Fasting is an age-old practice, done for religious, political or weight loss goals. Prolonged fasting, especially the do-it-yourself kind, can be hazardous to your health according to experts. But now researchers have discovered that a new, modified fast, called the fast mimicking diet, or FMD, could help slash disease risk and even slow down aging.

A preliminary study suggests the FMD may reduce dangerous belly fat, rejuvenate the immune system, lower the risk of cancer, diabetes and heart disease and improve memory.

Even better news for people who like to chew regularly, the FMD is just followed once in a while. It's typically done once a month for five days. On the other days, you eat normally. Depending on what shape you're in and what your disease risk factors are, you might be advised to go on the FMD every few months or just once a year, says Valter Longo, PhD, the Edna M. Jones Professor of Biogerontology at the University of Southern California Davis School of Gerontology. Longo also directs USC's Longevity Institute.

His research suggests the diet pushes your body into what he describes as a ''rebooting, regenerating, reprogramming period." Longo and his colleagues recently reported the results of their FMD study in the journal Cell Metabolism.

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FMD: A closer look

During the five-day FMD, dieters cut their normal calorie intake between 34 and 54 percent. So if you usually chow down on about 2,000 calories, that number will decrease to somewhere between 680 and 1,080. You're instructed to get a specific percentage of your calories from the three major macronutrients, as follows:

  • 9 to 10 percent from protein
  • 34 to 47 percent from carbohydrate
  • 44 to 56 percent from fat

In his study, Longo randomly assigned 19 men and women to the FMD group and another 19 to a control group (they simply continued their normal diet). The FMD group did the modified fast once a month for five days three times.

The people on the FMD saw a 3 percent reduction in body weight and some belly fat reduction. The researchers also found that risk factors or biomarkers for diabetes, cardiovascular disease, cancer and aging improved. For instance, blood sugar levels were reduced by more than 11 percent and stayed nearly 6 percent lower than when they started the study even after the FMD ended. Inflammation, which is linked to most chronic disease you can think of, was lowered, too.

Prior to this study, Longo studied the diet in animals. In mice, the FMD improved cognitive functioning, decreased bone loss and cancer incidence and lengthened lifespan.

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How it works

The diet basically ''fools the system into thinking or sensing a starvation mode while it's not [actually] starving," Longo says. “The starvation mode is causing the body to destroy what we suspect are bad cells, whether they are in the liver, muscle, nervous system or blood system. During the time of low to very low caloric intake, the system shrinks, basically. It tries to save energy. To do that, it has to get rid of cells, billions of cells. Again, we don't have proof they are the bad ones, we still have to prove that. But we hypothesize that the bad ones are destroyed."

Next, the body rebuilds. "You have to make new cells, and the new ones…work better," says Longo. This process leads to a rejuvenation of the immune system, Longo says.

FMD in perspective

"In the last decade, scientists have been taking another look at intermittent fasting and found some health benefits," says Benjamin Horne, PhD, a researcher at Intermountain Medical Center Heart Institute in Murray, Utah. He has researched intermittent fasting’s effects on diabetes and heart disease risk. "We found people who have been fasting routinely, historically through their lives, had about a 40 to 45 percent lower risk of coronary artery disease and about the same, or a little more, [reduction] for diabetes risk."

While more research is needed, Horne says the Longo study is valuable partly because it had a control group to compare the fasting intervention to. "Very few studies have done that," he says.

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What's next?

The researchers are studying more people on the FMD. Based on those results, Longo says he hopes to design specific, individualized ways to use the diet in people with different health problems, such as diabetes or heart disease.

FMD should be done only under medical supervision, Longo says. People can do it at home, but they need to be monitored, he notes. Restrictions include no strenuous exercise and avoiding high temperatures during the five-day program.

How easy it is to adhere to the FMD is one question experts raise. "The concern with a prolonged fasting program would be the sustainability of the diet,” says Crissy Kaleekal, MS, RD, LD, director of nutritional service at The University of Kansas Hospital. “While it may work in a research setting, it may be very difficult for the average person to follow and still get the same important nutrients that the body needs."

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Kathleen Doheny is a Los Angeles journalist specializing in health, behavior and fitness topics.