Your Definitive Guide To Intermittent Fasting
It seems like we can’t go more than a day without seeing an article on intermittent fasting (IF) and its purported health benefits—which can sound pretty darn miraculous (improved cognitive function, weight loss, and extended lifespan to name a few). But with so many popular iterations of IF out there, each with potential perks and pitfalls, you may find yourself wondering which option—if any—is right for you, or if it’s even safe.
The truth is, this area of science is still young—many IF studies have been done on rodents or small sample sizes of people—and there’s a lot that needs to be verified with further research. But that said, many reputable experts we’ve spoken to do see a potential benefit to various forms of intermittent fasting for most people—when done responsibly and in a way that’s in tune with your goals and current health status. Here, we outline the options (with some important caveats) and answer all your burning IF questions.
What is intermittent fasting?
According to Jason Fung, M.D., fasting expert and author of the book The Complete Guide to Fasting, when it comes to fasting "There's actually infinite variability. It can be any time. Any time that you don't eat—that's fasting. It's the flip side, the B side of eating. It's really that simple." But it can start to feel complicated the more you think about the logistics: Is fasting the same thing as intermittent fasting? Am I allowed to drink water or other zero-calorie beverages? Is fasting actually safe for humans over the long term?
To clear things up, let's start with some definitions. There are a variety of intermittent fasting options (some vastly different than others), so familiarizing yourself with the popular terminology is key to finding the best option for you:
Time-restricted eating (16:8 fasting, 18:6 fasting, and 20:4 fasting)
While intermittent fasting, or IF, is a term used to describe a lot of different protocols, it’s most often used to describe something called time-restricted eating (or TRE)—meaning, restricting the period of time you eat to a set number of hours each day. Time restricted eating, also called time restricted feeding in research settings, typically consists of confining all your eating to a 12-hour, 10-hour, 8-hour, 6-hour, or 4-hour window, and fasting the remainder of the day.
Many experts view a 12-hour eating, 12-hour fasting window (think: eating breakfast at 7 a.m. and wrapping up dinner by 7 p.m.) as a great, safe place to start for anyone. “This plan is a piece of cake, since you’ll hopefully be sleeping for about 8 of those 12 hours,” says Vincent Pedre, M.D., a functional medicine physician and gut health expert who often recommends IF to his patients. And, at least in animal studies, it shows promise for weight loss compared to a shorter fasting window.
One of the most popular versions of time-restricted eating is the 16:8 fasting plan, which consists of a daily 16-hour fast and confining your eating to an 8-hour window. So, you could start eating at noon and finish your last meal by 8 p.m.; or, you could shift your window earlier, having breakfast at 7 a.m. and finishing your last meal by 3 p.m. Studies have shown that a 16:8 fasting window is associated with weight loss and decreased fat mass (while maintaining muscle mass).
Also popular, but a bit more intense, is the 18:6 fasting plan, which is a daily 18-hour fast where you confined your eating to a 6-hour window. If your goal is weight loss and you’ve experienced a plateau on a 16:8 plan, this is the logical next step. While more research is needed, an 18:6 fast likely helps your body burn through stored carbohydrates (glycogen) faster so you start burning fat (in the form of ketones) for fuel, and some believe it may be enough to activate autophagy—a cellular clean-up process that’s associated with longevity.
The most restrictive of the popular TRE regimens is the 20:4 fast (sometimes called the “warrior diet”), which is a daily 20-hour fast where you confined your eating to a 4-hour window. This essentially breaks down to one meal a day and is not for beginners—you need to work your way up. Compared to a 16:8 or 18:6 fast, it’s speculated that you will burn more fat, lose more weight, and experience greater autophagy on a 20:4 diet, although more research is needed comparing all of these TRE plans.
Where to place your TRE window.
The ideal placement of your eating and fasting window will likely depend on a number of factors—work schedule, social obligations, fitness routine, and simply what feels best for your body—but a growing body of research seems to suggest that an earlier eating window may be better. A small but rigorously controlled randomized crossover study from 2019 found that when participants ate between 8 a.m. and 2 p.m., they fared much better than when they ate the same three meals on an 8 a.m. to 8 p.m. schedule.
The perks? For one, they had lower blood glucose levels during the day and overnight, lower insulin levels, and an ideal cortisol pattern (with levels higher in the morning and lower at night)—all of which suggests an improvement in circadian rhythm, says fasting and longevity expert Peter Attia, M.D.. More surprisingly, though, says Attia, was that after just four days, this earlier eating schedule increased the expression of the SIRT1 gene (associated with longevity and healthy aging) and the LC3A gene (a biomarker of autophagy).
Sometimes called a “Monk Fast,” this more advanced version of intermittent fasting typically consists of skipping one entire day of eating. For example, you finish up dinner around 7:30 p.m. on day one, skip food entirely on day 2, and break your fast at 7:30 a.m. on day three. For obvious reasons, this type of fast may be very difficult to sustain and is not for beginners. In Fung’s clinic, he typically recommends up to three 36-hour fasts per week for type 2 diabetes (always under a physician’s supervision). “Since type 2 diabetics have more insulin resistance, the longer fasting period is more effective than more frequent shorter fasting periods, although we have had good results with that too,” he says. Additionally, because less restrictive fasts have been shown to upregulate markers of autophagy, this process may be induced with a 36-hour fast as well. However, people with diabetes should be careful intermittent fasting so they don’t become hypoglycemic.
Also under the umbrella of intermittent fasting is alternate-day fasting (ADF). ADF is just how it sounds: You only eat every other day (which essentially works out to a 36-hour fast, followed by a 12-hour eating window). While some purists only consume water, herbal tea, and moderate amounts of black coffee on fasting days, others employ the 25 percent rule. In this version, you consume 25 percent of your normal caloric intake on fast days.
While it shows promise for weight loss and reducing inflammation, alternate-day fasting is a more extreme approach to IF that may be hard to sustain over the long term. “In my practice, I’ve found this tricky and definitely for more advanced intermittent fasters,” says Pedre. Women in particular should proceed with caution. One study found that after three weeks of alternate-day fasting, women actually had worse blood sugar control (while men did not), and some experts believe that these intense forms of fasting may interfere with optimal women’s hormone balance (more on that later).
A slightly easier variation of ADF, the 5:2 plan allows you to eat normally for five days every week while eating only 500 to 600 calories on the other two days. Among its benefits, one small study on 24 women found the 5:2 diet induced biochemical changes in breast tissue that researchers interpreted as potentially protective against breast cancer (the women also lost weight).
Those calorie-restricted days might be more of a challenge, though, especially if you have a hard time controlling yourself when you eat, says Pedre, and keep in mind that "on these restricted days, the type of calories consumed is just as important as the number of calories." So, you should aim to eat lots of protein, healthy fats, and vegetables.
While not a widely established protocol like these others, Benjamin Horne, PhD, a renowned expert on fasting and cardiovascular disease, suggests that simply not eating for one day (24 hours) once a month may be one of the simplest, most effective ways to reap fasting’s benefits. “We think that long-term fasting of about one day, once a month, over a period of decades is [enough to] activate beneficial mechanisms,” Horne said in a recent news release. Mechanisms that may help prevent chronic disease.
In two 2019 studies authored by Horne, researchers examined how fasting one Sunday a month affected the health of 2,000 Mormons (for whom this type of fast is typical) who had undergone a cardiac catheterization procedure. In the first study, “routine fasters” had a 45% lower mortality rate than non-fasters after four years; and in the second study, using the same data, these fasters had a 71% lower rate of developing heart failure. While data is limited overall, we’re excited to see more research on once-a-month fasting.
Prolonged fasting and fasting mimicking diets, while still fasting protocols, are not exactly the same as intermittent fasting. According to longevity expert Valter Longo, Ph.D., "prolonged fasting and fasting-mimicking diets are very different since they last two or more days (usually four or more) and they are not intermittent—meaning that they can be done when needed and not at particular intervals." Attia, for example, does a 7-day fast four times per year. What’s the point? While any type of fasting is likely to promote healthy aging, these longer fasts may be particularly beneficial for targeting longevity and cellular repair mechanisms such as autophagy.
Can’t forgo food that long? The ProLon 5-Day Fasting Mimicking Diet, developed by Longo at the USC Longevity Institute, essentially mimics the effects of fasting without entirely eliminating food. On the plan, you decrease calories to around 1,100 on the first day and then to around 800 the next four days (while eating specific, lower-carb, nutrient dense whole foods). The point of this program is to "allow the body to enter a full ketogenic mode, meaning it will break down and [recycle] damaged cells and cellular components, activate stem cells, and preferentially decrease visceral and abdominal fat."
More research is needed on prolonged fasts, and they’re likely not appropriate for all. Prolonged fasts that forgo all food should be done under the guidance of a medical professional, especially if you have any health issues.
Which type of fasting is right for you?
When it comes to IF, there is no one plan that works for everyone. But certain plans are probably going to be more sustainable and safe over the long-term than others. While more research is needed, certain time-restricted eating plans, once-a-month fasting, and 5:2 fasting may be easier to maintain and less likely to cause side effects than more restrictive plans like ADF.
“Figuring out what works for you takes some trial-and-error, and a professional [like a registered dietitian] can help you tweak and troubleshoot,” says Pedre. “If you go it alone, I suggest starting with a 12-hour daily fast and easing into a 16-hour fast. Leave alternate-day and 20-hour fasts to people who've done it for a while.”
To help you evaluate which plan is right for you, identify your goals:
- Are you looking to generally promote healthy aging, curb cravings, have more energy, and maybe lose some weight? Time-restricted eating may be a good start—with a fasting window on the shorter side that you can gradually extend (reign it back if you start to feel crappy!).
- Want to promote overall health, but don’t want to commit to a daily fasting regimen? Consider once-a-month fasting.
- Already a time-restricted eating pro? Consider switching things up with an alternate-day fast if you feel pretty good but think you may have hit a plateau with your benefits.
- Obsessed with the idea of boosting longevity and up for a bit of a challenge? Ask a medical professional (like a functional or integrative physician, or a dietitian) if you’re a candidate for a prolonged fast and if they can guide you through one.
There are also some people who should likely avoid intermittent fasting and prolonged fasting altogether, including pregnant and breastfeeding women, those with a history of disordered eating, and anyone (particularly women) with excessive levels of stress in their life—more on that below.
Does intermittent fasting help you lose weight?
Short answer: probably. While there are many benefits of intermittent fasting, one of the most strongly established perks for a variety of IF plans (both anecdotally and research-backed) is weight loss. In fact, alternate-day fasting has been shown to be as effective as traditional calorie restriction diets, while being better at preserving muscle mass; and a 2015 systematic review of six studies on IF for weight management reported an impressive average weight loss of nearly 3% after the first month and 9% after six months.
There are a variety of possible reasons for this weight loss benefit. For one, periods of fasting have been shown to reduce insulin levels, which may push you into a ketogenic state where you start to burn stored fat for fuel. This can help stabilize blood sugar and have an appetite suppressing effect. Tapping into your fat stores while fasting can also make your metabolism more “flexible,” says Eva Shelub, M.D., meaning, you can more easily switch between burning glucose (sugar) and fat for fuel. “The longer the fasting period, the more the body will eventually revert to lipolysis, or the breakdown of fat to fatty acids to use for fuel,” she says.
Additionally, intermittent fasting may simply help you eat fewer calories. Studies show that people who intermittent fast unintentionally decrease total daily caloric intake by 300-400 calories, depending on how big the fasting window. But, as you’ll learn in the next section, the health benefits of intermittent fasting extend well beyond weight loss.
What are other health benefits of intermittent fasting?
The health benefits of intermittent fasting will vary a bit depending on the specific type of fasting regimen you choose, and more robust human data is needed before we can claim any benefits with certainty. That said, these are some of the most promising science-supported perks to date:
Several human studies demonstrate fasting’s effectiveness at fighting oxidative stress and inflammation—two drivers of aging and chronic disease. In one study, people fasting for Ramadan (no food during daylight hours for a month straight) experienced significant reductions in inflammatory markers of inflammation including interleukin-6 and C-reactive protein. Another study on obese patients with asthma found that an 8-week alternate-day fast noted “striking reductions in markers of oxidative stress” and inflammation.
Improved heart health
In the same Ramadan study mentioned above, fasting individuals also experienced a drop in homocysteine levels—a major cardiovascular disease risk factor—and improvements in their total cholesterol to HDL cholesterol ratio. Additionally, a new study found that people who regularly fasted one full day a month had a significantly lower risk of heart failure, and several studies suggest intermittent fasting may help lower blood pressure.
Upregulation of autophagy + longevity genes
Intermittent fasting has been shown to extend the lifespan and reduce age-related diseases in animals. And a recent study from 2019 found that people who followed a time-restricted eating plan in which all meals were consumed between 8 a.m. and 2 p.m., multiple genes associated with longevity (including one that upregulates autophagy) were enhanced.
Balanced blood sugar and reduced insulin levels
In the same time-restricted eating study as above, participants also experienced lower blood glucose levels during the day and overnight, lower insulin levels, and an ideal cortisol pattern (with levels higher in the morning and lower at night)—all of which can help keep cravings at bay, make you less likely to store fat, help prevent type 2 diabetes.
Growth hormone gains
In addition to insulin, another hormone that is positively regulated with fasting is human growth hormone—studies have shown that after two or three days of fasting, HGH can double, but less intense fasts may have a positive effect too. “Growth hormone helps preserve muscles and bone density, and helps us use fats for fuel, but unfortunately, secretion decreases steadily with age,” says Shah. “One of the most potent stimuli to growth hormone secretion is fasting.”
Improved gut health and diversity
Studies on patients with irritable bowel syndrome have found that fasting can be an effective way to ease symptoms like diarrhea and abdominal pain; and recently, a fasting mimicking diet was shown to reduce inflammation and completely reversed some irritable bowel disease symptoms. Additionally, researchers say intermittent fasting may elicit changes in the gut that are protective against metabolic syndrome.
Improved brain health
Anecdotal claims abound regarding intermittent fasting’s ability to boost mental energy, focus, and cognitive function. And some researchers suggest that IF can prolong the health-span of the nervous system, potentially minimizing risk of neurodegenerative diseases like Alzheimer’s and Parkinson’s, by regulating key pathways that help neurons cope with stress and resist disease. Other research suggests the ketones generated during a fasted state have a neuroprotective effect, guarding brain cells against degeneration.
How does intermittent fasting work?
How can one simple practice possibly do all this? There are a variety of mechanisms that are likely at play, and too many to write about here, but here are a few key mechanisms that help explain some of the perks:
Periods without food may push you into a ketogenic state, as mentioned above, which not only helps with weight loss, cravings, and blood sugar regulation, but may benefit the brain as ketone bodies have a neuroprotective effect. This may also help regulate certain hormones. There’s some debate about how long it takes to enter ketosis while intermittent fasting, but research suggests that—depending on a person’s physical activity levels—12 to 24 hours of fasting is typically enough to decrease liver glycogen levels (a form of glucose that serves as stored energy for your body) to the point the body starts using ketones as an energy source.
Fasting gives your body a break from digestion, which helps it focus on other things. Specifically, the migrating motor complex (MMC), a pattern of electromechanical activity in the digestive system, that works in between meals and during fasts. It acts as an intestinal house cleaner, sweeping residual, undigested material through the digestive system.
Fasting upregulates autophagy, a cellular process that researchers believe plays a role in modulating aging, improving brain health, decreasing inflammation, and improving immunity. Autophagy refers to a “self eating” process by which cells disassemble and clean out unnecessary or dysfunctional components—organelles, proteins, and debris that are no longer efficient. Autophagy also encompasses mitophagy, the removal of damaged mitochondria. This improves the functioning of cells throughout the body and promotes all-around optimal functioning. According to integrative neurologist Ilene Ruhoy, M.D., Ph.D, most age-related neurodegenerative disorders like Alzheimer's are associated with the accumulation of misfolded proteins or pathologic proteins—so upregulating autophagy may help combat them.
Like ketosis, researchers suspect that there’s a significant uptick in autophagy once you’ve burned through all your glycogen. While some experts debate how long you have to fast before autophagy ramps up—Attia speculates three days while integrative physician Amy Shah, M.D., says as quickly as 14 hours—it likely depends on the individual, and a number of factors such as their weight and activity levels (exercise also upregulates autophagy). One study on humans showed that markers for autophagy were present after 18 hours of fasting (on an 18:6 time-restricted eating plan), but as we really have no direct way to measure autophagy in the body
What is the best way to break a fast?
While intermittent fasting is not a diet per se, it is still important to emphasize healthy foods during your eating window, especially what you eat to break your fast. It's completely counterproductive—and even dangerous for insulin and blood sugar balance—to fast and then immediately binge on unhealthy foods. In other words, don't break your fast by going through the drive-thru or eating a bunch of carbs like white bread or sugar. "A meal too high in carbohydrates will result in an unpleasant glucose crash," warns Attia. Your best bet: something with protein, healthy fats, and vegetables.
That said, what exactly counts as "breaking" your fast, anyway? Can you indulge in a splash of almond milk in your coffee, or will that counteract everything? Can you even have coffee? Many fasting protocols allow a small amount of calories, typically 50, to be consumed during the fasting window, but whether or not you should probably "depends on your objective," says Attia. Let's break down some of the specifics of what breaks a fast:
Does tea or coffee break the fast?
Calorie-free beverages such as water, and extremely low-calorie beverages like herbal tea and black coffee, are often permitted during fasting protocols. In fact, many studies of intermittent fasting have allowed these beverages as part of the fasting window. "If the purpose of your fast is losing weight, I don't think black coffee is impairing that, particularly," Attia explains. While skipping the milk or cream is preferable, a splash probably won't hurt. Caffeine may even help you stick to your fast, as it is reportedly an appetite suppressant.
When it comes to fasting for longevity, definitely hold the cream. According to Horne, while some mechanisms behind fasting's benefits, like ketosis, remain active with the consumption of a small number of calories (especially if they’re from fat), autophagy may not. "In humans, it appears that autophagy does not remain as active when any food is consumed," he says. There is some evidence to suggest consuming coffee itself may enhance autophagy, but more research is needed in humans to confirm this.
If you are fasting as a way to rest your digestive system, it is important to note that coffee can stimulate gastric acid secretion and counteract some of fasting’s healing properties. Therefore it is not advisable for this purpose. But most herbal tea, with the exception of mint, is likely okay.
Do supplements break the fast?
While there is not a lot of specific research on this topic, it’s unlikely that supplements will negate fasting’s benefits (except, perhaps, downregulating autophagy). To be safe, though, simply take your supplements during your eating window. This is a good idea anyway since some supplements, especially fat-soluble vitamins like A, D, E, and K, will be better absorbed with a meal. And with prolonged fasts, Attia personally recommends holding off on supplements until it’s over. If you are taking prescription medication, you need consult with your doctor before starting a fast (particularly fasts that exceed 24 hours, which, for example, will almost definitely require you to adjust your insulin intake if you have diabetes).
Should you work out fasted?
This is ultimately a personal preference, and not required to reap the benefits of fasting. But, according to a recent study, working out before your first meal could provide added benefit—at least for men.
The study found that men who worked out before breakfast burned double the amount of fat as the group who exercised after breakfast. This increase in fat burning is super important, but not for the reason you might think—both of the exercise groups in this six-week experiment lost the same amount of weight.
The fasted workout was significantly more beneficial in terms of balancing blood sugar, which is crucial for mitigating health concerns like diabetes and heart disease. The reason these men were able to burn more fat during their workout is that their bodies were better able to respond to insulin. "This suggests that performing exercise in the overnight-fasted state can increase the health benefits of exercise for individuals, without changing the intensity, duration or perception of their effort," said study author Gareth Wallis, Ph.D..
More research needs to be conducted on women, but if working out fasted feels good, have at it. It's worth noting, however, there’s definitely an adjustment period to working out on an empty stomach. If you’re not used to it, your blood sugar dip and you may experience side effects of hypoglycemia (feeling faint, dizzy, sweaty, etc). So when you’re first starting out, skip the HIIT workout and consider some yoga or a light jog. As with everything fasting related, listening to your body is key and white-knuckling it is not advised.
Is fasting safe for women?
While intermittent fasting has many benefits, there are conflicting opinions about whether women should adopt this eating pattern. Some experts argue that intermittent fasting may negatively impact female hormones and fertility. And, as mentioned above, an alternate-day fasting study found that women’s blood sugar actually worsened after three weeks on the plan.
So why could intermittent fasting (particularly more intense forms like ADF) disproportionately cause issues for women? While more research on women is needed to really tease this out (most fasting research has been done on male mice!), one reason may be that "women are hardwired to be more sensitive to external stressors, and intermittent fasting is a small hormetic stressor," says Shah.
Normally, a little bit of this hormetic, or pulsatile, stress is actually a good thing. In fact, many health-promoting practices (e.g., exercise, a nutrient-dense keto diet, using a sauna) are also considered hormetic stressors and, as such, help the body become more resilient over time. But, when your allostatic load (the cumulative impact of stressors, both physiological and mental) becomes too great—say, for example, you're chronically stressed, under-sleeping, practicing intermittent fasting, and following a low-carb diet—your body is simply overtaxed and hormonal imbalances can result.
This is especially the case if women are underweight or don’t have much body fat, as hormones are produced from fat. “What happens if you extend your fast and you don't have ample body fat is that your body kind of goes into freak-out mode,” says integrative and functional medicine dietitian Ali Miller, R.D.. “You actually make norepinephrine or adrenaline, because the body's trying to survive. It’s sending you signals that it's starving, because it is.”
All of this may cause some women to experience, "ovarian dysfunction," says Shah, such as missed periods and trouble conceiving. While this hasn’t been proven in humans, a study on female rats found that after two weeks of alternate-day fasting, the rats stopped having menstrual cycles and their ovaries shrunk. When your body is in a stress-induced state, hormones in the thyroid (Ft3) drop. The body is protecting you from what it thinks is starvation mode.
This doesn't mean you need to skip intermittent fasting—anecdotally, many women have experienced great benefits—but it should make you think twice about more intense plans or being so dogmatic about following “the rules.” Many experts suggest women start with a shorter fasting window, and if you feel okay, work your way up. Shah suggests checking in with yourself—if your energy levels are good, your periods are still regular, your hunger isn't out of control, and you're sleeping well, then you are probably okay to continue intermittent fasting.
Miller personally practices a 16:8 fasting plan, eating her first meal around noon, but because she already had multiple stressors in her life, she tweaks things to make it more sustainable. “With my current percent body fat, my activity factor, my stress levels, and being a mom of a toddler, I find that I need to blend up some fat with my morning coffee or matcha,” she says, “otherwise my body goes into anxiety mode.”
Are there any risks or side effects associated with intermittent fasting?
Regardless of which plan you choose, there are a few caveats. Fasting for longer periods of time when your body isn’t prepared can yield serious consequences. If you feel lightheaded or weak, or you feel like fasting is impairing your day-to-day activities, always eat something, says Pedre.
There are also some people who should avoid fasting altogether. First on that list? Pregnant and breastfeeding women. “Fasting promotes fat burning, but women in pregnancy are supposed to be creating and storing fat, not burning it," says Felice Gersh, M.D., OB/GYN, who is generally pro-fasting. This could lead to inadequate weight gain of the growing fetus and hormonal imbalances that could cause significant harm. Fasting can also create a brief time of hypoglycemia (very low blood sugar), which is bad for the fetus even if happening briefly. And fasting during pregnancy could lead to reduced milk supply and inadequate nutrient intake for both mom and baby.
Second is anyone with a history of an eating disorder. The goal of fasting is improved well-being, but it can be triggering for anyone with a history of eating disorders, so it may be best to steer clear if that’s part of your health history.
According Pedre, you should also exercise some caution if you have gut issues, food sensitivities, a sleep disorder, anxiety, or chronic stress. Going without food can sometimes trigger your fight-or-flight response, which may exacerbate these issues.
To be safe, talk to your doctor before starting an intermittent fasting plan, especially if you have any kind of chronic health condition.
The bottom line on intermittent fasting.
While more high-quality intermittent fasting studies on humans are needed, the current body of research, the first-hand anecdotal accounts, and the input we’ve gotten from physicians (who either personally practice IF or guide their patients through the process) is strong enough to suggest that this “trend” is here to stay—and that, for many people, it’s worth exploring.
But we can’t emphasize enough how important it is to listen to your body when adopting a fasting regimen. While the potential benefits are significant, so too are the potential risks if you choose a plan that’s too intense or that doesn’t honor your personal biology, your stress levels, and where you are (right now) in your health journey. And remember, "fasting isn’t a magical "cure-all",” says Pedre, “it's simply another tool in your weight-loss and optimal-health arsenal."
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