Are you wondering which fasting approach is right for you? Different fasting protocols exist, such as the 16:8 fast and 20-hour fast, and they’re all linked with weight loss, reduced blood pressure, and increased autophagy. Maybe you’ve heard of the Fasting Mimicking diet (or FMD for short), an option for people who cannot tolerate a no-calorie fast.
This article covers how the FMD diet works, who should do it, potential side effects, and commonly asked questions on the topic.
The Fasting Mimicking Diet is a modified fast that varies from traditional fasting because it allows for some calories, but you’ll still reap the benefits of a calorie-free fast. It’s essentially low-calorie, low-protein, low-sugar, moderate-carb, and moderate-fat.
The FMD diet runs for 5 days, and it’s actually a program that was first developed by Dr. Valter Longo, the Director of the Longevity Institute at the University of Southern California (*).
The ProLon Diet — Dr. Longo’s program and the first fast mimicking diet on the market — is not a keto-friendly plan because it allows for moderate carbs, but it can still lead to ketosis and fat-burning.
If you’re on a keto diet and want to try the FMD approach, you can tweak it a bit by lowering carbs and increasing fat while keeping protein low (as with the standard FMD plan).
For those who are not keto and would rather try the fasting mimicking diet without ordering ProLon’s 5-day fasting nutrition program, you’re also free to do it. Just keep in mind to prepare your food in advance based on the recommended calorie intake.
Throughout a 5-day period, you are only allowed to consume about 34-54% of your usual calorie intake (*).
As for the fasting mimicking diet calories:
On day 1, you eat 1,090 kcal with the following macros: 10% protein, 56% fat, and 34% carbs.
For days 2-5, you drop your calorie intake to 725 kcal with the following macros: 9% protein, 44% fat, and 47% carbs.
See the table below for your reference:
Another thing to note is that all your foods (breakfast, lunch, dinner, and snacks) should be plant-based. For those who subscribe to the ProLon’s kit, you can expect to receive:
- Nut bars (e.g., macadamia, coconut, honey)
- Dietary supplements, such as vitamins, minerals, and omega-3 fatty acids
- A fruit-flavored drink
- Herbals teas with no caffeine
- Easy-to-prepare soups
- Snacks, such as olives and kale crackers
While Dr. Longo’s 5-day FMD protocol discourages replacing the above-mentioned foods and drinks with other options, some may prefer doing this if they dislike the taste.
After the 5th day, you’re going to resume your normal diet.
A fasting mimicking diet offers similar advantages to regular intermittent fasting. The ProLon website mentions various key benefits, from cell renewal to fat loss. Let’s discuss each below:
1. Cell renewal or autophagy
Autophagy is expected to happen on the third day of a fasting mimicking diet as stated on the ProLon website’s FAQs. This is after you’ve depleted your glycogen stores and entered a metabolic state of ketosis (* , *).
Autophagy is your body’s “cleansing system” in which damaged cell parts that contribute to chronic diseases are removed and replaced (*). In other words, autophagy plays a vital role in keeping you healthy.
You may not feel autophagy physically, but you may notice symptoms like reduced appetite and weight loss.
2. Reduced blood pressure
A FMD diet was introduced to a small cohort of patients with prostate cancer.
Post-intervention, many of their metabolic factors improved, including their blood pressure. Their systolic BP was reduced by 9.52 mmHg while their diastolic BP was reduced by 4.48 mmHg (*).
In another study, participants ages 20-70, who were all generally healthy, were placed on a five-day fasting mimicking diet for 3 months.
The results showed that their systolic BP dropped by 4.5 mmHg while their diastolic BP dropped by 3.1 mmHg. Additionally, their IGF-1 (or insulin-like growth factor 1) decreased, helping to reduce cancer risk (* , *).
3. Enhanced mental clarity and mood
For those seeking the mental benefits of fasting, the FMD is capable of improving brain function through autophagy (*).
According to a mini-review article in Frontiers in Psychiatry, restricting your calories short-term induces autophagy, which has anti-depressant effects in humans (*).
Another study focusing on intermittent fasting found that one week of fasting significantly increases spatial planning and memory (*).
4. Increased fat loss
Since the FMD puts your body in a fasted state, it promotes weight loss — a common reason people follow Dr. Longo’s program.
Although keep in mind that intermittent fasting strategies like the 16:8 diet also result in weight loss, followed by a sustained fat loss if you make fasting (plus eating healthily, exercising, and other healthy habits) part of your lifestyle.
In a 2017 randomized controlled trial, Dr. Longo and other researchers divided 100 generally healthy participants into two groups. One group followed a diet with no restrictions for 3 months, while the other group followed a fasting mimicking diet for 5 days per month over the course of 3 months (*).
Based on the results, the FMD group experienced a reduction in their body weight and total body fat (*).
5. May change your hunger response
A common misconception about fasting is that it makes people extremely hungry, resulting in bingeing.
The reality is that fasting leads to temporary hunger, but this happens only a few hours after your last meal — let’s say 4-8 hours. This hunger wave will peak but it eventually disappears the longer you fast.
Ultimately, fasting decreases ghrelin (the hunger hormone) and your desire to eat, while increasing your feeling of fullness (*).
The difference is that with the fasting mimicking diet, you’re able to eat meals and snacks for 5 days, only that they’re limited in calories.
6. Lessens the risk of prolonged water-only fasting
Doing a water-only fast (no food and drinks except for water) for more than 48 hours is generally well-tolerated and safe under medical supervision (*).
However, a lot of people who are new to extended fasting may experience fatigue, electrolyte imbalances, nausea, and headache.
The good news is that you get to avoid these potential issues with the fasting mimicking diet since you’ll still consume a small number of calories each day.
As with traditional calorie restriction and fasting strategies, the fasting mimicking diet can result in short-term side effects. These include increased hunger, cravings, tiredness, headache, brain fog, and low blood sugar.
Minimize these side effects by avoiding any strenuous workout during your 5-day fast. Pairing fasting and exercise is fine as long as you keep them light. Walking, gentle yoga, and stretching are great examples.
The ProLon program also mentions (on their FAQs page) that people may experience bloating, cramping, gassiness, constipation, and diarrhea. These symptoms could result from any sudden change in your diet, including adding high amounts of fiber too quickly (*).
If you are taking medications for lowering blood sugar and blood pressure, speak with your doctor before you do a fast mimicking diet.
The FMD diet is an option for those who cannot tolerate or choose not to undergo extended fasts without some food.
It’s for individuals who love the idea of a plant-based diet or are already following it since the FMD protocol involves consuming plant-based foods and beverages.
Last but not least is anyone who is eager to achieve weight loss and other FMD benefits discussed earlier in this article, such as autophagy, better mood, and lowered blood pressure.
In a nutshell, yes, it’s safe for most people — although it’s important to discuss the FMD with your healthcare provider if you have any concerns. This is especially true if you have a medical condition or you’re taking medications.
For those who subscribe to the ProLon program, stop consuming your plant-based products right away if you experience allergy symptoms.
Some plant-based ingredients (nuts, for example) may trigger allergies. Watch out for edema (or swelling), skin redness, tingling of the mouth, runny nose, digestive symptoms, and trouble breathing (*).
The fasting mimicking approach focuses on low-calorie, low-protein, low-sugar, moderate-carb, and moderate-fat options that are plant-based. Within a 5-day period, you consume the following:
- Soups: quinoa, mushroom, tomato, butternut squash, etc.
- Nut bars: almonds, macadamia nuts, pecans
- Snacks: olives with herbs, olives with sea salt, almond and kale crackers, etc.
- Supplements: vegetable powder with vitamins and minerals, algal oil (oil from marine algae) which has omega-3 fatty acids
- Drinks: hibiscus tea, spearmint tea, spearmint lemon tea, and L-drinks (ProLon offers this as fruit mixes with vegetable glycerin, which is supposed to protect your lean mass while in a fasted state).
Some people may not find these foods and drinks appealing, including those embracing a meat-based diet. Alternatively, you could start a ketogenic diet, which also mimics a fasting state, or try other types of fasts.
If you decide to proceed with the FMD plan but do not want ProLon’s 5-day kit, you can go the do-it-yourself route. You could swap some of the options from the list above with homemade versions. Feel free to replace some of the veggies and grains with those you can tolerate.
Preparation is key to a successful fast. Before starting the FMD, talk to your healthcare provider and ask if there are changes to be made to your medications.
Since the fasting mimicking diet involves meals with specific calories and macros, prepare these 5 days’ worth of meals in advance so you’ll simply heat them or open them.
It’s also a good idea to ease your way into a 5-day fasting mimicking diet by slightly reducing your calorie intake (especially calories from carbohydrates) days before the fast. This will help you get used to fewer carbs in general and avoid blood sugar swings during the fast.
Consider doing lab tests before and after your fast to note any differences. You might also want to check blood glucose, ketone levels, and changes in your weight.
Here’s what your meals would look like on the FMD plan. Feel free to make substitutions based on what you’re not allergic to and what’s available. The general rule is to keep them plant-based, unprocessed or minimally processed, and sugar-free.
Day 1 — 1,090 kcal
Breakfast: nut butter bar and herbal tea (spearmint tea, hibiscus tea, or ginger tea)
Lunch: tomato soup and kale crackers
Afternoon snack: nut butter bar
Dinner: cauliflower and spinach soup
Day 2-5 — 725 kcal
Breakfast: nut butter bar and herbal tea (spearmint tea, hibiscus tea, or ginger tea)
Lunch: broccoli and asparagus soup
Afternoon snack: nut butter bar
Dinner: mushroom soup and green olives
Note that you can follow the standard 10% protein, 56% fat, 34% carbs on day 1 and 9% protein, 44% fat, 47% carbs on day 2 if you’re not on the keto diet.
For keto dieters, lower your carbs and slightly increase your fat without necessarily affecting the recommended calories in a day.
The fasting mimicking diet is meant to be practiced for only 5 days once a month, for 3 months to experience its weight loss and health benefits.
When you’re not fasting, it’s important to resume your usual calorie intake, but make sure to meet these calories from nutrient-dense foods.
You could also start eating a diet that’s lower in carbohydrates and higher in healthy fats so that your next FMD is easier. An added benefit is that this helps you continue losing body fat without having to restrict your calories.
Who Should Not Do a Fasting Mimicking Diet?
This fasting plan should be avoided by pregnant and breastfeeding women, children, and those who are underweight and have a history of an eating disorder.
Remember to discuss it with your doctor if you’re taking any medications for blood sugar and blood pressure.
Here are frequently asked questions on the fasting mimicking diet:
How much weight can I lose on a fasting mimicking diet?
It’s possible for you to lose as much as 5 pounds doing the FMD once a month for 3 months.
However, everyone loses weight at different rates even when they consume the same number of calories due to other factors, such as the overall quality of their diet, sleep, age, and current health condition.
What’s the difference between fasting mimicking and intermittent fasting?
The main difference between these approaches is that fasting mimicking involves eating foods that are low in calories throughout the day, while intermittent fasting alternates between periods of eating and not eating.
Furthermore, fasting mimicking is plant-based while intermittent fasting foods (during your feeding window) can include a wide variety of options, such as meat, seafood, eggs, avocados, and berries.
Does a fasting mimicking diet put you in ketosis?
Yes, it can induce a metabolic state of ketosis since it’s a prolonged fast. The same is true for regular short-term intermittent fasts, especially when you start and break these fasts with meals that are low in carbohydrates.
Can you drink coffee on a fasting mimicking diet?
Black sugar-free coffee is allowed on fasts; however, you need to limit yourself to only one cup of 8-12 ounces of black coffee on a fasting mimicking diet. You may sweeten your coffee with stevia or monk fruit while avoiding creamers.
The fasting mimicking diet is a powerful tool to experience the benefits of fasting without completely giving up food. Although this is done only once a month, for up to 3 months, it’s best paired with a healthy diet in between, such as keto.
As mentioned in this guide, this method leads to weight loss, autophagy, lower blood pressure, and many other benefits of fasting in general. You might want to consider other types of fasts as well if you can comfortably go without food for hours.
Overall, the fasting mimicking diet is safe and tolerable, but consult a healthcare provider if you have concerns and you’re currently on medication.
Myers T et al. Potential Effects of Prolonged Water-Only Fasting Followed by a Whole-Plant-Food Diet on Salty and Sweet Taste Sensitivity and Perceived Intensity, Food Liking, and Dietary Intake. 2022 May 3