Frequently Asked Questions

About Fasting

Fasting Generally

Who would benefit from fasting and why?

What’s the best way to fast?

Daily Fasting

What is daily fasting, and why would I want to do it?

How long is the ideal daily fast?

Can a longer overnight fast keep you slimmer?

Does it matter when our eating window is?

So are you saying it’s healthier to skip dinner rather than breakfast?

I’ve been eating dinner for decades. If it’s so bad, how come I’m not sick?

Does it matter what I eat during my window?

But aren’t there studies that say time-restricted feeding doesn’t work?

The flawed UCSF TRF study of 2020

The flawed China Southern TRF study of 2022

If you can explain what’s wrong with these studies, why can’t other reporters?

Is there any reason to be concerned about daily fasting?

Is intermittent fasting the same thing as daily fasting?

Are 5:2 fasting and alternate-day fasting healthy?

If you eat in a 5:2 or ADF pattern, can you eat whatever you like?

Prolonged Fasting

What is prolonged fasting?

Why do a prolonged fast?

Which diseases can prolonged fasting reverse?

Which diseases are least likely to be cured by prolonged fasting?

Can prolonged fasting reverse cancer?

So is fasting useless against most cancers?

How do I do a prolonged fast?

So who might be able to do a prolonged fast on their own?

You fasted twenty days on your own, as you recount in The Oldest Cure in the World. Would you do that again or recommend it to others?

What’s the biggest danger of a prolonged fast for a healthy person?

How do I find a clinic that will supervise a fast?

Does health insurance cover a supervised fast?

How long should a prolonged fast be?

How often should I do a prolonged fast?

Are shorter prolonged fasts—fasts of two, three, or four days—helpful?

So should you never fast for two, three, or four days?

What’s the longest I can safely fast?

 
 

Fasting Generally

Who would benefit from fasting and why?

Virtually everyone who wants better health. The best research we have suggests that unless you have an extremely rare disorder that makes it dangerous to go several hours without food, a fast will induce metabolic repairs that will make you healthier and might help you live longer.

 

What’s the best way to fast?

There are two main types of fasting: daily fasting and prolonged fasting. Just about everyone would benefit from a healthy fast each day, and most people would probably benefit from a prolonged fast of a week or more every once in a while. For the majority of people, the prolonged fast should be done under medical supervision, but the daily fast can be done on your own. Let’s take these two types of fasting one at a time.

Daily Fasting

 

What is daily fasting, and why would I want to do it?

Most people don’t realize it, but they already fast every day. When you stop eating overnight (or during the day if you’re a shift worker), you’re fasting until you take your first calories or caffeine the following morning. The crucial question that researchers only recently thought to ask is whether the length of the fast affects our health. What they’ve discovered is that people who fast longer overnight are healthier, evidently because a longer fast gives the body more time to make repairs.

 

Each night the body does a great deal of mending, reconstructing, and tidying up—work that it mostly can’t do during the day because it’s busy processing the nutrients from our meals, firing all the neurons that make up our thoughts, supplying our muscles with the energy to move about, and doing everything else that adds up to our waking lives. So out of necessity the body puts off many of its deepest repairs until night when we’re no longer eating or stirring around.

 

The overnight repairs are important indeed. To take just one example, every night our gut has to patch vast sections of its whisper-thin lining that got abraded as food slid across it during the day. If the lining isn’t fixed, we can get leaky gut syndrome, in which food, bacteria, and viruses leak out of the gut and into the abdominal cavity, where they cause all sorts of havoc. To take another example, our brain has to clear out the waste products of the day’s countless chemical reactions each night. If it doesn’t, we can develop ghastly neurological disorders like dementia. Our bodies make untold numbers of repairs like these night after night to keep us free of cancer, cardiovascular disease, diabetes, kidney disease, and on and on, and when they have more time for repairs, the healthier we are—which is why a longer overnight fast helps.

 

How long is the ideal daily fast?

Most scientists would say at least twelve hours and maybe up to about eighteen hours. Researchers recently discovered that the body doesn’t get to work on most of its deeper repairs until about six hours after we’ve eaten our last calories. They think the body waits because there’s a metabolic cost to switching from eating and processing nutrients to fasting and making repairs. Apparently the body doesn’t want to make that costly metabolic switch if we’re just going to force it to switch back an hour or two later by putting more food in our mouths. So about six hours after our last calories, the body seems to be confident enough that we’re done eating to begin its repairs in earnest, albeit at a slow rate. With each hour that passes, the rate increases, as if the body is gaining more confidence that we’re really fasting and won’t disrupt its work by taking more food. By twelve hours after our last calories, the rate of repair doubles with each extra hour of fasting.

 

Unfortunately, most people rarely go twelve hours a night without food. Studies have found that people in the developed world often eat across fourteen or fifteen hours a day. A typical person who finishes her bedtime snack around 10 p.m. and breaks her fast the next day with coffee at 7 a.m. is fasting only nine hours a night, which means she’s getting just three hours of modest repairs and never reaching the exponential rate of repair. If, however, she took her last food at 6 p.m. and broke her fast at 9 a.m., her fifteen-hour fast would give her nine hours of repairs, three of them in the exponential phase. Scientists think those extra hours of repair will keep her healthier over the course of her life. We don’t yet have long-term human trials to confirm (or disprove) that assumption, but short-term human trials and long-term studies of lab animals point strongly in that direction.

 

Although researchers don’t know for sure how long the ideal daily fast should be, human studies generally show health benefits after about twelve hours of nightly fasting, and the benefits increase markedly with every hour beyond that. Eating windows of as few as six hours have yielded excellent improvements in biomarkers that predict long-term health. My sense from the researchers I’ve talked to and whose studies I’ve read is that most think the ideal daily fast lies around sixteen or eighteen hours, for an eating window of eight or six hours. A few researchers think the ideal fast might be as long twenty hours, but most think cramming an entire day’s worth of food into your stomach in a four-hour window might exact too high a toll on the body. Most TRF trials have an eight- or ten-hour eating window because researchers have found that almost all volunteers can eat comfortably in those times. For the same reason, many researchers themselves eat in an eight- or ten-hour window. I’ve eaten in a six-hour window for a couple of years now and have found it both very easy and, I believe, great for my health, as I discuss in chapter 16 of The Oldest Cure in the World.

 

Can a longer overnight fast keep you slimmer?

Probably. Mostly this seems to be because people who narrow their eating window tend to eat fewer calories—and they aren’t any more hungry than people who graze all day; in fact, studies show they may be less hungry. A longer fast also helps the body burn a little more fat overnight, and the reason is similar to that for overnight repairs. To oversimplify just a little, the body has two modes with regard to fat—fat-making and fat-burning—and it can be in only one of them at a time. During the day, the body is mostly in fat-making mode in order to store fat from the foods we eat. When we stop eating long enough, the body switches to fat-burning mode and burns some of its fat stores. As with the overnight repairs, this burning of fat doesn’t start in earnest until about six hours after our last calories. And just as with repairs, the fat burning starts to increase dramatically after about twelve hours of fasting. The person in my example above who fasts only nine hours a night is not only limiting herself to three hours of repairs and no exponential repairs; she’s also limiting herself to three hours of fat burning and none of the “overdrive” burn. But if she fasted fifteen hours, she’d get nine hours of repair and fat burning, including three hours in overdrive.

 

Does it matter when our eating window is?

Yes, the timing is crucially important, with an earlier window almost certainly healthier than a later one. A great many studies show that our bodies were built to process nutrients most efficiently early in the day and that good health flows from eating in sync with that biology. One reason is that by late afternoon our circadian rhythms force our digestive organs and other nutrient-processing apparatus to slow down. We can of course still eat late in the day or at night, but when we do, the food lingers longer in the stomach and gut, glucose dawdles in the bloodstream, and the nicks and dings that we suffer as a result add up over time. Eating at night appears to be especially damaging, as it disrupts some of the vital overnight repairs I mentioned above. I discuss this at some length in my book.

 

In the best trials we have so far, volunteers have been healthiest when eating more of their calories earlier in the day and taking few or no calories after the late afternoon. The science is still young, and it would be premature to trumpet a specific window with certainty, but the eating windows that have produced the best health results in humans tend to be around 9 a.m. to 3 p.m., or 8 a.m. to 4 p.m. That said, the best window for any particular individual will vary slightly. It depends on factors like when you wake up and, after waking, how long it takes your hormones to get your food-processing machinery to crank into gear. To account for individual variability, scientists usually instruct their volunteers to start eating one to two hours after getting out of bed in the morning.

 

Trials of later eating windows, like noon to 8 p.m., have shown inconsistent benefits, and still later windows, like 4 p.m. to 8 p.m., have shown almost no benefit and possibly some harm.

 

So are you saying it’s healthier to skip dinner rather than breakfast?

Yes. Or rather, move dinner earlier in the day. Most people who practice time-restricted feeding make the mistake of skipping breakfast. I certainly did. But because evolution has primed our bodies to process foods most efficiently in the morning, we skip breakfast at our peril. The best evidence we have says good health flows from putting dinner in the middle of the day, as many people around the world do. I eat dinner around noon or even earlier. Whenever you eat your last meal, it’s unhealthy to snack later in the evening. The Salk Institute’s Satchin Panda, one of the world’s leading experts in the field, says eating at night is one of the most harmful metabolic assaults you can make on your body.

 

I’ve been eating dinner for decades. If it’s so bad, how come I’m not sick?

The damage that comes from eating late, like the damage that comes from smoking a single cigarette, is small and progresses to disease only after doing it many times. A diagnosable disorder can take quite a long time to manifest. That said, people who narrow their eating window often notice within a few days that they feel less sluggish and more alert, just as people who quit smoking often notice right away that they can breathe more easily.

 

But here’s another point to consider: Are you sure you aren’t sick? Do you, like half of all American adults, have high blood pressure? How about high cholesterol? Although many doctors will tell you these and similar disorders are a normal result of aging, they aren’t. Hypertension and hypercholesterolemia are diseases, almost always avoidable through good diet and other habits, and they usually signal worse trauma to come, like heart attack, stroke, diabetes, kidney disease, dementia, cancer, and much more. If you have such a disease, you’re sick, and it’s possible that the timing of your eating may be contributing to your sickness. The good news is the power to reverse disease often lies in our own hands. As I discuss in my book, most people can get healthier with a few lifestyle changes, chiefly by eating plants, and fasting can often help.

 

Does it matter what I eat during my window?

Absolutely. A narrow eating window can overcome some of the harm of a bad diet, but we have no evidence that it can completely counterbalance unhealthy eating. No matter when you eat, the best research suggests a healthy diet—one that is mainly or, better still, entirely of minimally processed plants—is needed for optimal health. For more information, see “Sources on Diet” at the end of my book.

 

In The Oldest Cure in the World, you base a lot of your argument for a narrow and early daily eating window on a couple of studies from a single lab. Are there any other studies?

At the time I wrote the book, the studies into early time-restricted feeding (eTRF) by Courtney Peterson, now of the University of Alabama at Birmingham, were just about the only rigorous human studies on daily fasting and certainly the only rigorous human studies on eTRF. Since the book has gone to press, scientists elsewhere have published results from careful-seeming eTRF trials that have strongly corroborated Peterson’s findings. In October of 2021, for example, researchers at Tianjin Medical University in China reported that when they asked type-2 diabetics to eat for twelve weeks either in a ten-hour eating window from 8:00 a.m. to 6:00 p.m. or whenever they liked, those who ate in the ten-hour TRF had better functioning beta cells (the insulin-making cells of the pancreas), less insulin resistance, lower triglycerides, lower total cholesterol, and lower LDL cholesterol (the most harmful kind). As in Peterson’s studies, these were impressive changes from merely changing when, not what, the volunteers ate.

 

In another study published in February of 2022, researchers at the Chinese Academy of Medical Sciences asked healthy volunteers to eat for five weeks in one of two eight-hour windows. Half the volunteers ate in an early window (eTRF) while the other half ate in a midday window (mTRF). The early volunteers were allowed to choose an eight-hour window between 6:00 a.m. and 3:00 p.m. (the slight flexibility in the window was meant to accommodate different wake-up times), while the midday volunteers chose an eight-hour window between 11:00 a.m. and 8:00 p.m. The eTRF group saw improvements in a range of biomarkers—insulin sensitivity, fasting glucose, total body mass, adiposity, inflammation, and diversity of gut microbes—whereas the mTRF group improved very little (chiefly in insulin sensitivity, and even then not as much as the eTRF group).

 

Both of these Chinese studies were randomized controlled trials, so they added considerable weight to Courtney Peterson’s finding that eating earlier, with the last calories in mid-afternoon, significantly improves health.

 

But aren’t there studies that say time-restricted feeding doesn’t work?

Yes, researchers behind two prominent studies have claimed, or the media has claimed for them (often vociferously), that TRF doesn’t work. But to my eyes both studies had deep flaws and don’t refute TRF at all. Let’s look at each.

 

The flawed UCSF TRF study of 2020

In September of 2020, a team led by researchers at the University of California at San Francisco published the results of a trial in which they divided overweight or obese volunteers into two groups, one of which ate for twelve weeks in a midday TRF window of noon to 8 p.m., the other eating whenever they wanted. The biomarkers of the mTRF group improved hardly at all. The study so shocked the lead researcher, UCSF’s Ethan Weiss, that he stopped eating in an mTRF window, a practice he adopted several years earlier. “There is no benefit to eating in a narrow window,” he said flatly to one reporter.

 

But the facts, including the facts of his own study, don’t come close to supporting his statement. Here are five pertinent facts:

 

First, it’s probably true, as the Chinese Academy study from February of 2022 suggests, that eating in a later window (like noon to 8 p.m.) is less healthy than eating in an earlier window (like 8 a.m. to 4 p.m.). But as we’ve seen (above and in chapter 16 of my book), eating in an early window has been proven in multiple randomized controlled trials to have impressive benefits. So even if it’s true that a later eating window doesn’t improve human health, Weiss’s condemnation of all TRF windows is unjustified at best, sloppy at worst.

 

Second, it may not even be true that a midday eating window confers no health benefits. I say this for two reasons. First, most of the mTRF volunteers in the UCSF trial probably weren’t truly fasting during their fasting hours. How do I know? Because during their daily fasting period they were allowed to drink caffeinated beverages, and caffeine disrupts a fast. The Salk Institute’s Satchin Panda, who, to repeat, is one of the world’s foremost researchers in the field, has found that a single caffeinated drink, even a non-caloric one, can jerk the body out multiple aspects of fasting metabolism. We don’t know all the ways caffeine alters a fast, but as Panda wrote in his 2018 book The Circadian Code, “Even if you have your morning coffee by itself, without breakfast, it still counts as the moment when you break your overnight fast.”

 

So although Weiss and colleagues claimed (and no doubt believed) that their mTRF volunteers were fasting for sixteen hours a night, if their volunteers took coffee, tea, a zero-calorie energy drink, or a calorie-free caffeinated soda in the morning, they were to some extent breaking their fast after only ten or eleven hours. We don’t know how many of the mTRF volunteers did so, but since 80 to 90 percent of American adults drink caffeinated beverages regularly, we can safely assume quite a few of them did, which compromises the study’s findings.

 

Sadly, the researchers’ failure to account for a basic fact of fasting metabolism—that caffeine disrupts a fast—is a historical commonplace among doctors and scientists doing fasting research. Many scientists who have dabbled in fasting research over the years have been only minimally learned in the science of fasting and have often been wholly ignorant of its clinical practice. It’s the rare fasting scientist who has consulted a doctor experienced in supervising fasts—an expert who might have spared the scientists some of their blushes. Had the UCSF team bothered to talk to, say, Alan Goldhamer, a doctor who has fasted tens of thousands of patients at the TrueNorth Health Center just an hour north of San Francisco, he might have warned them of the dangers of a caffeinated “fast.”

 

The other reason I say a midday TRF window may confer health benefits is that the UCSF trial actually found modest health improvements in one of two subgroups who ate in the midday window. The volunteers in the trial came from two pools. One pool lived in or near San Francisco, were supported in person by the research team, and got a little healthier. The other pool lived elsewhere, participated in the trial almost entirely via a mobile app, and didn’t get healthier. We don’t know for sure why the first pool got healthier and the second didn’t, but other researchers, including Courtney Peterson, have speculated that the app pool, lacking in-person support, may not have been able to stick as closely to the TRF window as the better-supported San Francisco pool. If so, the reason the app pool didn’t benefit from mTRF was probably because they simply weren’t doing it.

 

Here’s a third fact about what was wrong with the UCSF study: the UCSF researchers told the control group and the mTRF group to eat in different ways, which further compromised the results. Specifically, the researchers told the mTRF volunteers to eat however they liked but told the control group to eat three structured meals a day and snack lightly if they needed to. The mTRF volunteers, if they were like most Americans, would have grazed throughout their eight-hour eating window. Because research suggests grazing is less healthy than eating in distinct meals, the scientists’ instructions may have set the mTRF group on a course for worse health while setting the control group on a course for better health. (I should note, however, that we don’t know whether the troubles of grazing throughout a sixteen-hour day would also show up when grazing in an eight-hour window.)

 

The contrast between this aspect of the UCSF study and Courtney Peterson’s studies could not have been sharper. Peterson ensured an apples-to-apples comparison by requiring both her eTRF group and her control group to eat identically. Both groups took all their food in three meals, and at each meal they ate the same food as each other. Her team also prepared every morsel that went into their volunteers’ mouths, so there was no doubt they were eating as identically as possible, whereas the UCSF study left volunteers free to make their own meals. Consequently, when Peterson’s volunteers got healthier on eTRF, she knew the difference was due to eTRF, not to variations in how or what her volunteers ate.

 

Fourth, Weiss seems to have overstated the case when he said the mTRF volunteers in his trial lost muscle mass, which concerned him. Set aside for the moment that many of the mTRF volunteers, drinking caffeine and without in-person support, may not have truly done mTRF. The trouble with Weiss’s statement is that while the mTRF group did lose a small but significant amount of lean mass (as determined by a DXA body scan), lean mass isn’t the same as muscle mass. Lean mass consists of many bodily components, like organs and water, not just muscle. I assume this is why the text of the USCF study doesn’t say the mTRF volunteers lost muscle, only that they lost lean mass. It also says, “the DXA analysis of lean mass did not factor in muscle hydration, so it is possible that changes in hydration could confound the lean mass calculations.” That is, the lost lean mass could have been mostly water. But even if much of the lost lean mass was muscle, the amount lost was well below the threshold for clinical concern—not the strongest evidence for abandoning a useful therapy.

 

Unfortunately, the above sort of analysis hardly penetrated media reports on the UCSF study, most of which implied or said outright that all forms of TRF were of no use to human health.

 

The flawed China Southern TRF study of 2022

In April of 2022, a second study generated another round of bad press for TRF. A headline in the New York Times was typical of the lot: “Scientists Find No Benefit to Time-Restricted Eating.” In fact, the scientists behind the study said no such thing. A truer, if less sexy, headline would have been “Obese People Improve on a Calorically Restricted Diet, But They Don’t Improve Further When Eating in a Restricted Window.” In the yearlong study from China’s Southern Medical University, obese people were asked to eat a calorie-restricted diet either at whatever hours they liked or in a daily feeding window of 8 a.m. to 4 p.m. Both groups lost weight and enjoyed improvements in their biomarkers like lower cholesterol and lower blood pressure, but for the most part the TRF group didn’t do any better than the non-TRF group.

 

The findings, while useful, weren’t a large surprise, at least not to people who know something about nutrition research. Caloric restriction, if severe enough, is one of the most potent—maybe the most potent—of interventions for the health of humans and nearly every other creature on the planet. Calorically restricted animals consistently live much longer with less disease, so it’s not a big shock that volunteers who were undertaking serious CR and already getting impressive health benefits weren’t additionally helped by TRF.

 

Then again, we can’t be sure they weren’t being helped by TRF because the study had some hefty flaws. The heftiest was that the non-TRF group may in fact have been doing TRF. If so, the TRF group wasn’t being compared to free-eating controls but rather to a TRF group much like themselves. The reason the non-TRF group may have been doing TRF is that those volunteers apparently weren’t told to spread their eating over the entire day. Instead, they seem to have been told just to eat when they wanted to, and since they reported before the trial that they usually ate all their food in about ten hours, they probably weren’t eating in a longer window during the trial. (The study didn’t say when they ate during the trial.) If so, it’s no surprise that the eight-hour TRF group didn’t enjoy major improvements compared to the ten-hour TRF group, although one might have expected slightly more improvements than the study found.

 

This brings us to the next flaw. The non-TRF group may have been eating in a window even shorter than ten hours. These volunteers were, after all, people who habitually overate who had been asked to cut several hundred calories a day from their diet. When calories are cut so steeply, dieters don’t usually spread their eating over the same number of hours as before. They wake up hungrier and often take a greater share of their calories earlier. I can only speculate, but a reasonable hypothesis is that overeaters who normally ate in a ten-hour window might eat in a nine- or even eight-hour window when their calories were cut by a quarter. If so, there may have been almost no difference in the eating windows of the two groups.

 

Finally, the TRF volunteers in the China Southern study, like the mTRF volunteers in the UCSF study, were allowed to drink caffeinated beverages during their fasting hours. Since the majority of Chinese adults drink caffeinated tea with some regularity, a portion of the TRF volunteers may not have been fasting each day for as long as the researchers imagined. If they took their tea a few hours before or after their eating window, they may have been practicing not an eight-hour TRF but a ten- or even twelve-hour TRF.

 

If you can explain what’s wrong with these studies, why can’t other reporters?

A lot of obstacles stand in the way of good science reporting, and when the topic is complex, it’s the rare reporter who has the institutional support to do it well. One of the biggest structural problems is that even at large papers like the New York Times, reporters have to cover a broad swath of fields and can’t immerse themselves deeply enough in any one field to know its particular pitfalls. They get a press release from a scientific journal with a sensational headline—“Fasting Method Practiced by Millions Is a Flop”—and they run with it. Sure, they’ll call an expert or two to get some perspective on the study, but sometimes they’re talking only to the experts whom the journal points them to, or to the most quotable but not necessarily best informed experts, or to the experts whose opinions match their own biases. In any case, these chats only sometimes give reporters a broad enough understanding about what the study might have gotten wrong.

 

In addition to structural problems like these, there are other, more malign problems, mostly having to do with clickbait and bias. Stories that yield sexy headlines (“Hot Trend Debunked”) entice reporters and editors for the obvious reason: more clicks mean more advertising revenue. Then, too, reporters have their own biases, never more so than where food is concerned. Like the rest of us, most reporters eat unhealthily and don’t particularly care for evidence that suggests they would do well to change, and they also know that many of their editors and readers aren’t eager to hear they should consider changing the way they eat. When it comes to early time-restricted feeding, I suspect few reporters want to bear the bad news that eating dinner at the usual hour may be taking years off our lives.

 

Bias in favor of unhealthy habits is rampant in dietary reporting. Just look at the many stories claiming that butter and bacon are healthy (neither one is), that drinking a little alcohol each day is healthier than drinking none at all (it’s not), or that being a little overweight is healthier than being slender (it’s not). It’s the rare reporter or editor who will tell you the “science” behind such claims is deeply flawed. So while it may be frustrating that many of the same phenomena are playing out with fasting, it’s hardly surprising.

 

Is there any reason to be concerned about daily fasting?

Probably not, but some scientists have raised one potential area of concern. A few observational studies from the 1970s and 1980s noted that people who regularly fasted for more than twelve hours a night were at higher risk of developing gallstones than those who fasted for shorter periods. Those findings, however, were only correlations, and no causation has yet been found between fasting and gallstones. Probably the people who developed stones did so from unhealthy habits like eating too much cholesterol. Still, since most people eat too much cholesterol, it’s reasonable for TRF researchers to keep an eye on stone formation.

 

That said, to my knowledge, not one of the dozens of interventional trials of TRF (as opposed to those observational studies) has documented a single gallstone in humans or other animals. Additionally, over the past century, multiple fasting doctors reported that prolonged fasting (which, admittedly, is different from daily fasting) could dissolve gallstones and kidney stones. For reasons such as these, the great majority of fasting scientists and virtually all fasting doctors confidently recommend adults fast every day for at least twelve hours, and many recommend a fast of more like sixteen or eighteen hours. The jury is still out on whether adolescents, older children, and pregnant women would benefit from TRF, while scientists unanimously agree that infants, young children, and others with a biological necessity to eat frequently shouldn’t eat in a time-restricted window.

 

Is intermittent fasting the same thing as daily fasting?

Intermittent fasting is a vague term that different people use to mean quite different things, from daily fasting to weekly fasting to prolonged fasts of several weeks. The vagueness is one reason I avoid the phrase. Most people use “intermittent fasting” to mean either time-restricted feeding of the kind I’ve been talking about above, like fasting sixteen hours per day and eating across the other eight hours (16:8 fasting) or a pattern of eating normally five days a week and fasting the other two days (5:2 fasting).

 

Another reason “intermittent fasting” is a bit of a pointless term is that all fasting is intermittent. The alternative, continuous fasting, would lead to starvation and death, so “intermittent” is redundant when used with “fasting.”

 

Are 5:2 fasting and alternate-day fasting healthy?

There’s a case to be made for certain forms of 5:2 and alternate-day fasting (ADF), and there’s a case to be made against other forms of them. There are couple of ways to do 5:2 and ADF. One way is to eat no food whatsoever on fast days, while another is to eat a reduced number of calories on “fast” days, like 500 to 800 calories. We don’t have data to say conclusively, but it would surprise me if going completely without food for two or more days a week proved healthy. The reason is that our circadian rhythms are set in part by when we eat, so if you’re constantly shifting your eating pattern back and forth between eating and not eating for a full day, you may be jerking your circadian rhythms around too much. Again, Salk’s Satchin Panda is one of the leading experts on this, and he has concluded that subjecting your body to these sorts of shifts is like putting yourself through jet lag every few days. My own miserable experience with thirty-six-hour fasting every other day for a month (see chapter 7 of my book) made me a believer in Panda’s conclusion.

 

On the other hand, eating lighter for two or three days a week (in contrast to eating nothing) may be quite helpful, especially if you’re not eating healthily enough the rest of the time. Short-term human trials have shown significant metabolic benefits from this pattern of eating. Similar trials have shown benefits in eating lightly for four or five consecutive days once a month. To keep from throwing yourself into the circadian jet lag that Panda warns about, people who eat this way should probably maintain a consistent eating window across both “fast” days and feast days.

 

If you eat in a 5:2 or ADF pattern, can you eat whatever you like?

No, but you might hear otherwise from some people who should know better. One of my biggest concerns with 5:2 and ADF is that some of the proponents of these ways of eating claim if you follow the rules for “fast” days strictly, you can eat whatever you like on feast days. They believe the health benefits that you accrue on your light-eating days will protect you from wanton feasting. Krista Varady, a nutrition researcher at the University of Illinois at Chicago and a leader in the field, is one of the most prominent voices behind this claim. (Check out the sprinkled donuts, pepperoni pizza, and cheeseburger on the cover of her book The Every Other Day Diet.) Unfortunately, I’ve never seen anything like good data to support the argument, which strikes me as terrifically injudicious. The data we do have strongly suggests that those who eat Big Macs on their feast days will be a lot less healthy than those who eat salad.

Prolonged Fasting

 

What is prolonged fasting?

Different practitioners define “prolonged fasting” different ways, but it usually means fasting for more than a day. A prolonged fast can be done on water only or on vegetable broths totaling no more than 250 calories a day. Water-only fasts tend to make sick people healthier more quickly, but the side effects, like headaches, nausea, and fatigue, can be more intense. Fasting on broths, known as modified fasting, tends to be gentler, with fewer side effects and more energy for the faster, but it may take longer to reverse a disorder on a modified fast.

 

Why do a prolonged fast?

When you go days or weeks without food, your body makes even deeper repairs than it does on its overnight fasts. For healthy people, these powerful repairs probably help prevent future disease. For unhealthy people, the repairs can often partly or completely reverse their disorders.

 

Which diseases can prolonged fasting reverse?

It’s a long list. Dr. Otto Buchinger, Germany’s most influential fasting doctor, used to answer this question by saying, “Better to ask me what fasting can’t cure.” Over the past century or so, fasting doctors in multiple countries have reported that prolonged fasts have partially or completely reversed cases of arthritis, gout, type 2 diabetes, asthma, bronchitis, allergies, hay fever, skin diseases like psoriasis and eczema, migraines, gastritis, amoebic dysentery, cirrhosis, non-alcoholic fatty liver disease, follicular lymphoma, fibromyalgia, hypertension, endocarditis, irregular heartbeat, Reynaud’s disease (poor blood flow to the extremities), enlarged prostate, impotence, various disorders of the eye including early glaucoma, gallstones, kidney stones, and autoimmune diseases like rheumatoid arthritis, ankylosing spondylitis, and ulcerative colitis. This is only a partial list. For a few of these conditions, we have peer-reviewed studies confirming the reversals, but we await randomized, controlled trials to verify or refute the vast majority of them. Those trials, however, are unlikely to happen soon because the drug and device companies that fund most medical research can’t make large profits from fasting.

 

Alan Goldhamer, America’s most experienced fasting doctor, believes fasting succeeds best against diseases caused by dietary excess. In his experience, if a disease comes from eating unhealthily (as many diseases do), fasting stands a good chance of reversing it. Similarly, nearly all fasting doctors believe that to keep diseases from returning after a healing fast, patients must eat more healthily than they did when they got sick. Abundant research suggests the healthiest diet is one of minimally processed plants.

 

Which diseases are least likely to be cured by prolonged fasting?

For a start, any disease of long standing. As a general rule, the longer the patient has had the disease, the less thorough the reversal is likely to be. Someone who has had type 2 diabetes for twenty years is much less likely to completely reverse her disease than a diabetic who fasts two years after contracting the disease. But even people with deeply rooted disease often get a measure of relief from a prolonged fast.

 

Other conditions don’t seem to improve at all with fasting. Tuberculosis, type 1 diabetes, and multiple thyroid disorders like hyperthyroidism and Graves’ disease don’t appear to benefit much from fasting. Nor will a fast regrow a torn ligament, repair ripped cartilage, or set a broken bone. Nor has fasting ever been shown to reverse end-stage heart failure or severe dementia, although in the early phases of those diseases, a fast may yield some benefit.

 

Can prolonged fasting reverse cancer?

Yes, but rarely entirely. The only cancer that prolonged fasting has been shown to completely reverse (in peer-reviewed case studies) is follicular lymphoma, as I relate in the prologue of The Oldest Cure in the World. We still await the randomized, controlled trials that would prove the matter conclusively, but since conventional medicine has no cure for lymphoma, a prolonged fast under medical supervision, followed by a diet of minimally processed plants, seems to me the best hope for sufferers of this otherwise terminal disease.

 

Prolonged fasting, however, isn’t nearly as successful against other cancers, and against a few cancers it may even be harmful. Fasting doctors over the last century have reported that fasting can often shrink cancerous tumors, but it doesn’t eliminate the tumors, which usually grow back. Studies in cancerous rodents show something quite similar: during fasting, the cancer struggles in the absence of its preferred fuel, glucose, and the tumors shrink. But in time the cancer finds other fuel, the tumors regrow, and the rodents succumb. Although most cancers seem to have difficulty in the absence of glucose, there’s evidence from petri-dish studies that ketones, which the body runs on during a prolonged fast, can fuel the growth of certain cancer cells, including breast cancer cells.

 

So is fasting useless against most cancers?

No, quite the opposite, as I discuss in chapter 12 of my book. Scientists think that because of fasting’s tumor-shrinking power, a fast of two or three days (on water or broth) or a very low calorie fasting-mimicking diet can be a potent part of an anti-cancer regimen when coupled with chemotherapy, radiation, or immunotherapy. In lab animals, fasting makes chemotherapy and other cancer therapies more effective at knocking out tumors, and trials in humans, while still in the early stages, point in the same direction. More thorough human trials are underway.

 

Fasting also minimizes the side effects of conventional cancer therapies. In multiple clinical trials, volunteers who fasted (either on water only or on vegetable broths) while undergoing chemotherapy routinely reported far less nausea, fatigue, and other side effects than patients who received chemotherapy without fasting.

 

How do I do a prolonged fast?

The first thing to know is that most people shouldn’t undertake a prolonged fast unless they’re under the supervision of a doctor skilled in the practice. Prolonged fasters often experience uncomfortable symptoms like headaches, nausea, vomiting, lower back pain, insomnia, or skin rashes. These symptoms are usually short lived and benign, a normal part of what fasting doctors call a healing crisis (the consequence, they believe, of the body ridding itself of the substances that are making it sick). Other times, however, such symptoms are signs the body is responding poorly to the fast and heading for trouble. The average faster can’t tell a mild healing crisis from real danger, and if someone who’s approaching trouble pushes a fast too far, she can damage her organs or, in rare cases, die. A skilled fasting doctor can distinguish harmless from harmful symptoms by taking a thorough medical history and conducting a baseline exam before the fast and then performing regular checkups, including of blood and urine, during the fast.

 

Another concern with unsupervised fasting is that a couple of tiny groups of people are physically incapable of fasting for long. People in one of these groups are unable to burn their own fat for fuel, while people in the other group can’t clear the waste products of their protein as it’s burned for fuel, which inevitably happens at the start of a prolonged fast (see below). People with either disorder can slip into a coma and die when fasting, and some people with these rare disabilities have no idea they have them. Although their numbers are truly minute, the fact that such people exist leads some fasting doctors to say nobody should fast on their own for more than about eighteen hours. Other fasting doctors, however, are more lenient in their advice to do-it-yourself fasters.

 

So who might be able to do a prolonged fast on their own?

First, let’s make clear who else (in addition to the groups mentioned above) shouldn’t fast. All of the fasting doctors I interviewed or whose work I’ve read say you shouldn’t fast for more than about eighteen hours on your own if you aren’t in good health. If you have a diagnosable disorder (even something as supposedly minor as mild high blood pressure), or if you’re taking medications of any kind, or if you even suspect you might have a health problem, you shouldn’t do a prolonged fast on your own. Fasting doctors also advise against prolonged fasting for infants, children, and pregnant women, although some doctors make exceptions for brief supervised fasts when a specific disorder might be reversed by a fast. Fasting doctors also warn against fasting if you have kidney disease or another ailment that compromises the body’s ability to expel the toxins that will be mobilized by a fast. Finally, people with an eating disorder and people who are afraid to fast are also usually discouraged from fasting unsupervised, although they might fast successfully in the supportive setting of a clinic.

 

Fasting doctors are more divided about whether healthy people can fast for a short time without medical supervision. Some doctors say people in good health, with no diagnosed or suspected conditions and taking no medications, can safely fast on their own for up to a week. But other doctors, as I said above, think nobody should fast for more than about eighteen hours.

 

You fasted twenty days on your own, as you recount in The Oldest Cure in the World. Would you do that again or recommend it to others?

Emphatically not. I was pretty ill informed when I did that fast more than a dozen years ago. Knowing what I know now, I would never make a fast that long unsupervised, even though I’m now a much more experienced faster. I’d recommend anyone who’s considering a prolonged fast to heed the cautionary opinions of fasting doctors in the preceding few paragraphs and in my book.

 

What’s the biggest danger of a prolonged fast for a healthy person?

A concussion caused by fainting. Blood pressure drops during a fast, and a faster who stands up too quickly runs the risk that her blood won’t stand up with her, which can cause her to pass out and conk her head. Fasting doctors say the remedy is simple: pump your legs before you rise to improve blood flow throughout your body, stand up in stages, and if you feel even a little lightheaded, sit back down immediately.

 

Dehydration is another common but easily avoided danger of the prolonged fast. Food contains a lot of water, so when we don’t eat, we need to drink more than we otherwise would to stay hydrated. Fasting doctors recommend drinking several tall glasses of purified water daily while on a prolonged fast. A typical recommendation is for a total of 64 ounces (half a gallon) per day.

 

How do I find a clinic that will supervise a fast?

As of 2022, the United States, where I live, has just four fasting clinics staffed by doctors trained to supervise fasting. I offer no endorsement of these or any other fasting clinics, but the information at their websites may help you decide which, if any, is best for your needs:

TrueNorth Health Center, Santa Rosa, California

Fasting Escape Retreat Center, Yorba Linda, California

Dr. Gracie’s Wellness Retreat, West Farmington, Ohio

Clínica Back to the Garden, Caguas, Puerto Rico

 

Each of these clinics specializes in inpatient fasting, and TrueNorth and the Fasting Escape may also offer to supervise fasters remotely in their own homes, provided they can meet a few requirements like getting bloodwork analyzed and possibly involving a local doctor. TrueNorth is by far the largest and oldest of the bunch, with something like 1,500 inpatients a year, compared to a fewscore patients annually at the other clinics. For my experiences at TrueNorth, see chapters 15 and 17 of my book. Clínica Back to the Garden is apparently the only doctor-supervised fasting clinic in Latin America. Its doctors speak both Spanish and English.

 

Europe has many more fasting clinics than the United States, with the greatest concentration in Germany. For my experiences at Germany’s Buchinger Wilhelmi Clinic, the world’s largest fasting clinic, see chapters 10 and 13 of my book. Buchinger Wilhelmi also has a sister clinic in Marbella, Spain. I’m unfortunately not well versed on clinics in the rest of the world, of which, however, there aren’t many.

 

Does health insurance cover a supervised fast?

Typically not in the United States, although some insurers will cover the initial physical exam and any routine bloodwork and urinalysis. Insurers in other countries, notably Germany, sometimes cover the entire cost of inpatient fasting.

 

How long should a prolonged fast be?

That depends on what you’re trying to achieve. If you’re hoping to reverse a grave disorder, you may need to fast for several weeks, and you may need multiple fasts (with long periods of refeeding on healthy foods in between). If the disorder has persisted for many years, you may get only a partial reversal, and in any event the time it takes to get a result can vary widely by the individual. At your intake interview or telehealth consultation, a fasting doctor can often tell you how long similar cases have taken to resolve.

 

If you’re in good health and are fasting for prevention—that is, fasting in the hope that the deep repairs of a prolonged fast will wipe out unseen metabolic dangers that might otherwise turn into a disease like cancer or Alzheimer’s—the question of how long to fast is trickier because we lack the science that would inform us. Most fasting doctors today recommend healthy people fast once a year for about either seven days on water or ten days on broth. If the fast comes off with few side effects, most fasting doctors declare the faster presumptively healthy. If the faster experiences a prolonged healing crisis, doctors typically recommend either prolonging the fast until symptoms resolve or fasting more frequently until the patient can fast free of symptoms.

 

How often should I do a prolonged fast?

Again, if you’re trying to fix a specific illness, fasting doctors have found the condition dictates the answer: you fast as often as needed to find relief, provided you have the fat stores to sustain the fasting and your exams and lab work show no signs of trouble. That might mean a fast of a few weeks every six months or even every three months. For people in good health fasting preventively, most fasting doctors, as noted above, recommend a prolonged fast once a year.

 

Are shorter prolonged fasts—fasts of two, three, or four days—helpful?

They may not be. The potential problem is that although long fasts can initiate deep repairs, they come with a price to the body in the early part of the fast. It’s probably a small price and is almost certainly worth paying, but we’re not quite sure just how expensive it is, so a bit of caution would be prudent. What we know is that when we shift from eating to prolonged fasting, the body initiates a great many metabolic changes that allow it to switch from running on its preferred fuel, glucose, to running on the byproducts of its fat stores, ketones. The body doesn’t make this momentous switch overnight. It shifts gradually, from roughly the second through the fourth days of a fast. During this transition, as the body moves from running on glucose to running on ketones, it has to burn something else for fuel, and the something it turns to is protein.

 

This period of protein catabolism—burning proteins—isn’t as scary as it might sound, but until we know more about it, neither is it a trivial consideration. When most people hear “burning proteins,” they think of muscles withering away, but that doesn’t happen on a fast. Muscle volume can indeed shrink, but much of the shrinkage is due to lost water and intramuscular fat (the fat inside muscle cells, which, not incidentally, is behind type 2 diabetes—which is one reason fasting can reverse diabetes). The proteins burned during the transition period seem mostly to come from parts of the body other than muscle. You may recall from high school biology that we have enormous quantities of proteins in literally every part of our bodies in the form of enzymes, hormones, antibodies, and much, much more. These proteins are constantly wearing out and being broken down and replaced. At the start of a prolonged fast, the body increases the rate of breakdown and converts the proteins into glucose for fuel. Although scientists aren’t certain, it seems probable that the fasting body selectively harvests the oldest and most damaged proteins to use first. If so, getting rid of them during a fast is almost certainly a good thing, especially since they’re replaced with new, presumably healthy proteins when we refeed. Moreover, even if the fasting body suffers some minor temporary harm from burning proteins, that harm is almost certainly outweighed by the repairs made during a long fast. Just ask any patient whose fast has rid them of fibromyalgia or inflammatory bowel disease or follicular lymphoma.

 

But it’s important to know that the scenario I’ve sketched above isn’t perfectly clear. This process seems to be what’s going on, but we don’t know for sure because it’s not been thoroughly researched. With this uncertainty in mind, many fasting doctors urge their patients not to fast for two, three, or four days because people who fast for so short a period are putting their bodies through the possible trauma of burning proteins without reaching the start of the deeply restorative repairs.

 

So should you never fast for two, three, or four days?

I don’t think the evidence supports so total a ban. To take one contrary example: over the past two centuries, both fasting doctors and fasters have reported that some acute illnesses, like the common cold and certain fevers, seem to heal more quickly with a fast of a few days. According to their accounts, if a sufferer, at the first sign of illness, stops eating or reduces her nutrient intake to 250 calories a day of vegetable broths (while drinking plenty of water in either case), the illness appears to heal more quickly. I’m not aware of any formal studies to back these claims, but the claims have been made consistently by credible doctors and fasters of just about every age and sex in multiple countries across multiple eras. Today, fasting doctors usually say that when you’re acutely sick, the drawback (if any) of burning proteins on a short fast is outweighed by the gain of getting rid of the illness more quickly.

 

What’s the longest I can safely fast?

It depends on how healthy you are and how much fat you have. For most people, fasting doesn’t become dangerous until it crosses over to starvation, which can be defined metabolically. If the start of prolonged fasting is defined as the point when the body switches from running on glucose to running on ketones, the start of starvation is the point when the body runs too low on stored fat to meet its energy needs and switches its metabolism once again, this time to burning essential proteins like cardiac muscle for fuel. Even slim people can usually fast more than a month before beginning to starve, while the average well-padded American would take months.

 

In the past, fasting doctors sometimes fasted patients for months at a time, and the longest fast on record is a 382-day effort by an obese Scotsman who slimmed, under the supervision of doctors, from 456 to 180 pounds. (See chapter 11 of The Oldest Cure in the World.) But fasting doctors today worry about the stress that super-long fasts might inflict on the body, so most of them limit their patients’ fasts to about 40 days. If a sick patient can’t reach their health goal with a fast of that length, doctors often refeed them (or send them home to refeed) for a number of weeks or months and then fast them again for up to 40 more days, and sometimes again, and yet again.