What is the Fast-Mimicking Diet (FMD) and What Are Scientists Saying About it?


Imagine if you could diet for just five days a month and see results. Results that extend far beyond weight loss and promise to “rejuvenate” your body at a cellular level. This is the basis of the fast-mimicking diet, also known as FMD.

Intermittent fasting has skyrocketed in popularity in recent years, with programs like the 5:2 diet and time restricted eating receiving endorsements from celebrities and nutritionists around the world. All of these different diet plans are characterized by a cycle of fasting and feeding on a regular, cyclical schedule. But unlike most other fasting diets, which rotate on a daily or weekly basis, the FMD revolves around a monthly cycle.

“It’s really another form of intermittent fasting but it’s unique in the sense in the way it is organized,” Adam Collins, associate professor of nutrition at the University of Surrey in the U.K., told Newsweek.

What is the Fast-Mimicking Diet (FMD)?

The FMD diet, designed by gerontology professor Valter Longo, consists of five days of restricted eating, where calorie consumption is kept at around 700 calories per day. “The FMD […] is a low calorie nutrition program that last 5 days with a specific low sugar, low protein, high fat plant based composition, designed to match or surpass the effects of water only fasting, without the safety and compliance issues,” Longo, director of the Longevity Institute of the University of Southern California, told Newsweek.

“For people without [specific] diseases, it is designed to be used for only 5 days for 2 to 3 times per year (10 to 15 days per year [in total]) and otherwise not impose a lifestyle change.”

What Scientists Say About the FMD Diet
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Like any energy-restricting diet, people do tend to lose weight on FMD. But weight loss isn’t the only benefit of this type of cyclic fasting. In 2018, Collins and colleagues at the University of Surrey investigated how the health benefits of intermittent fasting compared to other restrictive dieting strategies.

“We had one group doing the 5:2 diet and then we had another group doing traditional continuous energy restriction,” Collins said. “So they had the same overall energy restriction if you average it out across the week. Then what we did was we waited until they had lost 5 percent of their body weight and brought them back into the lab.”

Interestingly, those on the 5:2 diet didn’t lose the weight any faster than those undertaking more traditional energy restriction. But what the researchers did see was a significant difference in various metabolic markers between the two groups.

“Our metabolism is designed to operate in a cycle of feeding and fasting. Your body is able to cope with these changes and adjust accordingly. You’re going to start tapping into energy reserves, and that essentially boils down to us shifting from predominantly using carbohydrates for energy to now using mainly our body fat. So that fast-feed cycling is having metabolic effects.

“From a cellular point of view, if you’re changing the availability of energy, your cells are going to have to change their priorities. So you’re going to change from synthesizing and building new molecules to breaking down and recycling old ones instead. And this process, called autophagy, is where all of the longevity stuff comes in, because all that breaking down and recycling is like spring cleaning for your cells.”

The difference between the 5:2—which involves restricted eating twice a week—and FMD is the window over which fasting takes place. “[FMD] is a way to fast where you’re not doing it every week but when you do it, you’re doing it for a cumulative period of time,” Collins said.

What Results Are Scientists Seeing From the FMD?

Preliminary studies have already suggested that FMD can reduce inflammation, improve cognition, reduce our risk of cancer and heart disease, and possibly event protect against Alzheimer’s. Most significantly, in a paper published by Longo’s team in the journal Nature Communications on February 20, it was shown to potentially reverse biological aging.

“In [our] studies the FMD was shown to decrease liver and abdominal fat without reducing muscle mass, it reduced A1C [a measure of average blood sugar levels], it caused changes in the immune system cells consistent with a rejuvenation of the immune cell profile and it caused a 2.5 year median biological age decrease in both of the clinical trials which enrolled a total of 184 participants,” Longo said.

Health meal diet
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It is worth noting here that two thirds of the study participants were overweight or obese at baseline so some of these positive results may have resulted from simple weight loss among these participants. “It seems it was most beneficial for the obese, as would be expected, and those with higher systolic blood pressure etc at baseline,” David Clancy, a lecturer in biogerontology at Lancaster University in the U.K., told Newsweek.

Even so, the beneficial effects persisted for several months after the last FMD cycle.

However, as encouraging as these results are, 184 is a fairly small sample size, and most of our prior understanding of this topic has come from research in animals. “While preliminary findings are encouraging, more research is needed, particularly more human studies with meaningful sample sizes,” Caroline Susie, a dietitian and spokesperson for the Academy of Nutrition and Dietetics in the U.S., told Newsweek.

With all that in mind, who is best suited to this diet?

Who is Best Suited For a FMD?

“Those who are overweight or obese, those with high blood pressure or high cholesterol, those with fatty liver, those with pre-diabetes or high A1C, those with high CRP or other inflammation markers,” Longo said.

Clancy added: “It’s not unreasonable to think that, during ages 40 to 60 at least, this regime twice per year may add 3 to 4 years of healthy life, maybe more, in those with higher BMI, blood pressure, blood sugar etc. This is the age range where we accumulate problems that will harm us later.

“That said, a sensible exercise habit could achieve the same, or even better, and is maintainable into older age. Starving oneself in older age is risky because immune responses to infections need ready resources.”

The elderly are not the only ones who should be cautious of restrictive dieting. “There are concerns about this dietary approach for those living with certain medical conditions,” Susie said. “This diet is not appropriate for women who are pregnant or breastfeeding; people with a history of disordered eating are not appropriate for FMD; people who are currently underweight and or already living with vitamin/mineral deficiencies are not appropriate. Additionally, if you are living with cancer, diabetes, or congestive heart failure, FMD is not appropriate.

“Always check with your doctor before you start any diet or make any changes to your diet.”

The FMD program is commercially available under the brand name ProLon.

Is there a health problem that’s worrying you? Do you have a question about fasting? Let us know via [email protected]. We can ask experts for advice, and your story could be featured on Newsweek.