Let’s Talk About Intermittent Fasting
We’ve got the expert-approved scoop on everything you need to know about the health benefits and how-to’s of intermittent fasting. Find out if it’s right for you.
It seems like everyone’s talking about intermittent fasting (IF) these days. From celebrities to social media influencers, claims about the effectiveness of IF as a fast track to better health are everywhere. But is it? And what actually is intermittent fasting—and how do you do it? We took your questions to the pros for the bottom line on this growing food trend.
Our Pro Panel
We consulted some of the nation’s top nutritionists and doctors to bring you the most up-to-date facts on intermittent fasting.
Vanessa Rissetto, R.D.
Registered dietitian and co-founder
Culina Health nutrition and health clinic
New York City
Christine Bishara, M.D.
Internal medicine doctor, certified in integrative medical weight loss
New York City
Steven Gundry, M.D.
Cardiothoracic surgeon; medical director
International Heart and Lung Institute Center for Restorative Medicine
Palm Springs, CA
Intermittent fasting is not a diet in the traditional sense. It doesn’t call for restricting calories or eliminating certain foods or even food groups. Instead, it is a pattern of eating organized around time.
Technically speaking, you can eat whatever you want, but common sense is encouraged. Treats and indulgences are allowed, but you should focus on consuming nutrient-dense foods with sufficient dietary fiber during your eating window in order to get the nourishment your body needs to stay fuller longer.
No, one type of intermittent fasting isn’t better than another. The best plan for you is the one that’s most compatible with your lifestyle and eating preferences. Beginners often start with a time-restricted plan (like the 16:8), but you might find that you prefer another pattern. Trial and error is the best way to figure out what works best for you and fits most seamlessly into your lifestyle.
No—intermittent fasting isn’t for everyone. Experts caution that people who have a history of eating disorders or are underweight should not experiment with time-restricted eating patterns. They also recommend that those with diabetes or who participate in intensive athletic training should consult their doctor before starting an intermittent fasting program.
What Is Intermittent Fasting, Anyway?
Intermittent fasting isn’t a diet. It’s a way of eating based on time instead of calories. In other words, it doesn’t restrict what you eat, but rather when you eat. Intermittent fasting calls for consuming your daily intake of food within a compressed window of time, which leaves the body more time within a 24-hour period to metabolize calories while also having more time to rest and repair. The goal: Helping your body make better use of the fuel you feed it for optimal health.
Fasting has been used in both religious and medical contexts since ancient Greek and Roman civilizations. In addition, medical studies done as early as 1914 credit fasting as an effective treatment for diabetes. More recently, a 2020 survey conducted by the International Food Information Council found that IF is the most popular way of eating, edging out gluten-free and low-carb regimens. Maybe that’s because celebrities like Twitter CEO Jack Dorsey and actress Gwyneth Paltrow rave about how great IF makes them feel—or maybe it’s because IF has been linked to weight loss. Either way, the idea of structuring a day around eating and resting time slots has been popular for centuries—and is still going strong.
How Does Intermittent Fasting Work?
The science works like this: After eating, your stomach sets about breaking down food into amino acids, fatty acids, and glucose. The insulin secreted by the pancreas (the organ which produces digestive enzymes and hormones) during this process ushers glucose into cells, which your body, in turn, uses for energy. Excess glucose that isn’t needed for energy at that moment is redirected to the liver where it’s converted into glycogen and stored for later use. Once the body has used all its readily-available glucose, it starts sending hunger signals to try to get you to eat again. However, if those signals go unanswered, the body enters a fasted state, and burns the stored glycogen in the liver. When that’s depleted, the liver breaks down fatty acids into ketones, a compound that the body can use for energy.
To put it another way: Fasting prompts your body to fuel itself from the fat it has stored after using all the carbohydrates it has ingested and digested. This is what can help trigger weight loss. Plus, the longer the body remains in a fasted state, the more time there is for health benefits such as reduced inflammation and cell rejuvenation (the body’s process of removing or recycling damaged cellular material).
What Are Some of the Most Common Ways to Practice Intermittent Fasting?
You eat, then you fast. Sounds simple, right? But the devil’s in the details, and there are numerous ways you can work this food/fast cycle. Let’s take a closer look at the most popular types of fasting:
Time-Restricted or 16:8 Intermittent Fasting
The best-known plan for intermittent fasting is 16:8, or 16 hours of fasting followed by an 8-hour eating window. For example, this can mean you stop eating by 7pm and don’t consume anything (except tea, water, or black coffee—they don’t have enough calories to trigger your body’s digestion process) until at least 11am the next day. At that point, your eating window opens, and you’re free to eat when and how you’d like—two meals and some snacks; three smaller meals—until 7pm when you start fasting again.
While you should aim for a healthy, nutrient-rich diet, there technically isn’t any restriction on what foods you can have or how much when doing IF. The idea, according to experts, is to stop eating at least four hours before going to sleep so that the digestion process can start while you’re still awake and active. It also starts the clock on a fasting period that stretches until your first meal the following day, which is enough time for your body to switch from using carbohydrates to ketones for fuel. Plus, eating early is a good idea since it has also been shown to reduce the risk of obesity and diabetes by increasing insulin sensitivity which results in better blood sugar control.
Time-restricted plans are perfect for newcomers to IF because they only require giving up late-night snacking and delaying your first meal by a few hours. If sixteen hours of no food sounds like a refrigerator raid ready to happen, here’s the good news: You don’t have to start out with a full 16-hour fast. You could aim for a 14-hour fast and 10-hour eating window (14:10) or even a 12-hour fast and a 12-hour eating window (12:12) and work your way up from there.
Modified-Calorie or 5:2 Intermittent Fasting
Another way to fast is by limiting both when and how much (but not necessarily what) you eat. An example of this is the 5:2 intermittent fasting plan, which allows for eating normally five days out of the week and then restricting your calorie intake to only about 500 to 600 calories on two fasting days. (The two days don’t need to be consecutive, but some experts believe this back-to-back approach is more effective at kickstarting IF’s benefits because it keeps the body burning fuel from fat for a longer time period.)
Here’s how this approach works: You’ll stop eating at, say, 7pm on Sunday night. Then on Monday, you consume up to 500 calories for women or 600 for men during small meals or snacks at your preferred time. Healthy, easy options that stick to this calorie recommendation are a salad and a yogurt, or lentil soup and a piece of bread. The trick is to aim for a meal comprised of nutrient-dense, fiber-rich options like vegetables, and then some lean protein to help keep you feeling full longer. If you choose to do your second fasting day consecutively, you’d repeat the pattern the next day. Otherwise, you return to a day or two of normal eating before repeating a fasting day with reduced caloric intake. Because you’re restricting calories in this plan (while 16:8 doesn’t actually require you to restrict calories), you may notice more weight loss on this plan.
Alternate day fasting is exactly what it sounds like: you alternate between a day of normal eating and a day of fasting (except for water, coffee, or tea).
OMAD (One Meal a Day)
Another popular method is to fast for a longer period of time. OMAD (One Meal a Day) is a 23:1 intermittent fasting plan, in which you fast for 23 hours and then eat your only meal of the day at your preferred time during a one-hour window. This type of fasting is often used by more advanced IFers who have been doing some form of fasting already—and is usually only practiced occasionally, such as once a month, if it’s not being done as part of a 5:2 regimen that includes a normal eating routine on most days.
Keep in mind that no matter which plan you choose, IF isn’t a free pass to gorge yourself on junk food during your eating window. While specific foods aren’t forbidden like they might be in a traditional diet, the idea is to consume a normal amount of balanced foods (plus a few treats!). Intermittent fasting works best when you fill up on a satisfying mix of whole grains and lean proteins during your eating window, which keep you full longer than the empty calories found in sugary snacks and processed foods.
The Benefits of Intermittent Fasting
If we didn’t lose you at “an hour a day,” good news! You might be a candidate for trying intermittent fasting. And in fact, along with the recent celebrity-driven hype over this eating approach, there are some very legit health benefits to IF. Here are some of the compelling reasons you might want to try it.
Intermittent fasting can be a powerful tool for weight loss because it causes your body to switch from burning carbohydrates for energy to burning stored fat. In fact, a systematic review of 40 studies found that intermittent fasting can yield a typical loss of 7 to 10 pounds over 10 weeks. In addition, people who practice intermittent fasting naturally tend to consume fewer calories since their eating time is compressed into eight hours or less, rather than the standard 14-hour (or longer!) eating window.
Think about it: If you kick off the day with breakfast at 7am and don’t stop noshing until after a late-night snack before hitting the hay at 11pm, your body is never in short supply of easy carbs for fuel, so it never gets the chance to burn stored fat. In contrast, building a fasting window into your day allows your body to use all the readily available carbohydrates it’s taken in, and then dig into the energy it has stored in both the liver and fat cells.
Bonus: Many folks who practice intermittent fasting report being less hungry once their bodies have adapted to eating in a compressed time window. The upshot? You may end up consuming fewer calories while on an intermittent fasting plan, which may help you achieve weight loss goals.
Improved Brain Function
Talk to people who fast and they’ll rave about how sharp and focused they are—especially early in the day. But is it all in their heads? Nope. Science actually backs up their claims. In fact, a 2019 study in the New England Journal of Medicine found that intermittent fasting prevented short-term memory loss and helped stave off age-related memory decline.
When the body is in a fasted state, it breaks down and cleans out damaged cells in a process called autophagy. This clears the way for cellular renewal, which can help promote better skin and healthier hair in addition to protecting against degenerative diseases such as Parkinson’s and Alzheimer’s.
What Are the Potential Drawbacks of Intermittent Fasting?
While intermittent fasting can offer notable health benefits, there can be some drawbacks (hello grumbling stomach, crabby mood, and general fatigue), especially when you're first starting out. (Read more about the downsides.) Plus, there are some groups of people that should avoid intermittent fasting all together. Because IF calls for restricting calories during some hours of the day, it’s not for everyone. And it could be particularly harmful if you fall into any of these categories:
Anyone with a history of an eating disorders such as anorexia or bulimia: Intermittent fasting calls for ignoring or pushing past certain hunger cues, which can trigger a complicated relationship with hunger and food for those with eating disorders or predisposition to eating disorders.
Pregnant or breastfeeding women: Although there hasn’t been sufficient research on pregnancy or breastfeeding and fasting, these conditions generally require increased caloric and nutrient intake and are therefore not considered compatible with intermittent fasting.
People who are underweight: If you’re underweight or have trouble gaining weight, then intermittent fasting is not recommended since those who fast often consume fewer calories than those who don’t.
Extreme exercisers: Expert opinions vary when it comes to exercising in a fasted state. Some believe working out before eating might result in a little extra fat burning, but that it could also lead to diminished performance, weakness, or instability.
Given how much buzz is out there about intermittent fasting these days, it’s no surprise that many people are exploring it as a potential new path toward better health and positive aging. Does that mean you should try it? Well, that depends on if you’re ready to let go of the idea that three meals a day, served morning, noon, and night, is the only way to eat. If you’re ready to start an IF plan, then talk to your health care provider to see if intermittent fasting could be right for you.
Nighttime Eating and Obesity: The Journal of Obesity. (2019.) “The Association of Having a Late Dinner or Bedtime Snack and Skipping Breakfast with Overweight in Japanese Women” https://www.hindawi.com/journals/jobe/2019/2439571/
Intermittent Fasting and Weight Loss: Molecular and Cellular Endocrinology. (2015.) “Do intermittent diets provide physiological benefits over continuous diets for weight loss? A systematic review of clinical trials.” https://www.sciencedirect.com/science/article/abs/pii/S0303720715300800
Nutrition Recommendations for Pregnant Women: Medical Clinics of North America. (2016.) “Nutrition Recommendations in Pregnancy and Lactation.” https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5104202/
Intermittent Fasting and Alertness: Annals of Thoracic Medicine. (2018.) “The Effects of Diurnal Intermittent Fasting on the Wake-Promoting Neurotransmitter Orexin-A” https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5772108/
Health Benefits of Intermittent Fasting: The New England Journal of Medicine. (2019.) “Effects of Intermittent Fasting on Health, Aging and Disease.” https://www.nejm.org/doi/full/10.1056/NEJMra1905136