Intermittent Fasting: What is it, and how does it work? (2023)

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

What is intermittent fasting?

Many diets focus on what to eat, but intermittent fasting is allabout when you eat.

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

Johns Hopkins neuroscientist Mark Mattsonhas studied intermittent fasting for 25 years. He says our bodies haveevolved to be able to go without food for many hours, or even several daysor longer. In prehistoric times, before humans learned to farm, they werehunters and gatherers who evolved to survive — and thrive — for longperiods without eating. They had to: It took a lot of time and energy tohunt game and gather nuts and berries.

Experts note that even 50 years ago, it was easier to maintain a healthyweight in the United States. There were no computers, and TV shows turnedoff at 11 p.m.; people stopped eating because they went to bed. Portionswere 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 andchildren stay awake for longer hours to watch TV, scroll through socialmedia, play games and chat online. That can mean sitting and snacking allday — and most of the night.

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

How does intermittent fasting work?

There are several different ways to do intermittent fasting, but they areall 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 forthe remainder. Or you might choose to eat only one meal a day two days aweek. There are many different intermittent fasting schedules.

Mattson says that after hours without food, the body exhausts its sugarstores and starts burning fat. He refers to this as metabolic switching.

“Intermittent fasting contrasts with the normal eating pattern for mostAmericans, who eat throughout their waking hours,” Mattson says. “Ifsomeone is eating three meals a day, plus snacks, and they’re notexercising, then every time they eat, they’re running on those calories andnot burning their fat stores.”

Intermittent fasting works by prolonging the period when your body hasburned through the calories consumed during your last meal and beginsburning 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 studies published 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.

Williams stresses that 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 with type 1 diabetes who 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.

But, Williams says, people not in these categories who can do intermittent fasting safely can continue the regimen indefinitely. “It can be a lifestyle change,” she says, “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.


How do you do intermittent fasting? ›

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.

What exactly happens in 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 are the best hours for intermittent fasting? ›

Consider a simple form of intermittent fasting.

Limit the hours of the day when you eat, and for best effect, make it earlier in the day (between 7 am to 3 pm, or even 10 am to 6 pm, but definitely not in the evening before bed). Avoid snacking or eating at nighttime, all the time.

What can you eat or drink while intermittent fasting? ›

Technically speaking, eating or drinking any calories breaks a fast. That means it's generally considered OK to drink black, unsweetened coffee or tea, as well as water.

What is the first rule of intermittent fasting? ›

The first rule of intermittent fasting is … don't break your fast! Without a strong, intact, periodic fast, you won't reap the benefits of intermittent fasting.

Why is 16 hours the magic number for fasting? ›

Both 12-hour fasting and 16-hour fasting can be effective for weight loss, but 16-hour fasting may be more effective because it may lead to a greater reduction in overall caloric intake (through a more condensed eating window) and promote more significant reliance on fat burning.

What happens after 16 hours of fasting? ›

If you don't eat for 10–16 hours, your body will go to its fat stores for energy, and fatty acids called ketones will be released into the bloodstream. This has been shown to protect memory and learning functionality, says Mattson, as well as slow disease processes in the brain.

Should I intermittent fast every day? ›

Fasting on either a daily or weekly basis helps your body burn fat. There are different approaches to intermittent fasting but the most popular include: Alternate day fasting includes normal eating every other day, with days of restricted eating (500 calories) in between.

Does sleeping count as fasting? ›

And yes, sleeping counts as fasting! If you are looking for significant weight loss, you might consider working up to 18-20 hours of daily fasting (OMAD or one-meal-a-day), alternate day fasting (fasting every other day, with up to 500 calories on fasting days) or a 5:2 schedule (fasting for two days per week).

What food can I eat when fasting? ›

Foods you can eat while fasting
  • Water. Plain or carbonated water contains no calories and will keep you hydrated during a fast.
  • Coffee and tea. These should mostly be consumed without added sugar, milk, or cream. ...
  • Diluted apple cider vinegar. ...
  • Healthy fats. ...
  • Bone broth.
Sep 26, 2019

Is it better to 16 8 fast in the morning or evening? ›

The easiest way to follow the 16:8 diet is to choose a 16-hour fasting window that includes the time that a person spends sleeping. Some experts advise finishing food consumption in the early evening, as metabolism slows down after this time.

What can I drink while fasting? ›

The best kind of drinks for your fasting hours are calorie-free. That way, they will not activate an insulin response or break your fast. Acceptable beverages for intermittent fasting include water, teas, coffee, and apple cider vinegar. These zero-calorie beverages are suitable for your fasting window.

Does lemon water break a fast? ›

Long story short - the answer to the question “Does lemon water break a fast?” is no, lemon water does not break a fast. Lemon water contains almost no calories and zero sugars, it doesn't raise insulin levels, which means it will not break your fast (1).

What not to eat after fasting? ›

Raw vegetables, beef, and occasionally eggs and nuts can be difficult for some people immediately post-fast, says Fung. And anything fried or greasy should also be avoided until you get your digestion revving with gentler foods.

What not to take during intermittent fasting? ›

Drinks to avoid while intermittent fasting
  • Cappuccinos, latte and milk teas. Ingredients like milk and sugar, impact our metabolism and blood sugar levels and are not suitable during fasting hours. ...
  • Coconut water. Yes! ...
  • Freshly squeezed juices. ...
  • Energy/sports drinks and sodas. ...
  • Protein powder. ...
  • Gummy multivitamins.
Feb 8, 2023

How much weight can you lose in a month with intermittent fasting? ›

If you consistently stay active while following intermittent fasting, you can lose as much as 6-8 pounds per month. This estimation is shockingly high when you consider that all you have to do is not eat for select hours throughout the day.

How long are you supposed to do intermittent fasting? ›

There is no predetermined intermittent fasting program length. It is an individual's choice based on one's objectives for fasting. If weight loss is the goal most people start seeing results within 2-3 weeks.

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