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A new study looks at the benefits of a fasting-like diet. andresr/Getty Images
  • Five-day cycles each month of a fasting-like diet improved markers of prediabetes and reduced fat in the liver.
  • People following this fasting-mimicking diet also showed signs of reduced immune system aging and biological age.
  • The diet involves eating a lower-calorie, higher-fat diet five days in a row each month, with people following their normal diet the rest of the time.

Monthly cycles of a diet that mimics fasting can improve insulin resistance and other markers of prediabetes, and reduce fat in the liver, a new study shows. Researchers say this diet also reduced signs of immune system aging and biological age.

The fasting-mimicking diet (FMD) involves five-day cycles of a diet low in overall calories, protein and carbohydrates; and high in unsaturated fats, such as the kind found in olive oil, nuts and seeds.

The diet was developed by study author Valter Longo, PhD, a professor at the USC Leonard Davis School of Gerontology in Los Angeles.

“This is the first study to show that a food-based intervention that does not require chronic dietary or other lifestyle changes can make people biologically younger, based on both changes in risk factors for aging and disease and on a validated method developed by the [Morgan E. Levine] group to assess biological age,” Longo said in a news release.

The new study was published Feb. 20 in Nature Communication. Previous research led by Longo has also found that the FMD can lower risk factors for cancer, diabetes and cardiovascular disease in people.

The FMD is a primarily plant-based diet designed to produce fasting-like effects on blood levels of glucose, ketone bodies and other molecules, while still providing calories and nutrients and making it potentially safer and easier to complete compared to conventional fasting.

Day one of the FMD provides around 1,100 calories, with 11% coming from protein, 46% fat and 43% carbohydrates. Days two to five provide around 720 calories per day, with 9% protein, 44% fat and 47% carbohydrates.

On average, adult females need 1,800 to 2,400 calories a day to maintain weight, while adult males need 2,400 to 3,000 calories a day. This varies based on activity level and other factors.

Research has shown that eating 15% to 20% fewer calories per day — known as calorie restriction — can improve risk factors for diabetes and cardiovascular disease in people.

However, doing this every day for months or years is challenging.

One advantage of the FMD is that it is done five days in a row during the month, with people eating a normal diet for the rest of the month. In the new study, participants did three to four cycles of the FMD.

In the study, researchers provided participants with all the food they needed, with the food for each day individually boxed to avoid people eating meals and snacks on the wrong day.

The food boxes included vegetable-based soups, energy bars, energy drinks, chip snacks, tea and a supplement that provided minerals, vitamins and essential fatty acids.

Longo is the founder of ProLon, the company that provided food for the study.

The FMD is not the only fasting-light option out there. Versions of intermittent fasting and time-restricted eating also provide some of the benefits of extended fasting, without requiring a complete change to someone’s eating habits.

Research on these types of eating patterns is ongoing, with some studies showing strong benefits and others showing fewer or none.

In the new study, researchers analyzed blood samples from participants from two previously conducted clinical trial populations, aiming to uncover new insights. Participants who were randomized to the FMD in each trial practiced the FMD for five days each month, followed by their normal diet for 25 days.

After 3 months, people on the FMD diet had fewer risk factors for diabetes, including less insulin resistance and lower HbA1c levels, researchers found. HbA1c is a measure of long-term blood glucose control.

“Although the number of examined study participants is small, taken together with our previous study, these results indicate that the FMD can reduce insulin resistance and help restore normal glucose tolerance in individuals that had fasting glucose levels indicative of pre-diabetes,” the authors wrote.

People following the FMD also had a decrease in abdominal fat and fat within the liver, as measured with magnetic resonance imaging (MRI). Both of these are linked to a lower risk of metabolic syndrome.

Metabolic syndrome is a group of five risk factors that, if untreated, increase the chance of developing heart disease, diabetes and stroke.

“These [measures] are things that we use as parameters of people’s overall health,” said Dr. David Cutler, family medicine physician at Providence Saint John’s Health Center in Santa Monica, Calif.

In addition, researchers found that people in the FMD group had signs of a more youthful immune system, and a reduced biological age, as indicated by how well the cells and tissues are functioning.

The measure of biological age researchers used in this study was based on a number of markers for functioning of the metabolism, heart and circulation, lungs, liver, immune system and inflammation, and blood cells.

Based on this, most participants in the study had biological ages younger than their chronological ages, which suggests they “may be healthier than the average American,” the authors wrote.

After three cycles of the FMD, the average biological age of participants had decreased by nearly 2.5 years, the findings showed.

The study had several limitations, including the small number of participants, with only 86 people following the FMD in total between the two clinical trials studied.

As a result, “you have to be very careful about drawing any kind of long-term conclusions based on a study of 86 people,” Cutler told Healthline.

“We’ve seen this many times before,” he said, “where you take a small population and you see very dramatic results. But when you try to apply it to the broader population, those results disappear.”

Another limitation is that people in the study generally had more “advantageous” social, economic, behavioral and health characteristics, the researchers pointed out, so the results may not apply to other groups of people.

Longer-term studies with larger numbers of people are needed, including studies that directly compare the FMD to other dietary interventions such as other types of fasting or diets such as the Mediterranean diet.

In addition, long-term studies are needed to understand whether the immediate benefits seen in the new study translate to a lower risk of heart attack, stroke, functioning, dying early, or other outcomes.

And whether the benefits — including the apparent decrease in biological age — continue to accumulate after years of following the FMD, or if there’s a plateauing.

Cutler, who often advises patients to eat mostly vegetables and eat less overall, questions whether the effort put into the FMD is worth the benefits seen in the study.

“How much do you want to devote — in terms of time, money and sacrificing things you like — to achieve these ends?” he said.

“I’m not saying don’t do it. I’m just saying, you have to temper your enthusiasm for something when you’re not really certain if it’s going to have significant long-term beneficial effects,” he added.

Researchers studied the effects of a fasting-mimicking diet (FMD) for three to four months in two clinical trial groups. The FMD involves eating a lower-calorie, higher-fat diet for five days in a row each month.

Those following the FMD had improved markers of prediabetes, lower abdominal and liver fat, and signs of reduced immune system and biological aging.

Longer-term studies are needed to know if these benefits continue to accumulate or if they plateau, and if they lead to reductions in cardiovascular disease, diabetes, dying early or other outcomes.