Healthy Fasting. One of the challenges people face when they use fasting as a tool for weight loss and other health benefits is that people around them may think fasting is more dangerous than it really is. In “4 Propositions on Weight Loss,” Proposition 2 and its gloss are:
For healthy, nonpregnant, nonanorexic adults who find it relatively easy, fasting for up to 48 hours is not dangerous—as long as the dosage of any medication they are taking is adjusted for the fact that they are fasting. …
The best evidence that fasting is not dangerous comes from the experience of those in religious traditions that encourage fasting. For example, the Mormonism I grew up in not only instructed everyone who could to fast for 24 hours once a month, it also encouraged people to fast for longer periods of time for special spiritual purposes. Mormon fasts often involved not drinking as well as not eating. Given the body of experience indicating that even fasts that risked dehydration were usually not that dangerous, nonreligious fasting that encourages the drinking of water should not be dangerous for those in good health who are not pregnant or anorexic. The biggest worry I have about people fasting is that it could easily throw off the dosage of medication they are taking. So anyone taking prescription medication should consult their doctor about medication interactions before fasting. And anyone taking nonprescription medication should think hard about lowering the dose when they fast.
In Amanda Mull’s Atlantic piece “What Billionaires’ Fasting Diets Mean for the Rest of Us,” she unfairly suggests that a short eating window every day is extreme enough to be suspect:
Twitter’s CEO, Jack Dorsey, doesn’t eat for 22 hours of the day, and sometimes not at all. Over the weekend he tweeted that he’d been “playing with fasting for some time,” regularly eating all of his daily calories at dinner and occasionally going water-only for days on end. In many cases, severe and arbitrary food restriction might be called an eating disorder. And while researchers are hopeful that some types of fasts may be beneficial to people’s health, plenty of tech plutocrats have embraced extreme forms of the practice as a productivity hack.
I am in line with Jack Dorsey’s practice, except I try to do my eating window around lunchtime when I can, which is often, since I spend my most research-intensive days working at home. I view fasting for 20-22 hours a day and occasionally for 48 or 72 hours as quite healthy. And as I wrote in “My Annual Anti-Cancer Fast,” I try to fast for a week once a year with the idea that, however hard that might be on me, it will be harder on any cancer cells or precancerous cells I might have.
Given my view that fasting is healthy, I also don’t see anything amiss in timing when I fast to when I most need some extra time. During my teaching semester—four months out of the year—when I teaching Monday, Wednesday, Friday, I fast throughout Monday and Wednesday (making for close to a 44-hour fast when combined with my short eating window on surrounding days) and eat lunch with my colleagues on Friday. Last week, in order to help get a grant proposal done, I fasted an extra day when I wouldn’t have otherwise. During the rest of the year, in addition to the short eating window each day, I find I need to skip eating at least one day every two weeks to keep my weight steady. I wish it weren’t so, but it is what it is. On the other hand, part of that is compensating for vacation days or days when guests are visiting, when I have a longer eating window.
I have to emphasize that if I were fasting this much and it were hard, that would indicate some sort of psychological pathology. But as I have emphasized often in my diet and health posts, if one is eating very low on the insulin index when one does eat fasting can be surprisingly easy, at least for some people, among whom I am one. (See “Forget Calorie Counting; It's the Insulin Index, Stupid,” “Obesity Is Always and Everywhere an Insulin Phenomenon,” and “Why a Low-Insulin-Index Diet Isn't Exactly a 'Lowcarb' Diet.”) And I promise, I am still well within the normal weight range. Putting the lowest weigh I have seen on the scale lately and rounding up my height to the nearest inch, I still get a BMI of 23.1, which is far from any danger zone of being too thin!
In the following paragraph, Amanda Mull summarizes some of the health benefits of fasting, but then argues that fasting raises the risk of anorexia and bulimia:
Intermittent fasting, like most health-and-wellness behaviors, can exist anywhere on a spectrum that runs from very dangerous to potentially beneficial, depending on who’s doing it and how it’s implemented. Fasting in one form or another has been a part of human eating behavior for millennia, and although scientific research on it is still preliminary, early studies suggest it might help reduce the risk of heart disease, cancer, and diabetes. For people with eating issues, though, fasting can be a very risky trigger for anorexia or bulimia. For most people, exploring Dorsey’s lengthy, everyday fasts without oversight from a doctor or nutritionist is probably unwise.
Bulimia. Amanda’s link on “bulimia” is to a Journal of Abnormal Psychology article, “Fasting Increases Risk for Onset of Binge Eating and Bulimic Pathology: A 5-Year Prospective Study.” The authors of this study are more careful in distinguishing correlation from causation in their abstract than in their title. They write:
We tested the hypothesis that fasting (going without eating for 24-hours for weight control) would be a more potent predictor of binge eating and bulimic pathology onset than dietary restraint scores using data from 496 adolescent girls followed over 5-years. Results confirmed that only 23% of participants with elevated dietary restraint scores reporting fasting. Furthermore, fasting generally showed stronger and more consistent predictive relations to future onset of recurrent binge eating and threshold/subthreshold bulimia nervosa over 1- to 5-year follow-up relative to dietary restraint, though the former effects were only significantly stronger than the latter for some comparisons. Results provide preliminary support for the hypothesis that fasting is a stronger risk factor for bulimic pathology than is self-reported dieting.
Wikipedia currently defines bulimia as “an eating disorder characterized by binge eating followed by purging.” One possibility, which I think the authors of this study would take seriously, is that girls who are likely to become bulimic later on in any case, also are prone to a variety of other behaviors. The other important possibility is that if one does fasting without eating low on the insulin index, it is quite dangerous for setting up a cycle of bad eating behavior because of the strong hunger that results. I do not recommend trying to do a lot of fasting without first moving to low insulin-index eating.
Those who are fasting in the way I do recommend should be careful to explain to others who see them fasting that the low-insulin-index eating is an important part of the practice. If people prone to anorexia could get fixated first on avoiding sugar, before they are tempted to do more than that, they would be less likely to do themselves harm.
Fasting is counted as one of the possible types of “purging” in bulimia. But the more common types of purging are vomiting and taking laxatives. Repeated vomiting can be quite dangerous. And laxative abuse is unlikely to be healthy. As far as these very dangerous behaviors go, for someone for whom fasting is easy because they are eating low on the insulin index, I don’t see why they would turn to vomiting or laxatives. And if they are eating low on the insulin index, they are less likely to do extreme binge eating because satiation kicks in fairly quickly if one is avoiding, say, sugar, rice, bread and potatoes and a few other high-insulin-index foods.
As near as I could make out from my online reading, those who induce vomiting or abuse laxatives who are underweight are called anorexics of the binge-purge subtype rather than being classed as bulimics. But clearly, the binge-purge subtype of anorexia shares a lot in common with bulimia. The National Association of Anorexia Nervosa and Associated Disorders gives a .9% lifetime risk for anorexia. Of this, from two studies I looked at on the fractions of each subtype in studies that were taking all the anorexics they could enroll, it appears that the binge-purge subtype is somewhat over half of anorexics, while the restrictive subtype is somewhat less than half.
It is possible that seeing other people fast might inspire someone prone to the restrictive subtype of anorexia to be very restrictive in eating. But it is also possible that if more people not prone to psychopathology were fasting regularly, that those prone to restrictive anorexia who were not fated to fall into anorexia no matter what could be better guided into helpful, rather than harmful patterns of fasting.
The simple fact is that we just don’t know whether having healthy fasting be more common in our culture would make things worse for those prone to anorexia or better. To me, one possibly powerful effect in the direction of making things safer for those prone to anorexia is that if were common knowledge in our culture that weight could be lost by a combination of low-insulin-index eating and fasting whenever someone wanted to, then the total amount of angst about weight in our culture might go down. That might help. One of the biggest ways it might help is if the common knowledge that the combination of low-insulin-index eating and fasting could bring effective weight loss helped everyone to set a bright line that inducing vomiting or abusing laxatives for weight loss is bad and unjustified, no matter how disgusted one is with one’s weight.
Anorexia. Amanda’s link on “ anorexia” is the second article above, Rebecca Ruiz’s piece “Silicon Valley preaches the virtues of fasting, alarming eating disorder experts.” Rebecca Ruiz relies on Andrew Walen making the immediate judgment that fasting is “disordered eating,” which is exactly the question at issue. Actually, in context, Andrew Walen and the other expert Claire Misko are saying something more subtle: that people who think of weight loss and eating well as a big achievement might be getting into a weird and unhealthy headspace:
"The connection between achievements and food and eating is very alarming," says Claire Mysko, CEO of the National Eating Disorders Association. "Not everybody who gets into this is necessarily going to spiral into a eating disorder, but if you are at risk, this is a really triggering framing."
In other words, Claire is saying it is notion of “achievement” that could trigger disordered eating, not having people around who are calmly doing fasting in the background of their lives as no big deal. And that is exactly how I do fasting: it isn’t a nothing, but for the most part it fades into the background of my life. The reason it can fade into the background is because the combination of low-insulin-index eating and fasting is relatively easy. It is not easy for things that are difficult to fade into the background of one’s life!
Tradeoffs. As I noted above, we just don’t know if having healthy fasting become more common would tip some of those prone to restrictive anorexia—or perhaps even those prone to binge-purge anorexia or bulimia—into worse symptoms. This is not something we have causal evidence of. But even if there is a causal effect in this direction, it needs to be counterposed to the health benefits people get from healthy fasting, including reducing the incidence of all the diseases associated with obesity.
As quoted in Rebecca Ruiz’s article, Geoffrey Woo makes this point:
Geoffrey Woo, who created a private HVMN Facebook group devoted to answering questions about fasting, is sympathetic to such criticism but calls it a "stretch."
He argues you could make a similar criticism of public health officials who promote hand-washing because some people, presumably who experience obsessive-compulsive disorder, might find such campaigns triggering. Woo knows it's not an equivalent analogy, but it speaks to his view of fasting as a public health intervention.
"The inspiration of fasting does come from realizing two-thirds of Americans are overweight or obese," he says.
Let me make another analogy. There is another behavior that is widespread in our society, whose main benefit is pleasure—with some believing that it might aid health based on evidence that is much weaker than the evidence for the health benefits of fasting. That behavior is moderate drinking of alcohol. I think people drinking some alcohol rather than none is much more likely to create a danger for those prone to alcoholism than people doing healthy fasting is to create a danger for those prone to anorexia. But how many of us wish that alcohol and similar drugs didn’t exist? I might, but I think I am in the minority.
The majority who are happy that alcohol exists have a responsibility to distinguish carefully between moderate drinking and excessive drinking. Just so, those who are happy that we have the powerful tool of fasting to aid health in numerous ways have a responsibility to distinguish carefully between healthy fasting and anorexic behavior. Let me try. First is the bright line that inducing vomiting and laxative abuse are bad. Second, I would say that those younger than 20 years old (or are pregnant) should definitely not be fasting more than 24 hours at a stretch, and should not be fasting more than 16 hours with any frequency. Third, it is a red flag if someone is doing a lot of fasting without also avoiding sugar. This could be conducive to a binge-purge pattern. Fourth, it is a red flag if someone is keeping their eating or purging patterns a secret. It can be annoying to have to defend what one is doing with fasting, but being open about what one is doing and defending it to other people—and sometimes recognizing that their worries are well-founded. is an important safeguard against doing something really stupid.
Doctors. What about the idea that one should only fast under a doctor’s supervision? Basically, this is impractical. Do we think every practicing Muslim should be under a doctor’s supervision throughout Ramadan every year, when they fast from sunup to sundown every day? Should every Mormon only do their once-a-month 24-hour fast under a doctor’s supervision?
As another comparison, think of how many lives could be saved if we insisted that no one should eat sugar except under a doctor’s supervision!
There are certain things we should develop a common wisdom about so that we all know what is a sensible way to do them without always needing to use expensive doctors to supervise them. Let me try my hand at beginning to develop that common wisdom:
If your body-mass-index is below 18.5, quit fasting! Here is a link to a BMI calculator.
Definitely people should not do fasting for more than 48 hours without first reading Jason Fung’s two books The Obesity Code (see “Obesity Is Always and Everywhere an Insulin Phenomenon” and “Five Books That Have Changed My Life”) and The Complete Guide to Fasting.
Those under 20, pregnant or seriously ill should indeed consult a doctor before trying to do any big amount of fasting.
Those on medication need to consult their doctor before doing much fasting. My personal nightmare as someone recommending fasting is that a reader who is already under the care of a doctor who is prescribing medicine might fail to consult their doctor about adjusting the dosage of that medicine in view of the fasting they are doing. Please, please, please, if you want to try fasting and are on medication, you must tell your doctor. That may involve the burden of educating your doctor about fasting. But it could save your life from a medication overdose.
Those who find fasting extremely difficult should not do lengthy fasts.
But, quoting again from “4 Propositions on Weight Loss”: “For healthy, nonpregnant, nonanorexic adults who find it relatively easy, fasting for up to 48 hours is not dangerous—as long as the dosage of any medication they are taking is adjusted for the fact that they are fasting.”
The Limits of My Knowledge. I am painfully aware of how little I know about anorexia and bulimia. Please, do educate me in the comments section below. Anorexia and bulimia are terrible. Despite some superficial similarities with fasting, I am questioning how much they have to do with healthy fasting. And no one knows what effect an increase in the fraction of those doing healthy fasting would have on those prone to anorexia and bulimia. It might well depend on exactly how those into healthy fasting do it and how they explain what they are doing.
Don’t miss my other posts on diet and health:
I. The Basics
II. Sugar as a Slow Poison
III. Anti-Cancer Eating
IV. Eating Tips
V. Calories In/Calories Out
VIII. Debates about Particular Foods and about Exercise
Julia Belluz and Javier Zarracina: Why You'll Be Disappointed If You Are Exercising to Lose Weight, Explained with 60+ Studies (my retitling of the article this links to)
IX. Gary Taubes
X. Twitter Discussions
XI. On My Interest in Diet and Health
See the last section of "Five Books That Have Changed My Life" and the podcast "Miles Kimball Explains to Tracy Alloway and Joe Weisenthal Why Losing Weight Is Like Defeating Inflation." If you want to know how I got interested in diet and health and fighting obesity and a little more about my own experience with weight gain and weight loss, see “Diana Kimball: Listening Creates Possibilities” and my post "A Barycentric Autobiography.