12 Burning Questions About Intermittent Fasting, Answered
Become an expert in everything related to IF: how it works, why it’s beneficial, who should avoid it, and what to know before you start.
Forget counting calories or swearing off carbs — the latest diet fad doesn’t put limitations on what you eat. Rather, it focuses on when. This way of eating is called intermittent fasting (IF), and in the past several years it has risen through the ranks of popular diets, until it’s now one of the most-looked-up diets on Google, with hundreds of thousands of searches on average each month.
What’s the reason for its popularity? “It seems a lot easier for some people. Traditional lifestyle changes tend to rely a lot on calorie counting or point watching or rules, and for some people — especially people with busy lives or who feel pulled in a lot of directions — that feels like a lot of work and effort. So for this, where they literally have to do nothing but skip a meal, it's just a lot easier for them to maintain,” says Elizabeth Lowden, MD, a physician in Warrenville, Illinois, who is board certified in obesity medicine, endocrinology, diabetes, and metabolism, and internal medicine.
Here, we explore the ins and outs of IF and answer all the questions you’re probably asking.
1. What Is Intermittent Fasting, and How Is the Diet Different From Starvation?
IF is a way of eating that calls for alternating between fasting (or significant reduction of calorie intake) and eating at specific times, according to Johns Hopkins Medicine. It’s different from other diets in that it’s not about eating specific foods. IF is not about depriving yourself, either. Rather, it’s about eating your meals during a certain time frame and fasting for the rest of the day and night.
2. What Is the History of Intermittent Fasting (and Fasting in General)?
Fasting has been around since ancient times and has predominantly been practiced within religions, according to a study published in the Journal of the Academy of Nutrition and Dietetics. But the version of IF that’s talked about today arose in the past decade or so. According to Harvard Health, IF became more popular around 2012 when the documentary Eat, Fast, and Live Longer aired. The Journal of the Academy of Nutrition and Dietetics study says many books on the topic were published around that time as well, including 2013’s The Fast Diet, which added to the buzz. Research followed. “Over the past five years, rigorous research has shown the remarkable benefits of intermittent fasting, which is behind this sudden interest,” says Sara Gottfried, MD, the Berkeley, California–based author of Brain Body Diet.
3. How Does Intermittent Fasting Work?
There are a few different versions of IF (outlined below), but each follows the basic premise of certain periods of time during the week (or day) meant for eating and for when food is limited (or avoided entirely), according to Harvard Health.
4. What Are the Different Types of Intermittent Fasting?
The following are the most popular.
This method calls for 16 hours of fasting and 8 hours of eating during the day. Followers of this method usually skip breakfast and eat between the hours of 11 a.m. and 7 p.m. or noon and 8 p.m. The rest of the night and morning is spent fasting. Dr. Lowden says this approach is commonly referred to as “time-restricted eating.”
This involves limiting your calories on fasting days and eating normally on other days. For instance, you might severely restrict calories on Monday, Wednesday, and Friday, and then eat normally on the other days. There are various calorie limits for fasting days, with some calling for 0 calories and others allowing up to 600 calories per day, according to a review published in December 2021 in JAMA Network Open.
A very low number of calories (around 400 to 500) is allowed on the two nonconsecutive “fasting” days of the week. The five other days have no eating restrictions, per Harvard T.H. Chan School of Public Health.
5. Can Intermittent Fasting Help You Lose Weight?
The short answer: probably. “IF gets a lot of press as a weight loss tool, and I recommend it in my practice for weight loss and weight management,” Dr. Gottfried says. It’s linked to weight loss because significant stretches of time between meals force the body to use the fat stored in cells for energy, according to Harvard Health. Insulin levels decline through this process as the body burns fat.
Lowden believes that any weight loss really comes down to calorie restriction. “Overall, people tend to consume fewer calories in a smaller window of time compared with eating all day, and that's what leads to weight loss,” she says. A study published in June 2018 in Nutrition and Healthy Aging involving 23 obese adults found that study participants took in about 300 fewer calories per day when participating in the 16:8 approach to IF. The National Institute on Aging notes that IF may work because there’s less time for eating, so you naturally take in fewer calories each day.
Versions of IF that restrict eating after a certain time, say 7 p.m., also eliminate nighttime eating, which has been shown to contribute to metabolic syndrome and obesity, according to a study published in December 2018 in BMC Public Health.
Some critics, however, say the amount of weight loss to expect from IF isn’t any more significant than what you’d see with other calorie-restrictive diets. A study published in the American Journal of Clinical Nutrition in November 2018 found that a diet that cut calories by 20 percent resulted in a similar amount of weight loss to the 5:2 version of IF after one year. Still, IF may be a good option if you find it easier to stick to than other diets.
6. What Are the Touted Benefits of Intermittent Fasting, and Are They Legit?
Here are some of the proposed benefits of IF.
Boosted Weight Loss
A review published in Current Obesity Reports in June 2018 found that most research on IF has supported its link to weight loss, and the data suggest that it can result in between a 5 and 9.9 percent loss of body weight. The 2018 study published in Nutrition and Healthy Aging suggested that alternate-day fasting may produce greater weight loss than time-restricted eating, but that alternate-day fasting may be harder to stick with than time-restricted eating. Ultimately, more research is needed on whether IF can result in sustainable weight loss.
A Lengthened Life
A study published in March 2020 in Cell used a lab model to analyze markers of cell aging, and suggested calorie restriction may slow the aging process by reducing inflammation in the body. Nonetheless, the study is preliminary; the same findings haven’t been shown in humans.
Reduced Insulin Resistance
This condition is the hallmark of type 2 diabetes, and overweight increases the likelihood of insulin resistance, according to the Centers for Disease Control and Prevention (CDC). IF can help with insulin resistance by reducing the number of calories consumed overall, per a study published in Nutrients in April 2019.
Improved Heart Health
A study published in Nutrition Journal found that IF helped study participants lose weight, lose fat, and lower their cholesterol levels, leading researchers to conclude eating this way may help people lower their risk of coronary artery disease.
Healthier Metabolic Markers
“What we do know is a lot of those metabolic parameters respond to weight loss in general,” Lowden says. “No matter how you lose the weight, you'll have decreased visceral (belly) fat, decreased fasting blood sugars, decreased triglycerides, blood sugar, all those things.” Lowden says this has been shown in a few animal studies, but “a lot of the things you'd physiologically expect from a longer fast time aren't necessarily panning out in population studies,” she says.
Lab research has shown that IF may bolster cognitive function. A study published May 2021 in Molecular Psychiatry that was performed on mice found IF was 10 percent better than traditional daily calorie restriction at enhancing long-term memory. This was an animal study, and it’s unknown whether humans would experience the same results. More research is ongoing.
7. Who Shouldn’t Try Intermittent Fasting, Because of Safety Concerns?
In general, health experts recommend that the following people avoid IF.
People With Type 1 Diabetes, and Those With Type 2 Who Are Taking Insulin
IF may improve insulin sensitivity, and therefore may be beneficial for people with type 2 diabetes, but it can also be risky for people on diabetes medications associated with hypoglycemia (low blood sugar), such as insulin and sulfonylureas such as glyburide, according to a study published in April 2019 in Nutrients. IF isn’t recommended for people with type 1 diabetes, who rely on insulin. If you have type 2 diabetes and are curious about the diet, be sure to consult your healthcare team first.
People With Other Chronic Diseases
Not much is known about how fasting will affect many chronic diseases, including diabetes, but negative side effects such as dizziness and nausea may be more pronounced for these people, according to the 2019 Nutrients study. “You have to be careful when you have medical issues that could be worsened by not eating regularly,” Lowden says. If you take medication for blood pressure or heart disease, you may be more prone to electrolyte imbalances that come with fasting, so fasting may not be recommended, per Harvard Health.
Those with a body mass index (BMI) of less than 18.5 are advised against any weight loss diet, including IF, according to a study published in Nutrients in March 2019.
Anyone With a History of Disordered Eating
IF may encourage an unhealthy relationship with food, according to Harvard T.H. Chan School of Public Health. A study published in June 2019 in Current Obesity Reports notes that some people may be tempted to use the end of their fast as an excuse to binge on unhealthy foods.
Fasting can increase the risk of cardiovascular disease, stroke, and arrhythmia among elderly people, according to the March 2019 Nutrients study.
Women Who Are Pregnant or Breastfeeding
That same study indicated that breastfeeding isn’t the best time to fast or reduce calorie intake, since women need an extra 300 to 500 calories per day to keep up their energy level and milk production, according to the Mayo Clinic.
Also, people who need to take medication with food should eat at regular intervals so they don't miss a dose, says Harvard Health.
It’s a good idea for everyone — whether you have any of the conditions listed above or not — to check with a doctor before starting a fast, per Johns Hopkins Medicine.
8. Is It Good or Bad to Exercise While Intermittent Fasting?
According to Harvard Health, one theory on the pro-exercise side is that exercising in a fasted state may make you burn fat. Lowden explains that the body needs sugar or some sort of energy to perform well while exercising. Normally, the energy comes from sugar molecules, which are stored as glycogen in the liver. “If you start exercising, you’re more likely to deplete those stores, and then your body has no choice but to go into a more anaerobic breakdown to give you the energy you need,” she says. Instead of burning through sugars that aren't available, your body is forced to burn through another energy source: fat.
But you may not have enough energy to exercise as intensely as you normally would. “Peak performance or even feeling good while exercising is much easier if you eat,” Lowden says.
9. What’s the Best Way to Manage Hunger While Fasting?
You will likely feel hungry as your body adjusts to IF, but Gottfried says it will adjust. “From my own experience and feedback from my patients, it gets easier,” she says. According to Harvard, research has found that IF doesn’t increase overall appetite. Gottfried says the 16:8 diet (or some variation thereof) seems to be the easiest for most people to integrate into their lives without feeling too hungry.
10. What Side Effects Can I Expect on an Intermittent Fasting Diet?
For many people, transitioning to eating this way is not easy. Per the April 2019 Nutrients study, IF can lead to migraine, dizziness, nausea, and insomnia. It can also make people feel hungry and weak in general, limiting their activity throughout the day.
11. What Is the Best Way to Get Started on a Fasting Diet?
In addition to considering your health goals, follow these steps before diving into IF.
Talk to Your Primary Care Doctor
He or she can determine if this eating style would be beneficial for your body. “If you have questions about what is right for you, it's important to talk to healthcare professionals,” Lowden says. “Everybody is an individual, and your specific doctor might have preferences about what you do and don't do based on your specific circumstances.”
Choose the Best Type of IF for You
For instance, if you often socialize late at night, 5:2 likely will be a better fit than 16:8. If you decide to try 16:8, Gottfried says to ease into it. “I recommend starting slowly at first and ramping up, with a 12-hour fast and a 12-hour eating window, then move to a 14-hour fast with a 10-hour eating window, and from there to 16:8,” she says. She says to keep in mind that people have different fasting windows. “Some like to eat at 10 a.m. and stop at 6 p.m.; others prefer to wait until 12 p.m. and stop eating at 8 p.m.,” she says. “Find what works for you.”
Make Sure You Have Water Handy for Proper Hydration
The 2019 Nutrients study suggested drinking plenty of water during the day to reduce your risk of dehydration and to help replace fluids that you normally would source from foods.
Limit Physical Activity
It’s also a good idea to limit your activities during your fasting windows until you know how your body will react, Gottfried says.
12. What Is the Best Way to Break a Fast and Begin Eating Again?
Don’t take the end of your fast as an excuse to go wild with unhealthy foods — that’ll undermine the potential success of the diet. “The principles of healthful eating and breaking a fast are the same whether or not it's a normal overnight fast or time-restricted eating,” Lowden says. Focus on breaking your fast with a healthy, balanced meal filled with lean proteins, healthy carbohydrates, and healthy fats.
Pay particular attention to protein, especially if you have diabetes. “In order [for people with diabetes] to maintain normal sugar levels and avoid worsening insulin resistance, we always recommend eating a form of protein with every meal, particularly when you're breaking a fast,” Lowden says. Protein doesn’t break down into glucose as efficiently as carbs, so it has a slower, less-immediate effect on blood sugar levels, according to Diabetes.co.uk.
Gottfried says a healthy source of carbs will restore depleted glycogen levels when you break your fast. She recommends a Mediterranean-style meal with 40 percent carbohydrates, 30 percent protein, and 30 percent fat.
Lowden says your timing when breaking the fast may make a difference as well. “There are studies that show people who eat their calories later in the day — even if it’s the same amount of calories — weigh more than people who do that earlier in the day,” Lowden says. According to an earlier study published in Nutrients, sleep-restricted adults with late bedtimes are more susceptible to weight gain because foods eaten at night tend to be higher in fat than those eaten earlier in the day. Take that into consideration when you’re determining your eating window.