What if adhering to a diet for a combined 15 days a year could wind back your internal clock nearly three years? The fasting-mimicking diet (FMD) claims to do just that. According to a new study, it may help lower your biological age and reduce your risk of age-related diseases—without drastically changing your eating habits.

As its name suggests, the FMD aims to mirror the effects of a water-only fast. Each cycle involves five days following the plant-based diet—high in unsaturated fats and low in protein, calories, and carbohydrates—and 25 days of your typical eating habits. In a study led by researchers at the University of Southern California Leonard Davis School of Gerontology, people who completed three monthly FMD cycles showed a 2 ½-year decrease in median biological age, independent of weight loss. The findings were published this month in the journal Nature Communications.

“[FMD] is making the system younger and by making it younger, making it more functional,” senior author Valter Longo, Ph.D., director of the USC Longevity Institute, tells Fortune. “It’s really repairing the problem from the source, rather than just putting a Band-Aid downstream of it.”

What is the fasting-mimicking diet?

Longo says he developed the FMD some 20 years ago as a standardized diet for people with cancer that could be used in clinical trials. Now marketed as ProLon, the five-day program sells proprietary FMD kits online for $195 ($175 for subscribers). Each contains:

  • Energy bars
  • Energy drinks
  • Herbal teas
  • Multivitamin supplements
  • Snacks
  • Vegetable-based soups

Study participants strictly adhered to these prepackaged items. Longo recommends following three FMD cycles for a total of 15 diet days, which he says is more achievable for people who may have difficulty changing their eating habits long term.

“It doesn’t require lifestyle changes,” he says. “You can look at it more as a medicine. It comes in a box, lasts five days, and then you go back to whatever it is that your lifestyle is.”

Both Longo and USC have disclosed financial interests in L-Nutra, the company that manufactures ProLon.

Mixed race man holding fork and knife at table
As its name suggests, the fasting-mimicking diet (FMD) aims to mirror the effects of a water-only fast.

JGI/Jamie Grill via Getty Images

How was the fasting-mimicking diet tested?

The research involved secondary analyses of two of Longo’s previously published, randomized clinical trials: a 2017 study in Science Translational Medicine and a 2023 study in npj Metabolic Health and Disease.

In the first trial, conducted at the USC Diabetes and Obesity Research Institute, 100 adults ages 18 to 70 were recruited in the Los Angeles area. Participants were “generally healthy” and hadn’t had any diagnosed medical conditions in the previous six months. Researchers noted Hispanics were underrepresented in the cohort compared to the greater LA population. About half of participants were assigned three FMD cycles, while the rest were asked to eat normally. Each cohort was 63% female.

The second trial, conducted at the Hypertension Institute in Nashville, Tenn., consisted of 84 adults ages 35 to 75. Participants had a body mass index of at least 28—25 to 29.9 is overweight and 30 or above is obese—and a diagnosis of either endothelial dysfunction or low small resistance artery compliance. The FMD cohort completed four diet cycles and was 73% female. The other cohort, which was 50% female, followed the Mediterranean diet.

How might the fasting-mimicking diet lower biological age?

While your chronological age is how long you’ve been alive, your biological age reflects how old your cells are and how well your body is functioning. Those two numbers don’t necessarily match up, and previous research suggests biological age is the better predictor of mortality risk.

Longo and his team used a combination of seven biomarkers, including systolic blood pressure and total cholesterol, to measure biological age. Across both trials, 86 people followed the FMD; after three months, their median biological age dropped by just over 2 ½ years. In the first trial, however, the majority of participants already had estimated biological ages younger than their chronological ages prior to starting the FMD.

While not everyone who followed the FMD showed a reduced biological age, the percentage of participants whose biological age increased was smaller than that of the control group.

Aging Process Of Banana On White Background
Each cycle of the fasting-mimicking diet (FMD) involves five days following the plant-based diet—high in unsaturated fats and low in protein, calories, and carbohydrates—and 25 days of your typical eating habits.

Getty Images

Study links fasting-mimicking diet to rejuvenated immune system

Aging and age-related chronic diseases are the result of cumulative cellular and molecular change and damage, Longo and his colleagues noted in their study. That’s why they suggest health interventions such as the FMD shouldn’t be limited to older adults.

In addition to reduced biological age, FMD participants showed:

  • Lower diabetes risk factors
    • Less insulin resistance
    • Lower hemoglobin A1C
  • Lower metabolic syndrome risk factors
    • Decreased abdominal fat 
    • Decreased liver fat
  • Rejuvenated immune system
    • Increased lymphoid-to-myeloid ratio, an indicator of immune system age and function

These results are “not groundbreaking,” according to Sander Kersten, Ph.D., director of the Division of Nutritional Sciences at Cornell University

“In a previous study by these authors, it was shown that the subjects lost body weight and body fat,” Kersten tells Fortune via email. “It is very expected that insulin resistance and liver fat improve accordingly. This is typically what we see when people lose weight.”

Longo and his team did acknowledge the small sample sizes for some of these measurements. Abdominal fat, for example, was analyzed in 15 study participants.

Even so, this research marks an exciting step in the science of aging, Longo tells Fortune, adding that additional FMD trials are in the works.

“Physicians and health care professionals like to recommend drugs, but I think this provides an alternative,” Longo says.

ProLon critic: FMD ‘not a complete diet at all’

Like Kersten, Joanne Slavin, Ph.D., a professor in the Department of Food Science and Nutrition at the University of Minnesota, was skeptical of the study results.

“This idea that they have put together this thing that is similar to starvation—but it’s healthy and it’s giving you a longer life—is just really misleading,” she tells Fortune. “It’s not a complete diet at all.”

ProLon soups range from 110 to 120 calories, and snacks from 35 (olives) to 260 (nut-based energy bar). The energy drinks are 20 calories. 

Though ProLon has been used in dozens of human trials, Slavin, a registered dietitian, notes this latest USC study had more of a gerontological than nutritional approach. She says she doesn’t support fasting other than for religious reasons, be it intermittent fasting or a diet that mimics a fasting state.

“I want to make sure everybody gets nutrients every day,” Slavin says. “Fasting as a mechanism to prevent diseases or improve health is suspect right away in my way of thinking.”

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