You've probably heard a lot about this dieting and weight-loss technique, but what is it exactly? Is it safe? And does it mean you can't eat?
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Scrolling through meal prep ideas on Instagram, chances are you've come across all sorts of meal plans that people follow and swear by—Whole30, keto, paleo, IIFYM. And now there's another eating style making that's generating a lot of buzz and, with it, a lot of questions. It's intermittent fasting (IF). But what exactly is intermittent fasting? How do you do it? And is it actually healthy?
Intermittent fasting isn't a diet.
IF doesn't have a meal plan in the sense that it's a prescribed diet of things you can and cannot eat. Rather, it's an eating schedule or pattern that dictates when you eat.
"Intermittent fasting is a means of cycling between periods of fasting and eating, following a specific and predetermined pattern," says Cara Harbstreet, M.S., R.D., of Street Smart Nutrition. "People may be drawn to this form of dieting because it does not specify what to eat." Plus, IF comes in many forms that you can modify depending on your schedule and needs.
"The amount of time that you spend eating and fasting can vary depending on which form of the diet you choose," says Karen Ansel, M.S., R.D.N., author of Healing Superfoods for Anti-Aging: Stay Younger, Live Longer. "Some may require that you fast for 16 hours out of the day and then eat during the remaining eight hours; others may recommend a 24-hour fast a couple of days a week; and others may simply require that you eat about 500 or 600 calories, two days a week and then eat as much and whatever you want on the others."
While the options for customization appeal to a lot of people, the lack of a menu or any food-related structure can be a struggle for others.
"One of the main drawbacks of intermittent fasting is that it doesn't provide any guidance relative to what you should be eating," says Ansel. "That means you could literally eat junk during your non-fasting periods, which isn't exactly a recipe for good health. If you do choose this kind of diet, it's key to make sure that you eat as healthfully as possible to make up for the nutrients you may be missing out on during fasting days."
The concept of fasting isn't new.
While the idea of having set eating windows isn't necessarily fresh, science on the potential health and weight-loss benefits mostly is—and it's pretty inconclusive.
"Fasting has been a part of human culture and religious practices for centuries," says Harbstreet. "Only recently, however, has research turned a focus to the potential health effects of fasting."
One study on mice linked intermittent fasting to lower insulin levels. Another rodent study suggested that IF could protect the heart from further injury after a heart attack. And rats who ate every other day for eight weeks lost weight over the course of another study.
But studies on humans are limited, as are studies that follow IF subjects for a long period of time. In 2016, researchers reviewed data from studies about intermittent fasting conducted on people and basically found that the effects were unclear or inconclusive. Not super helpful, and it leaves you wondering whether IF for weight loss works in the long run.
Intermittent fasting isn't for everyone.
This way of eating is definitely not the right option for certain people. If you have a condition that requires you to eat regularly—such as diabetes—IF could actually be dangerous. And the practice could also be harmful for people who have a history of disordered eating or obsessive behavior regarding food.
"By definition, intermittent fasting is the intentional and planned restriction of food," says Harbstreet. "For this reason, I absolutely would not recommend it to anyone with an active eating disorder, orthorexia, or other disordered eating behaviors. IF can be especially challenging for those who become preoccupied with food or struggle with rebound overeating after a period of fasting. If you find that you can't get your mind off food and end up eating more than you would if you had not fasted, it's likely that intermittent fasting is doing more harm than good. That goes not only for your health but also your relationship with food and how you nourish your body." (Related: Why the Potential Intermittent Fasting Benefits Might Not Be Worth the Risks)
Harbstreet also says that she would not recommend intermittent fasting to anyone who has trouble meeting their basic, minimal nutrition needs, noting that "if you aren't careful, you could shortchange yourself on important nutrients and your health could suffer as a result."
We still don't know everything about intermittent fasting.
Overall, it sounds like there is a ton that's just not totally understood about intermittent fasting right now.
Some people swear by it, while others may find it negatively affects them physically or mentally. "Until there is more research that supports health benefits as a result of fasting, I prefer to focus on supporting clients in choosing nourishing foods they enjoy eating and helping them reconnect to and trust their body when it comes to food," says Harbstreet. If you do choose to try it out, just make sure you are getting enough nutrients on your non-fasting days.