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. 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. Time-restricted eating, also called time-restricted feeding1 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. “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. 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 loss2 and decreased fat mass3 (while maintaining muscle mass). 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. 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. A small but rigorously controlled randomized crossover study4 from 2019 found that when participants ate between 8 a.m. and 2 p.m., they fasted 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 LC3A5 gene (a biomarker of autophagy). 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. 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 inflammation7, 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 study8 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). 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. “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 release9. 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. 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. 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 dietitian10] 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: That said, these are some of the most promising science-supported perks to date: In one study11, 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 study12 on obese patients with asthma found that an 8-week alternate-day fast noted “striking reductions in markers of oxidative stress” and inflammation. Additionally, a 2020 study13 of a cohort of 1,610 subjects who underwent a long-term fasting program found that blood pressure was significantly reduced, as well as weight and stress scores. Research also suggests that fasting-mimicking diets could make cancer cells more vulnerable to treatment. And a recent study4 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. “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.” Recently, a fasting mimicking diet was shown to16 reduce inflammation and completely reversed some irritable bowel disease symptoms. Additionally, researchers say17 intermittent fasting may elicit changes in the gut that are protective against metabolic syndrome. And some research suggests18 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 research19 suggests the ketones generated during a fasted state have a neuroprotective effect, guarding brain cells against degeneration. 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 systematic review24 of 40 studies on IF for weight loss reported an impressive average weight loss of 7-11 pounds over 10 weeks. 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 intake25 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. Several human studies demonstrate fasting’s effectiveness at fighting oxidative stress and inflammation—two drivers of aging and chronic disease. In one study11, 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 study12 on obese patients with asthma found that an 8-week alternate-day fast noted “striking reductions in markers of oxidative stress” and inflammation. 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: 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 suppressant26. 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 autophagy26, 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 secretion27 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. 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 K28, 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). Several studies29 show decreases in body weight with fasting during Ramadan, while others failed to observe any changes in body weight at all. One study30 found that regular exercise in the fasted state can stimulate adaptations in muscle cells to facilitate energy production via fat oxidation. The same study also concluded that regular fasted training is a useful strategy to stimulate physiological adaptations in muscle that may eventually contribute to improve exercise performance. More research needs to be conducted, 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 may 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. Some experts argue that intermittent fasting may negatively impact female hormones and fertility. While more research on women is needed to really tease this out, 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 cycles31 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.” 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 disturbances, 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. 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.”
