Alternate-day fasting diets, like the 5:2 diet, have become a popular way to quickly lose weight. The 5:2 diet made its way into the spotlight in 2013 when BBC aired a documentary entitled Eat, Fast & Live Longer. In this program, journalist Michael Mosley investigated the health benefits of fasting. Before attempting the various and attainable fasting methods himself, Mosley met with a series of doctors and industry professionals who assessed his current health condition. Mosley wanted to understand how to best protect himself against the negative effects of aging. From his story on alternate-day fasting, Mosley derived the 5:2 diet, which subsequently took the UK by storm.

The belief that fasting can improve your health shares similarities with the Paleo diet. Like Paleo dieters, Mosley looked to our ancestors for help when investigating fasting. When hunters and gatherers had a successful kill, they gorged themselves on the meat. This feast might last a few days and certainly was not restricted— however, if the hunters went days without a kill, they would be starved, surviving on minimal food and nutrients. Thus, our bodies are capable of functioning when we are underfed. But, bear in mind, our hunting and gathering ancestors put themselves in great peril, even wrestling mammoths to provide a feast. That is a lot of physical activity that we do not necessarily get today.

Throughout Mosley’s investigation, he interviewed a handful of researchers and specialists, one of them being Mark Mattson, an expert on the aging brain. Mattson, Chief of the Laboratory of Neurosciences at the National Institute on Aging and Professor at John Hopkins University, discussed the laboratory studies he performed regarding starvation. Based on the tests he had been conducting on mice, Mattson identified positive aspects of fasting. In one of these studies, Mattson found that when mice were given an unhealthy diet high in saturated fats and sugars, mice health declined much more rapidly, roughly 3-4 months sooner. On the other hand, mice given a diet lower in fat and subjected to intermittent fasting lived roughly 6 months longer. Thus, the mice maintaining a smaller size proved to live longer.

5:2 dieters argue that our bodies are not made to handle the modernization of food and that giving the digestive system frequent “breaks” helps to mend any issues with digestion.

Additionally, in his meeting with Mark Mattson, Michael Mosley learned that sporadic bouts of hunger help stimulate new neurons to grow in our brains. Mattson also looked to our mammoth-hunting ancestors to answer the question regarding cell growth. From a survival standpoint, hunger provides a survival advantage as it causes you to be more focused. Fasting’s effect on the brain is actually compared to exercising’s effect on your muscles…well, for mice anyway. In order to truly prove that these findings hold true for humans, human trials must be performed.

So how did this research and studies like it lead to Mosley’s famed 5:2 diet?

As Mosley attempted intermittent fasting, he realized how difficult this task is. Anyone can attest that we need food, and regularly! To accommodate this need, Mosley met with Dr. Krista Varady, author of The Every Other Day Diet and an advocate of alternate-day fasting. Like the 5:2 program, the “Every Other Day Diet” instructs participants to limit their caloric intake to 500 calories on fasting days. Although they are very similar in practice, on the “EODD” you are fasting slightly more than on the 5:2 diet. For example, one week you will fast 3 days and the next you will fast 4, then the following you are back to 3 days of fasting, and so on…

During Dr. Varady’s clinical studies of alternate-day fasting, researchers found participants decreased their levels of LDL cholesterol (bad cholesterol), triglycerides (fat), and blood pressure. Surprisingly, these scientists found it actually didn’t matter if you were eating a high-fat diet versus a low-fat diet on the given feast days—the LDL cholesterol and blood pressure were relatively the same for all participants.

Because they consumed 25% of their energy needs on fasting days, Dr. Varady predicted that most participants would consume 175% of their energy needs on a “feed” day. But, throughout the course of her study, participants were only consuming 110% of their energy needs on the feed days. Inevitably, there is a -65% consumption deficit.  This tells us that starving a few days a week and then feasting on cookies, pasta, pizza, and cheeseburgers will probably help you lose weight because you are reducing your overall caloric intake. However, your body will be missing proper nutrients. Additionally, if you are exercising regularly, your energy levels may be negatively affected by the significant decrease in calories on the fast days.

While the 5:2 diet and similar programs are not sensible dieting practices, the science behind fasting is worth a second look.

While we dismiss the 5:2 diet and similar programs, like the “Every Other Day Diet”, as viable dieting practices, we acknowledge that the science behind fasting and Mark Mattson’s research is worth a second look. Scientists have found that restricting caloric intake can help to regulate your body’s blood sugar levels. Research in mice has discovered that by reducing daily caloric intake, the body lowers its production of hormone IGF-1. A drop in the creation of this hormone is known to help your body go into repair mode—meaning, the body begins to protect itself against carcinogens, heart disease, diabetes, and other health issues.