Intermittent Fasting: What is it, and how does it work? (2024)

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.

Intermittent Fasting: What is it, and how does it work? (2024)

FAQs

Intermittent Fasting: What is it, and how does it work? ›

The idea behind intermittent fasting is that by restricting food, our bodies will more quickly and efficiently tap our fat stores for energy. While glucose from carbohydrates is our most direct fuel source, we burn fat for energy when glucose isn't available. This happens even more during times of food deprivation.

What is the simple way to explain intermittent fasting? ›

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.

What are the basic rules for intermittent fasting? ›

How to practice intermittent fasting
  • Eating during a 12-hour window every day (between 7 a.m. and 7 p.m., for example) and fasting for the other 12 hours.
  • Eating during an 8-hour window every day (between 11 a.m. and 7 p.m., for example) and fasting for the other 16 hours.
Dec 30, 2021

How to do intermittent fasting for beginners? ›

A good option for beginners, then, is to fast for a set number of hours each day. This option allows you to start with fasting for around 12 hours a day, most of which you will be asleep. After you get more comfortable with 12-hour fasts, you can slowly lengthen the time of the fast.

Can you eat whatever you want while intermittent fasting? ›

When deciding on a method, a person should remember that they do not need to eat a certain amount or type of food or avoid certain foods altogether. A person can eat what they want when following an intermittent fasting plan.

What can I drink in the morning while intermittent fasting? ›

Generally speaking, while fasting with time-restricted intermittent eating, you don't eat any food at all and only drink beverages with very few calories, such as water or unsweetened coffee and tea without milk. When you're not fasting, it's important to eat a healthy and varied diet that's good for your body.

What foods should you avoid during intermittent fasting? ›

It's best to limit ultra-processed foods like packaged snacks, deep-fried items, sugary drinks, and most frozen meals. These negate the positive effects of 16/8 intermittent fasting. To begin 16/8 intermittent fasting, choose an 8-hour window and limit your food intake to that time span.

What is the golden rule of intermittent fasting? ›

The absolute most important rule is “Don't break your fast”.

Beware of foods labeled zero calories, as most will have trace amounts of calories that can break your fast if you eat enough. Drink lots of water, add a fiber source with your meals, or use a supplement to help manage hunger cravings.

What foods won't break a fast? ›

Foods and beverages that do not break a fast include water, black coffee, unsweetened tea, and diluted apple cider vinegar.

What is the trick to intermittent fasting? ›

People who are starting intermittent fasting for the first time may find the 12 hour fasting method the easiest. This involves fasting for 12 hours every 24-hours period. People can include time when they are asleep in this fasting window, for example, fasting from 7 p.m. to 7 a.m.

What is the best first meal for intermittent fasting? ›

Start your fast correctly: The best way is to have a low-carb meal, with healthy fats and high-quality protein. This allows your body to activate satiety hormones that have long-term effects and mean you won't experience hunger for a long time.

What is the first meal of the day for intermittent fasting? ›

You can start intermittent fasting by skipping breakfast, having your first meal in the afternoon (about 12 p.m.), and eating your last meal before 8 p.m. Intermittent fasting is a popular type of eating plan that involves fasting for a period of time between eating periods.

What is the best intermittent fasting window to lose belly fat? ›

A 2023 review of research found that the 16/8 method and 16/8 combined with restricting calories were effective strategies for weight control in adults with overweight or obesity. An eating window starting before noon led to greater weight loss than one that began after noon.

How much weight can you lose with intermittent fasting in 2 weeks? ›

Effective Diet Plans for Rapid Weight Loss
Diet PlanHow much can I lose in 2 weeks?Example
Low-carb diet5-10 poundsAtkins, Ketogenic
Low-fat diet3-6 poundsOrnish, Pritikin
Intermittent Fasting4-8 pounds16/8 method, Eat-Stop-Eat
Sep 23, 2023

Can I eat sweets while intermittent fasting? ›

There are certain foods that aren't good to consume as part of an intermittent fasting regimen. You should stay away from foods that are calorie-dense and contain high amounts of sugar, fat and salt. “They won't fill you up after a fast, and can even make you hungrier,” Maciel says.

Why am I not losing weight on 16:8 fasting? ›

If you've tried intermittent fasting but aren't losing weight, possible reasons why include overeating during your eating window and poor food choices. To help, you can try eating fewer calories, work on balancing your meals, or create a smaller or larger eating window.

What is the simple plan for intermittent fasting? ›

People who are starting intermittent fasting for the first time may find the 12 hour fasting method the easiest. This involves fasting for 12 hours every 24-hours period. People can include time when they are asleep in this fasting window, for example, fasting from 7 p.m. to 7 a.m.

What is the nutshell of intermittent fasting? ›

Intermittent fasting is an eating pattern in which you cycle between periods of eating and periods of fasting. There are many types of intermittent fasting, such as the 16:8 and 5:2 methods. Numerous studies suggest that it can have powerful benefits for your body and brain.

What is the basic science behind intermittent fasting? ›

The idea behind intermittent fasting is that by restricting food, our bodies will more quickly and efficiently tap our fat stores for energy. While glucose from carbohydrates is our most direct fuel source, we burn fat for energy when glucose isn't available. This happens even more during times of food deprivation.

What's a good example of intermittent fasting? ›

Eat a normal diet five days a week and fast two days a week. Daily time-restricted fasting. Eat normally but only within an eight-hour window each day. For example, skip breakfast but eat lunch around noon and dinner by 8 p.m.

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