Intermittent Fasting: What is it, and how does it work? (2025)

Intermittent fasting is an eating plan that switches between fasting and eating on a regular schedule. Research shows that intermittent fasting is a way to manage your weight and prevent — or even reverse — some forms of disease. But how do you do it? And is it safe?

What is intermittent fasting?

Many diets focus onwhatto eat, but intermittent fasting is all aboutwhenyou eat.

With intermittent fasting, you only eat during a specific time. Research shows fasting for a certain number of hours each day or eating just one meal a couple days a week may have health benefits.

Johns Hopkins neuroscientistMark Mattsonhas studied intermittent fasting for 25 years. He says our bodies have evolved to be able to go without food for many hours, or even several days or longer. In prehistoric times, before humans learned to farm, they were hunters and gatherers who evolved to survive — and thrive — for long periods without eating. They had to: It took a lot of time and energy to hunt game and gather nuts and berries.

Experts note that even 50 years ago, it was easier to maintain a healthy weight in the United States. There were no computers, and TV shows turned off at 11 p.m.; people stopped eating because they went to bed. Portions were much smaller. More people worked and played outside and, in general, got more exercise.

With internet, TV and other entertainment available 24/7, many adults and children stay awake for longer hours to watch TV, scroll through social media, play games and chat online. That can mean sitting and snacking all day — and most of the night.

Extra calories and less activity can mean a higher risk of obesity, type 2 diabetes, heart disease and other illnesses. Scientific studies are showing that intermittent fasting may help reverse these trends.

How does intermittent fasting work?

There are several different ways to do intermittent fasting, but they are all based on choosing regular time periods to eat and fast. For instance, you might try eating only during an eight-hour period each day and fast for the remainder. Or you might choose to eat only one meal a day two days a week. There are many different intermittent fasting schedules.

Mattson says that after hours without food, the body exhausts its sugar stores and starts burning fat. He refers to this asmetabolic switching.

“Intermittent fasting contrasts with the normal eating pattern for most Americans, who eat throughout their waking hours,” Mattson says. “If someone is eating three meals a day, plus snacks, and they’re not exercising, then every time they eat, they’re running on those calories and not burning their fat stores.”

Intermittent fasting works by prolonging the period when your body has burned through the calories consumed during your last meal and begins burning fat.

Intermittent Fasting Plans

It’s important to check with your doctor before starting intermittent fasting. Once you get his or her go-ahead, the actual practice is simple. You can pick adaily approach, which restricts daily eating to one six- to eight-hour period each day. For instance, you may choose to try 16/8 fasting: eating for eight hours and fasting for 16.

Although some people find it easy to stick with this pattern over the long term,one research study that was not designed specifically to look at an intermittent fasting pattern found that limiting your daily time window of eating does not prevent weight gain over time or yield significant weight loss results. That study’s results showed that reducing the number of large meals or eating more small meals may be associated with minimizing weight gain or even with weight loss over time.

Another intermittent fasting plan, known as the5:2 approach, involves eating regularly five days a week. For the other two days, you limit yourself to one 500–600 calorie meal. An example would be if you chose to eat normally on every day of the week except Mondays and Thursdays, which would be your one-meal days.

Longer periods without food, such as 24-, 36-, 48- and 72-hour fasting periods, are not necessarily better for you, and may be dangerous. Going too long without eating might actually encourage your body to start storing more fat in response to starvation.

Mattson’s research shows that it can take two to four weeks before the body becomes accustomed to intermittent fasting. You might feel hungry or cranky while you’re getting used to the new routine. But, he observes, research subjects who make it through the adjustment period tend to stick with the plan because they notice they feel better.

What can I eat while intermittent fasting?

During the times when you’re not eating, water and zero-calorie beverages such as black coffee and tea are permitted.

During your eating periods, “eating normally” does not mean going crazy. Research shows that you’re not likely to lose weight or get healthier if you pack your feeding times with high-calorie junk food, super-sized fried items and treats.

But what some experts like about intermittent fasting is that it allows for a range of different foods to be eaten — and enjoyed. Sharing good, nutritious food with others and savoring the mealtime experience adds satisfaction and supports good health.

Most nutrition experts regard theMediterranean dietas a good blueprint of what to eat, whether you’re trying intermittent fasting or not. You can hardly go wrong when you pick leafy greens, healthy fats, lean protein and complex, unrefined carbohydrates such as whole grains.

Intermittent Fasting Benefits

Research shows that the intermittent fasting periods do more than burn fat. Mattson explains, “When changes occur with this metabolic switch, it affects the body and brain.”

One of Mattson’s studiespublished in theNew England Journal of Medicinerevealed data about a range of health benefits associated with the practice. These include a longer life, a leaner body and a sharper mind.

“Many things happen during intermittent fasting that can protect organs against chronic diseases like type 2 diabetes, heart disease, age-related neurodegenerative disorders, even inflammatory bowel disease and many cancers,” he says.

Here are some intermittent fasting benefits research has revealed so far:

• Thinking and memory. Studies discovered that intermittent fasting boosts working memory in animals and verbal memory in adult humans.
• Heart health. Intermittent fasting improved blood pressure and resting heart rates as well as other heart-related measurements.
• Physical performance. Young men who fasted for 16 hours showed fat loss while maintaining muscle mass. Mice who were fed on alternate days showed better endurance in running.
• Type 2 diabetes and obesity. In animal studies, intermittent fasting prevented obesity. And, in six brief studies, obese adult humans lost weight through intermittent fasting. People withtype 2 diabetesmay benefit: Most of the available research shows that intermittent fasting can help people lose body weight and lower their levels of fasting glucose, fasting insulin and leptin while reducing insulin resistance, decreasing levels of leptin and increasing levels of adiponectin. Certain studies found that some patients practicing intermittent fasting with supervision by their doctors were able to reverse their need for insulin therapy.
• Tissue health. In animals, intermittent fasting reduced tissue damage in surgery and improved results.

Is intermittent fasting safe?

Some people try intermitting fasting for weight management, and others use the method to address chronic conditions such as irritable bowel syndrome, high cholesterol or arthritis. But intermittent fasting isn’t for everyone.

Before you try intermittent fasting (or any diet), you should check in with your primary care practitioner first. Some people should steer clear of trying intermittent fasting:

• Children and teens under age 18.
• Women who are pregnant or breastfeeding.
• People withtype 1 diabeteswho take insulin. While an increasing number of clinical trials have shown that intermittent fasting is safe in people with type 2 diabetes, there have been no studies in people with type I diabetes. Mattson explains, “Because those with type I diabetes take insulin, there is a concern that an intermittent fasting eating pattern may result in unsafe levels of hypoglycemia during the fasting period.”
• Those with a history of eating disorders.

Butpeople not in these categories who can do intermittent fasting safely can continue the regimen indefinitely. It can be a lifestyle change ― and one with benefits.

Keep in mind that intermittent fasting may have different effects on different people. Talk to your doctor if you start experiencing unusual anxiety, headaches, nausea or other symptoms after you start intermittent fasting.