The popularity of intermittent fasting seems to only be picking up steam. It's not totally a surprise given the only rules, if you will, are about when you can eat, rather than what you can eat. The 5:2 diet is one of the many styles of intermittent that people are trying out for weight loss and overall health.
While traditional diets tend to require calorie restriction around the clock, the 5:2 diet asks you to restrict for just two days a week.
That means, during two consecutive days of the week, say, Monday and Tuesday, you consume 25 percent of your daily calories—600 calories per day for men—and the other five days, you eat normally, without restriction.
Celebrities are no stranger to trying out wild diet trends: Twitter's Jack Dorsey is known for his intermittent fasting—if you can call it that—habit, and Jimmy Kimmel credited his 25-pound weight loss to the 5:2 diet.
For many, the thought of severe calorie restriction for even two days doesn't sound incredibly appetizing, but adopters of the 5:2 diet don't see it as suffering at all. In fact, they credit the diet for increased focus and concentration, and an overall healthier lifestyle. Believe it or not, there is some science and experts to back up these claims.
Carolyn Williams, Ph.D., R.D., author of Meals That Heal and co-host of the Happy Eating Podcast, says, “I am a huge proponent of intermittent fasting and have tried several approaches.” She prefers the 16:8 approach (fast for 16 hours each day and eat for eight hours) over the 5:2 approach.
“I love food too much and just don't like waking up knowing I only get 500 calories that day,” says Williams. But many people find success on the 5:2 diet, especially guys who don’t have the time or energy to cook. For them, not worrying about meals may actually be a relief.
What is the 5:2 diet?
There are different methods to intermittent fasting. The 5:2 diet dictates that for two consecutive days, you consume 25 percent of your normal caloric needs, about 600 calories per day for men. The other five days? Eat as you normally would.
This style of fasting is popular among people trying to lose weight because the drastic reduction in calories makes it effective. People are also drawn to the non-diet days, or "fed" days, because they can eat without restriction. In theory, at least.
While you don't have to count calories, downing downing burgers, fries, pizza, and beer probably won’t help with your weight loss goals. Experts still recommend eating healthy foods on the days you are able to eat your normal caloric intake. After all, if you double your normal calories on fed days, your chances of losing weight are not so great.
The benefits of the 5:2 diet
Made popular by the best-selling book, The FastDiet, the 5:2 diet is a favorite with the weight-loss crowd. While intermittent fasting has been shown to be an effective weight-loss strategy, the 5:2 method of intermittent fasting hasn't been extensively studied. That said a recent review of research found that intermittent fasting protocols that include a time-restricted feeding window or fully fasted days (days you're restricting calories by more than 60 percent) both resulted in weight loss.
Williams also notes that intermittent fasting may support the function of insulin, and she credits that for part of the reason weight loss occurs. “A break from eating allows the body to shift from feasting to fasting mode, which supports the proper functioning of insulin and glucose, and supports a healthy body weight and insulin sensitivity,” she says.
The available research supports these claims, showing intermittent fasting can help reduce HbA1c—a measurement of blood sugar levels over two to three months—in people with type 2 diabetes.
“This down time or break in calorie intake also allows for autophagy, the cellular waste removal and repair that may play a role in reducing the risk of chronic diseases,” Williams says. Although studies are small, there is some evidence to suggest restricting feeding windows increases autophagy genes that clear damaged cells from the body.
But remember: Most of the available research look at fasting over the course of hours, not days.
The risks of the 5:2 diet
Like most diets, which are inherently restrictive, there are downsides to the 5:2 diet. “If you have an eating disorder, a history of eating disorder, or a challenged relationship with food, 5:2 fasting could actually be really dangerous,” says Ginger Hultin, M.S., R.D.N., owner of ChampagneNutrition and author of Anti-Inflammatory Diet Meal Prep, and How to Eat to Beat Disease Cookbook.
Hultin also adds that it may not be appropriate for athletes or people who engage in regular intense physical activity because the body needs food—calories—to fuel its activity. Working out on fasted days, for example, may result in poor performance and overall fatigue.
Williams also notes that without paying particular attention to choosing nutrient-dense foods during your normal and fasting days, you are more likely to miss out on important nutrients. Over the long term, that could result in vitamin and mineral deficiencies.
Should you try the 5:2 diet?
As with all things health and nutrition, every person is different. What works for one guy may not work for another.
“5:2 fasting can work really well for some people, but it's definitely not for everyone,” Hultin says. “In my practice, I'll focus much more often on a simple overnight fast with my clients—usually 10 to 12 hours—and avoid the more intensive fasting schedules like 5:2 because I find it's better for long-term success and stability."
If you’re an experienced faster, you may find success from the 5:2 diet. But if you’re new to intermittent fasting, experts suggest starting with a simple 10- to 16-hour fast to see how your body responds. You might find that drastically cutting calories for two days per week is too restrictive to sustain over the long term.
In the end, the best eating plan is the one that works for you and encourages healthy eating habits over the course of a lifetime.