Fasting is not safe for everyone, but some studies, mostly on cells and animals, suggest it may have health benefits, such as lowering the risk of certain cancers, type 2 diabetes, and cardiovascular disease.

Fasting or restricting calories is thought to induce autophagy, the body's process of clearing out old, damaged cells to make space for new ones. It begins when cells are stressed or deprived of nutrients and naturally decreases with age, meaning it plays an important role in slowing aging, the Cleveland Clinic said.

In the study, around half of the 100 participants were told to follow the fasting-mimicking diet, or FMD, for five days a month for three months and to eat their regular diet for the other 25 days. The other group just ate their normal diet. Participants were men and women between 18 and 70.

Blood tests and MRI scans of the participants showed those in the FMD group were less likely to develop diabetes and had lower levels of abdominal and liver fat, which are associated with a reduced risk of metabolic syndromes, such as liver disease, cardiovascular disease, and diabetes, the study said. It also increased their lymphoid-to-myeloid ratio — indicating a more youthful immune system. These factors can be used to measure biological age, the study's authors said in a paper published on January 20 in Nature Communications.

While chronological age refers to how long someone has been alive, biological age measures how well cells and tissues function. But experts still debate the term as younger cells may not necessarily indicate better health, and there is no "normal" way our cells should look at any given age, BI previously reported.

Participants could eat foods including soup and energy bars while 'fasting'

The FMD is a low-calorie diet high in unsaturated fats and low in protein and carbohydrates. During the five-day-fast period, participants consumed vegetable-based soups, energy bars, energy drinks, chip snacks, tea, and a supplement providing high levels of minerals, vitamins, and essential fatty acids, provided by L-Nutra, a nutrition-tech company founded by Longo. On day one of the FMD, participants ate around 1,100 calories, and around 720 on days two to five. They could eat at any time throughout the day.

Longo said in a press release: "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 Levine group to assess biological age."

The authors noted that the study involved only 100 participants, most of whom were healthier than the average person in the US population, meaning the results might not relate to wider populations.

"It would need to be rolled out to lots more people in different demographic and health groups to really understand how beneficial fasting is for everyone," Clare Bryant, a professor who researches innate immunity at Cambridge University and was not involved in the study, told Business Insider.

Previous studies have linked the FMD to stem-cell regeneration and reducing the side effects of chemotherapy in studies on mice.

David Clancy, a lecturer who ​​studies the biology of aging at Lancaster University and was not involved in the study, told BI: "It's not unreasonable to think that, during ages 40 to 60 at least, this regime twice per year may add three to four years of healthy life, maybe more, in those with higher BMI, blood pressure, blood sugar, etc."

But he said the diet is "pretty harsh" and that people may struggle to get through the working day toward the end. "Scheduling days four and five for weekend days would be sensible," he said.

Clancy said cultivating sensible exercise habits could achieve the same or even better results and is maintainable into older age.