What the Heck Is Intermittent Fasting?

These days, the term intermittent fasting is ubiquitous: Bulletproof founder Dave Asprey is chatting about it on Dr. Mark Hyman’s popular podcast, “The Doctor’s Farmacy;” Halle Berry is giving the practice (along with the keto diet) some credit for her enviable physique and her ability to age like Benjamin Buttons; and other celebs, like Jimmy Kimmel and Hugh Jackman, have said the practice helped to transform their bodies.

So, um, what exactly is intermittent fasting? The answer, along with what experts and science say about the practice, below.

So, what is intermittent fasting, anyway?

Intermittent fasting is exactly what it sounds like: fasting for periods of time. There are no hard and fast rules, but there are different approaches to the practice: 5:2, made popular by the book The Fast Diet, promotes eating a calorically normal diet for five days of the week and fasting for two nonconsecutive days of the week, consuming just 25 percent of your normal intake on those days.

Then, there’s the 16:8 approach, which recommends regularly fasting for 16 hours of the day, consuming all of your meals within an eight-hour window. An example of this would be eating all of your meals between the hours of noon and 8 p.m. and fasting for those other 16 hours, a good chunk of them coinciding with your sleep schedule.

Why are people doing it?

As the New York Times notes, fasting as a means of improving health has been around for thousands of years. That said, science studying intermittent fasting (IF) is relatively new and limited. Anecdotally, the claims are that fasting can aid in weight loss and maintenance of weight; can improve mental clarity and focus; and can boost longevity.

When it comes to hard science, research does back up the fact that fasting as a means of weight loss can be effective, but whether it’s more effective than other calorie-restricted diets remains iffy: One recent meta-analysis of clinical trials on IF found that intermittent fasting can be effective for weight loss, but not more effective than other traditional approaches, like counting calories.

But then there’s that whole aging thing. (Have you seen Halle Berry?) Another reason IF has become so popular in recent years is the claim that it improves longevity. This, too, has some science behind it. As Mark Mattson, chief of the Laboratory of Neurosciences at the National Institute on Aging and a professor of neuroscience at the Johns Hopkins School of Medicine, told TIME, based on his research, it seems that IF helps the body to fight stress and inflammation at the cellular level.

The idea is this: Fasting puts some stress on the body, causing your cells to adapt, and, as Mattson told TIME, “these cycles of challenge, recovery, challenge, recovery seem to optimize both function and durability of most cell sites.”

So, wait, should I take up intermittent fasting?

Before you throw away all your breakfast foods and declare Sundays “fast days,” just know that many experts—even experts with research in the ring—say intermittent fasting still needs to be studied more to determine how effective it truly is as a weight-loss approach. As researchers told TIME, most of the research on IF has been done with overweight and obese individuals, so it’s unclear how effective the strategy would be for someone who’s just trying to drop a few pounds.

Another issue with IF is that it’s difficult to maintain, which is likely one reason it scored so low on U.S. New & World Report’s annual “Best Diets” rankings. After all, if you do the 16:8 approach, you can pretty much say goodbye to happy hour with friends and dinner parties. Then, there’s the fact that there’s little-to-no focus on nutritional quality, and instead a large focus on calories, which can backfire on you—if the calories you are consuming aren’t nutrient-dense, what good is that doing you, right?

As Libby Mills, a registered dietitian and spokesperson for the Academy of Nutrition and Dietetics, told Health, if you’re considering trying IF, it’s important “to consider how you personally are affected by restriction.” For example, anyone who’s been prone to disordered eating in the past—whether that involves bingeing or extreme restriction—might want to be cautious of this approach. Most experts also suggest chatting with your doc before diving in.

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