The 5:2 diet is another way of intermittent fasting, but instead of breaking fasting periods up by time blocks, it has guidelines surrounding entire days. With the 5:2 diet, followers eat normally for five days of the week and for the other two days, they cap calorie intake at 25 percent (typically between 500 and 600 calories). This is an extreme style of intermittent fasting, and if you’re considering giving it a try, registered dietitians say it’s important to think carefully about how this will truly affect both your body and mind.

What Is the 5:2 Diet?

As mentioned, the 5:2 diet is a type of intermittent fasting where followers eat normally for five days of the week and restrict their calorie intake to 25 percent (typically between 500 and 600 calories) for the other two days of the week. It’s recommended to split up the two fasting days and not have them back-to-back. According to the scientific journal PLoS One, this eating style was made popular after a 2012 documentary by BBC Horizon and a book that was released shortly after called The Fast Diet. There aren’t many scientific studies on the 5:2 diet, but the study published in PLOS One found that it was successful for short-term weight loss, but not long-term weight loss.
“One of the selling points of this diet is that for five days a week, you can essentially eat whatever you want,” says Dr. Dara Ford, PhD, RDN, a registered dietitian and health studies lecturer at American University. Dr. Ford explains that there are no guidelines for these five days; someone following this diet can eat however many calories and whatever they want. As for those other two days? Once again there are no specific guidelines about what someone can or can’t eat; the only rule is that they have to cap their calorie intake at 25 percent of what they would normally consume.

What Dietitians Think of the 5:2 Diet

Simply put, Dr. Ford is not a fan of this eating plan. “Eating just 500 or 600 calories a day is not sufficient enough to meet your micronutrient needs,” she says, adding that this is especially dangerous if someone is physically active on those days.  David Gaviria, MPH, RD, LDN, a registered dietitian and doctoral student at the University of North Carolina Chapel Hill Department of Nutrition, agrees. “What also happens when calorie intake is this low is that the body goes into starvation mode. That means that when you can eat normally again, you will likely want to eat everything in sight and your body will make an effort to hold on to those calories because it thinks you’re starving,” he says. In the end, Gaviria says this is likely going to negate any weight loss benefits you may have gotten from the two-day-per-week fasting. Gaviria points out that research shows that while some do achieve short-term weight loss by following the 5:2 diet, it’s not any more successful than if they cut calories more moderately throughout the week. For example, he says that if someone eats 400 calories less each day as opposed to 1,500 calories less two days a week, the result will be the same. “It will be a lot more sustainable too,” he adds. Registered dietitian Jess Cording, RDN, and the author of The Little Book of Game Changers, says that she isn’t surprised that there is no evidence that the 5:2 diet can help with long-term weight loss. This is because, she says, it’s not sustainable. “The key to long-term weight loss is small changes, not very intense ones,” she says. She adds that this diet can be especially harmful for people with diabetes who must carefully manage their blood sugar levels or for people who are pregnant. Besides negatively affecting someone physically, Gaviria says the 5:2 diet can impact someone mentally too. On the two fasting days each week, the calorie intake is so low that he says it would be completely normal to feel fatigued, hungry and irritated. Cording also points out that it would be difficult to enjoy a dinner out with friends or even at home with your family while keeping your entire calories for the day under 500. “Eating is about more than just nutrients; it’s about enjoyment too,” she says. The intense nature of the 5:2 diet is dangerous in another way: Dr. Ford says that it can put someone at risk for disordered eating. Instead of any eating plan that drastically reduces calories—even if it’s just for two days a week—she agrees with Cording that small, sustainable changes are key for losing weight in a healthy way. Instead of obsessing over calories, all three dietitians say to focus on eating nutrient-rich foods. That means getting adequate protein, fiber and healthy fats at every meal. Eating a balanced diet will ensure you’re getting the vitamins and nutrients your body needs and will be more satiating than nutrient-poor foods that end up being high in calories and do little to fill you up. While some people may find some types of intermittent fasting beneficial, all three dietitians say that they aren’t fans of this particular style. Not only is the science lacking, but you’re bound to be downright miserable for two days every week. This is one eating plan that you can cross off your list.  Next up, see a list of 40 foods that help burn belly fat.

Sources

Dr. Dara Ford, PhD, RDN, registered dietitian and health studies lecturer at American UniversityDavid Gaviria, MPH, RD, LDN, registered dietitian and doctoral student at the University of North Carolina Chapel Hill Department of NutritionJess Cording, RDN, registered dietitian and the author of The Little Book of Game Changers