fat burning increases and cellular clean-up (autophagy) is enhanced. 
  • Day 3 - Cellular clean up continues, and many participants begin to reach Ketosis, now burning body fat as their primary fuel source. 
  • Day 4 - Cellular cleaning and renewal continues, along with fat burning.
  • Day 5 - The fast is completed, and re-feeding beyond day 5 is meant to ‘fuel overall wellness

What Are the Potential Benefits of FMD?

1. May Increase Weight Loss & Reduce Belly Fat

During a small study completed by Dr. Longo, the fasting group lost on average 6 pounds, and a greater proportion of belly fat compared to the control after three cycles of the ProLon Fasting Mimicking Diet[*]

The results are encouraging, however it’s important to note that this does not necessarily mean that the FMD is more effective than plain old calorie restriction or a standard Ketogenic diet where weight loss is concerned.

2. May Improve Cardiovascular Health Markers 

During the same study, subjects showed improvements in blood pressure and cholesterol levels over the three FMD cycles. This was particularly the case for those who started with higher levels to begin with.[*]

Heart disease and stroke are the world’s leading causes of death[*], so these improvements could prove promising with further research. 

3. May Support Healthy Blood Sugar Levels

It’s estimated that 415 million (1 in 11) people across the world are currently living with diabetes, a figure that’s set to rise to 642 million by 2040.[*]

Early research suggests that the FMD could potentially play a positive role where diabetes is concerned. Those who began with high blood sugar levels in the ProLon study saw their numbers drop into the normal range.[*]

There have also been positive results regarding the regeneration of β-cells - a type of cell found in the pancreas responsible for synthesizing insulin. Cycles of FMD have been shown to reverse β-cell failure and rescue mice from Type 1 and 2 Diabetes. The same study also observed β-cell regeneration in human Type 1 diabetic pancreatic cells.[*]

4. May Support Cancer Treatment

There’s also some promising research highlighting the potential use of the FMD in helping to manage certain types of cancer. 

One study suggests that the FMD can be as effective as water-only fasting in sensitizing breast cancer tumors to the effects of chemotherapy.[*] Animal studies have also shown a reduced cancer incidence in mice, as well as improved cognitive performance resulting from FMD cycles.[*]

Again, more research is needed in this area. 

5. May Reduce Inflammation & Help Treat Autoimmune Conditions

A 2016 study demonstrated that the FMD was able to reduce pro-inflammatory cytokines, increase corticosterone levels, and suppress autoimmunity.

Researchers concluded that: 

“Suggesting that an FMD or a chronic ketogenic diet are safe, feasible, and potentially effective in the treatment of relapsing-remitting multiple sclerosis (RRMS) patients.”[*

In another study, a series of 4-day FMD cycles helped to modulate the gut microbiome and promote intestinal regeneration, reducing inflammatory bowel disease pathology.[*] The FMD also helped to rejuvenate the immune system in an animal study.[*]

6. May Support Longevity

We know that short-term calorie restriction can lead to increased autophagy in nerve cells in mice.[*] Autophagy is the process of cleaning up old, defunct cells and replacing them with new, healthier ones. 

But what about in human studies?  

There’s evidence to suggest beneficial changes in risk factors of age-related diseases like cancer, diabetes, and cardiovascular disease in humans [*], which may well have an impact on health-span - the number of years one is able to enjoy with good health.  

7. May Help you Enter Ketosis More Easily

An intermittent fasting protocol like the FMD can be a nice gateway into a Ketogenic diet. 

It’s a way to get some of the potential benefits of fasting while also helping the body become fat-adapted. 

From there you can more easily transition into a healthy, high-fat diet. It’s less of a jarring transition compared to going from the standard western diet to full-blown Keto overnight. 

Learn more: Intermittent Fasting and Keto

8. May Help You Avoid the Pitfalls of Extended Water-Fasting

Although supervised water-only fasting is generally considered a fairly safe treatment option in certain situations, it does come with potential health risks, including pain, fatigue, insomnia, dehydration, and electrolyte imbalances.[*]

Because you still consume a small number of calories over the 5-day FMD protocol, it's proposed that you should be able to reap some of the rewards of water-only fasting, without the same level of risk, and without necessarily having medical supervision.

The bottom line: The results so far around the FMD are promising, but more long-term, large-scale human studies are needed before we can draw more definite conclusions about the benefits of the diet.

Potential Downsides of FMD:

1. Cost

Subscribing to the ProLon kit will set you back $240 for each cycle of the FMD. This can become costly, particularly if it's something you're looking to do every few months. A DIY FMD diet is an option - we’ll take a closer look in the next section.

2. Contraindications

You should get medical clearance before experimenting with the FMD. As we touched on above, the protocol is not suitable for pregnant or lactating women, children, those with a history of eating disorders, and anyone with certain health conditions.

3.  Difficulty

While the FMD is considered easier to adhere to than water-only fasting, it does take some planning and willpower to stick to. Some people report feeling fatigued during the 5-days, while others can go about their normal life without any trouble. It’s worth considering how the diet will impact your routine, and to schedule in the protocol during a quiet week if possible.

DIY Fasting Mimicking Diet: Can you do a Keto FMD?

Although the ProLon diet kit itself is technically not Keto-friendly, you can create your own DIY Keto FMD.

If you already follow a Ketogenic diet, the good news is that you’ll likely have a slight advantage heading into the FMD. You'll already be fat adapted, so may find you have an easier time burning body fat as a fuel early on in the fasting cycle.

As with the traditional FMD, you should aim to keep protein intake low, but decrease carbs and up your fat consumption.

You could follow a similar format to the ProLon plan, only swapping out the nut bars + soups, and replacing them with Keto-friendly foods, such as:

  • Bone broth soup
  • Whole nuts & seeds
  • Nut butters
  • Avocado
  • Coconut

The macros will shift away from the usual protocol, but you should still aim to consume a similar caloric intake (1090 kcal on Day 1, 725 kcal on Days 2-5).


There is some promising research emerging around the FMD in a number of areas. 

While more studies are needed, early evidence suggests it may help with weight loss goals and support overall wellbeing.

With some tweaking, it can also be combined nicely with a Keto diet. Just remember to consult your physician if this is something you are interested in experimenting with.