What Is Intermittent Fasting? (2024)

Intermittent fasting is a type of diet that specifies certain windows of time in which food intake is allowed, says Artur Viana, M.D., clinical director of a metabolic health and weight loss program and assistant professor of medicine at Yale School of Medicine in New Haven, Connecticut. While most strategic eating plans focus on what food is consumed, intermittent fasting is all about when you eat.

It doesn’t involve regulating the type of food eaten or the amount of carbohydrates, fat or protein in the food, making it unique to the mainstream diet world, according to Dr. Viana.

“Most people use intermittent fasting with the goal of losing weight,” says Dr. Viana. “For that purpose, intermittent fasting works by ultimately decreasing the overall amount of calories somebody takes in.”

Another suggested benefit is the prevention of metabolism slowdown that can occur with weight loss, but it hasn’t been proven, Dr. Viana points out. Meanwhile, other effects, such as improvement in cardiometabolic parameters like hypertension, don’t differ much from other forms of calorie restriction, like traditional dieting, according to Dr. Viana.

The concept of fasting—refraining from eating or drinking for set periods of time—dates back thousands of years. In fact, according to Britannica, fasting was first mentioned in the 5th century BCE by Greek physician Hippocrates, who touted it as a tactic for combating certain symptoms of illness. Additionally, it’s a common practice in a number of religions, including those of ancient civilizations and some Native American tribes, as well as Judaism, Christianity and Islam.

There are several intermittent fasting methods, which mostly differ in timing. However, most “work the same way, leading to overall restriction of calories,” says Dr. Viana.

16/8 or Time-Restricted Eating

One increasingly popular intermittent fasting method is referred to as 16/8 (also known as time-restricted feeding and time-restricted eating) “where an individual fasts for 16 hours and is allowed to eat only for eight hours,” says Dr. Viana.

Time-restricted eating—which may be slightly adapted by extending or decreasing the feasting period—may be easier to adhere to than some of the other types, which “is the key to any type of dietary method used for long-term results,” says Dr. Viana. It can be easier for some because the feast and fast periods can be adjusted to a person’s comfort level and aren’t necessarily as extreme as some of the other methods, in which the fasting period is much longer.

While some do find time-restricted eating to be beneficial, especially for managing a healthy weight and avoiding late-night snacking, it does have a few big pitfalls, says Jackie Newgent, a registered dietician nutritionist, plant-focused chef and author of The Clean & Simple Diabetes Cookbook. “It can feed into a disordered eating pattern, contribute to headaches and potentially hinder weight loss efforts,” she says.

OMAD

The one-meal-a-day (OMAD) method of intermittent fasting is also referred to as the 23:1 regimen. It involves fasting for 23 hours and feasting for just one hour. ​Dr. Viana points out that this is the same thought process of the 16/8 method, but it may be much harder to adhere to, since the eating window is so narrow.

“The key is being able to stick to a method in the long term and in the end, what matters overall is calorie restriction,” says Dr. Viana. “Most studies have failed to demonstrate a superiority of intermittent fasting over continuous calorie restriction when it comes to magnitude of weight loss and even cardiometabolic parameters.”

Another downside of the OMAD method, according to Newgent, is that it can exaggerate a yo-yo dieting cycle—a pattern of losing weight, gaining it back and dieting again—and magnify unhealthy relationships with food. “It can also lead to blood sugar fluctuation, as well as nutrient deficiencies—it’s already challenging for some to meet nutrient needs when they’ve got the entire day to eat,” she says. Additionally, active people and athletes might find the OMAD method to impact their performance negatively.

5:2

The 5:2 method is another popular intermittent fasting method. The concept is relatively simple: A person can eat without restrictions for five days and then consume a very low amount of calories (typically 25% of your total calorie intake on non-fasting days) during the remaining two days of the week. This is another way to decrease the amount of calories that one eats overall, says Dr. Viana.

“For some people, that [the 5:2 method] can adapt better to their lifestyle,” he says. “Someone could, for example, eat normally Thursday to Monday and markedly decrease the amount of calories they take in on Tuesday and Wednesday.”

Newgent says she wouldn’t intervene if someone finds the 5:2 method helps them eat healthier, but she cautions that low-calorie days should correlate with low-activity days since you won’t be giving your body adequate energy to fuel fitness properly.

This method is not ideal for everyone, “especially those who jump from one diet to another,” she says. “This will keep them on that never-ending rollercoaster ride of a dieting mindset.”

Eat Stop Eat

Eat Stop Eat, popularized by former bodybuilding supplement executive Brad Pilon, is very similar to the 5:2 method. However, instead of limiting calories for two days a week, a person eats or drinks nothing but water, coffee or other zero-calorie beverages for 24-hour periods once or twice a week. During the eating periods, the intention is to eat as you normally would.

This method is similar to the 5:2 method, but with more intense calorie-restriction during the two-day period.

“More intense calorie-restriction may mean that it is harder to adhere to but could lead to a higher degree of weight loss,” says Dr. Viana. However, he generally tries to keep his patients off “canned, fad-type diets,” explaining, “I think they tend to fail.”

Experts also point out that this method can lead to the regaining of weight loss once you resume your regular eating habits, as it doesn’t focus on healthy eating habits that tend to yield long-term results. Other potential pitfalls of this type of diet can include nutrient deficiencies, especially among those who exercise regularly.

FEATURED PARTNER OFFER

Partner Offers feature brands who paid Forbes Health to appear at the top of our list. While this may influence where their products or services appear on our site, it in no way affects our ratings, which are based on thorough research, solid methodologies and expert advice. Our partners cannot pay us to guarantee favorable reviews of their products or services

Enjoy 1 Free Month Of Membership At Sign Up

Lumen Metabolism Tracker

• World’s first hand-held, portable device to accurately measure metabolism
• Scientifically proven to meet the gold standard of metabolism measurement (RER)
• See how your metabolism is affected by your nutrition, physical activity and sleep
• Achieve sustainable weight loss with daily personalized meal plans
• $299 plus $19/mo, includes one free month of membership

Start Now

On Lumen's Website

See Also

Intermittent Fasting: Uitleg, 9 Voordelen en 10 Schema’s

Intermittent fasting is popular for good reason—research continues to support a handful of its health benefit claims. Below are a few backed by science.

It may lead to improvement in many health conditions.

A 2020 study funded by the National Institutes of Health and published in the New England Journal of Medicine analyzed decades of previous research and found that intermittent fasting can lead to improvements in health conditions, including obesity, diabetes, cardiovascular disease, cancers and neurological disorders^[1]Research on intermittent fasting shows health benefits. National Institutes of Health. Accessed 6/21/2021. .

It may increase human growth hormone.

Intermittent fasting can encourage fat burn and protect lean muscle in an alternative way—by raising your levels of human growth hormone (HGH).

“Levels [of HGH] start to decline in our twenties or thirties, leading to dry skin, thinning hair, increased belly fat and wrinkles,” says Kellyann Petrucci, M.S., a naturopathic doctor and author of Dr. Kellyann’s Bone Broth Diet. “Research shows that HGH may help decrease body fat and improve lean muscle mass.”

She points to 2011 research from the Intermountain Medical Center Heart Institute, which found that HGH increased an average of 1,300% in women and nearly 2,000% in men during a 24-hour fast^[2]Routine periodic fasting is good for your health, and your heart, study suggests. Intermountain Medical Center. Accessed 6/21/2021. . When fasting causes hunger or stress, the body responds by releasing more cholesterol—the researchers noted—encouraging it to burn fat instead of glucose as fuel and decreasing the number of fat cells in the body.

It can improve metabolic flexibility.

Another big benefit of fasting, especially for those who crave food every few hours, is associated with your metabolism. “Fasting for even short periods can increase your metabolic flexibility, which is your ability to switch between burning sugar and burning fat,” says Petrucci, citing research from the University of Alabama that found people who simply restricted their eating to the hours between 8 a.m. and 2 p.m. became more metabolically flexible^[3]Time-restricted feeding study shows promise in helping people shed body fat. The University of Alabama at Birmingham. Accessed 06/21/2021. . “This is important, because when you’re metabolically flexible, you don’t ‘crash and burn’ when you go for a long time between meals,” she says.

There are a few potential downsides of intermittent fasting, according to Newgent.

Contributes to Headaches

Headaches can occur on fasting days, “which may be due in part to blood sugar fluctuation,” says Newgent. “Dehydration causes headaches, too.”

Can Promote Disordered Eating

Both Newgent and Dr. Viana point out that intermittent fasting might not promote healthy eating habits, especially for anyone struggling with an eating disorder. “Any form of restriction or ‘rules’—including time-restricted eating—can exacerbate disordered eating, such as binging during eating ‘windows,’” says Newgent. “It also leads to a preoccupation with food.”

May Hinder Weight Loss

Intermittent fasting is commonly associated with weight loss. However, Newgent explains that many people actually gain weight due to overeating during non-fasting times. She also points out that “any long period of fasting can ultimately slow down your metabolism.”

Leads to Lack of Energy

Intermittent fasting can also zap your energy. “Simply put, a lower-than-normal calorie intake can lead to a lower-than-normal amount of energy,” says Newgent. “This may cause you to cut back on physical activity or hinder performance.”

Can Impact Medication Management

Some medications need to be taken with food, which wouldn’t be a fit for many intermittent fasting patterns. Unless done under medical supervision, intermittent fasting is typically not recommended for people with diabetes who need to take insulin or medication to manage their blood sugar levels.

Because intermittent fasting isn’t a diet, and it focuses on when—and not what—people eat, it can vary in risk.

Intermittent fasting is generally considered safe for most people, but it can be dangerous if someone takes in too little calories (800 calories or less is considered very low, and those types of diets need to be medically supervised), according to Dr. Viana. He also notes a risk of dehydration if water intake isn’t sufficient.

Newgent adds that eating patterns should be individualized. “In order to get all of the health-protective nutrients that your body needs to thrive, most methods of fasting are not advisable,” she says. “The human body is not designed for altered, time-focused eating patterns, especially a one-meal-a-day approach. Our cells need continual nourishment, not just when a fasting regimen says so.”

Dr. Viana advises people with medical conditions or who are pregnant or breastfeeding to consult their primary care doctor before trying intermittent fasting. People with diabetes might want to avoid intermittent fasting, as they might experience low blood sugar with certain diabetes medications. Additionally, anyone with a history of eating disorders may find intermittent fasting triggering.

Intermittent fasting isn’t for everyone. However, if you do decide to try it, Dr. Viana notes that consistency is key. “Intermittent fasting is a powerful tool to help change lifestyle, but before doing it, one should consider if they will be able to maintain it in the long run,” he says.

His overall view on eating is that it needs to be something natural and uncomplicated—noting that intermittent fasting can be too restrictive for many people. “I like the philosophy of mindful eating very much, which is essentially to get rid of the diet mentality. In my opinion, we tend to overcomplicate eating and that leads to failure.”

Also, keep in mind that what you eat should be your first priority over when you eat. “What is most important is to eat a healthy diet that contains little or no processed food and is rich in lean protein, whole grains and vegetables [among others],” says Dr. Viana.

If you do decide to incorporate intermittent fasting into your dietary plan, consult with your physician first—especially if you have any preexisting health conditions or are on certain medications.