September 14, 2016

Not So ... Fast ... (A Rant)

Hammers are great. Except for when you need a wrench.

Wrenches are great. Except for when you need a drill.

Drills are great. Except for when you need a crowbar.

Different job, different tool.
Different need, different tool.

With me so far?

With this in mind:

Fasting is great. Except when it’s not.

I think you’ll find I’m one of the more open-minded people in the low-carb/keto community. (I don’t know if this is good, bad, or meaningless. Maybe I should align myself with one camp and subscribe to that dogma and only that dogma. Then again, there’s enough of that out there already, right? And when Robb Wolf starts talking about eating lentils [as he did on this podcast], and Chris Kresser points out some of the potential dangers of going overboard with vitamin D supplementation, I guess I’m in good company if I’m able [and willing!] to see and respect the nuance some of this stuff requires.) I firmly believe in certain basic principles of nutrition that are forged in unalterable aspects of human physiology and biochemistry, but beyond that, if someone tells me they feel better eating a little bit of rice and potatoes now and then, and that they don’t feel so great loading up a cup of coffee with 4+ tablespoons of butter and coconut oil, who am I to insist they’re lying? Who am I to insist they force themselves to do something they insist makes them feel terrible? (Except bacon. If you’re not eating bacon, you are doing it wrong. [No, just kidding. Shout-out to anyone reading my blog who keeps kosher or observes the laws of halāl!])

So in my non-dogmatic perspective, what’s the deal with fasting?

Most of you are probably familiar with Dr. Jason Fung. Dr. Fung is one of my favorite people in the low carb health community. I love his salty language, razor-sharp wit, and pointed analogies. (I’ll link to some podcast interviews with him at the end of this post. As for the pointed analogies, listen for him talking about plane crashes and just needing “more lift and less gravity.” Classic!)

For those of you who live on Jupiter and have no idea who Dr. Fung is, he’s a nephrologist (kidney specialist) who regularly reverses obesity, type 2 diabetes, and kidney disease in his clinic. The humor, sarcasm, and snark in his blog posts puts mine to shame, and he manages to do so while being way more succinct than I am. (He should give lessons on how to do that...)

I respect Dr. Fung. I’m a huge fan. I loved his book and wrote a review of it here. In fact, we collaborated not long ago on a journal article that we’ll be submitting for peer review and publication. (Woohoo!) On his blog, in his YouTube videos, and in podcast interviews, he's written and talked about obesity, insulin resistance, hyperglycemia, and more. But what he is perhaps best known for is being a vocal proponent of fasting. Skipping a meal, skipping a whole day of meals, doing multi-day or even longer fasts: any and all permutations are okay, depending on what a patient’s goals are and what fits into their lifestyle. (He and Jimmy Moore are set to release a new book about fasting, and I can neither confirm nor deny that yours truly may have contributed some quotes…)

Dr. Fung has a multi-part blog series on the history and benefits of fasting as a religious, cultural, and health practice. He’s up to over 20 posts in the series, but he’s much more to the point than I am, and it’s well worth your time to read as your schedule permits. The first post is here, and each one ends (I think) with a link to the next one in the series. I am not at all doubting the success he’s had with patients. He is a doctor. I am not. As he says in interviews he’s done, when people ask him to address critics of his methods: he has supervised thousands of people through various permutations of fasting. People have generally only gotten better. Less medication. Lower insulin levels. Lower blood sugar. More energy. Greater fat loss. (And the critics? How many people have they supervised through fasting? How much proof do they have that fasting is dangerous? *Crickets.*)

Fasting has its place. I’m not going to write about the benefits of fasting. They are numerous, and if you want to know what they are, you can just check out the forthcoming book. And it should be obvious that, if I was quoted in a book about it, talking about it in a favorable way, then I do understand and respect these numerous benefits, and I think fasting can be incredibly therapeutic. (I talked about it in relation to neurological conditions toward the end of this podcast.) So I hope I’ve made it abundantly clear that I think Dr. Fung is awesome, and I think fasting can be awesome…

…when it’s applied intelligently and rationally, that is.

My fear is that many in the Paleo, low carb, and keto communities have sailed long past intelligence and rationality and have landed squarely in the twilight zone of insanity. Even worse, they are taking other people with them! People who should not fast!

Why do I care? Why does it affect me when someone who’s better off ingesting food on a regular basis chooses not to ingest food on a regular basis?

I care because I am the one who has to talk these people down from the proverbial ledge when they write to me complaining of fatigue, anxiety, loss of their menstrual cycle, or some other worrisome outcome.

I am concerned that the growing fascination (nay, obsession) with fasting is becoming a kind of orthorexia, wherein people become terrified of consuming food outside their self-prescribed window. Or consuming food before they were “supposed to.” People brag on social media about how long they’ve gone without food (and how much intense exercise they’ve done in that time). And even though no one comes right out and points a shaming finger at people who don’t fast, the result is that they still kind of make the rest of us feel like failures as human beings if we eat more than once every 24, 48, or 72 hours. Can we please stop this already?

Eating is a natural, biological imperative. We don’t have to do it multiple times a day, every day, but there’s not necessarily anything “wrong” with eating daily, either. At some point, we do need—and I mean need—to eat. That being said, many of us in the wealthy industrialized world probably do eat too much and too often. And I think Dr. Fung is doing people an important service by talking not just about what to eat, but when. It’s a big missing piece of the puzzle, particularly for people who struggle to lose body fat, or whose blood glucose and/or insulin isn’t coming down to healthier levels even on a “well-formulated” low carb diet. The conventional advice to “eat several small meals a day to keep your blood sugar up” has gotten us results about as nice as the ones that came from everybody basing their diets on starchy carbohydrates and using margarine instead of butter.

So yeah. Regardless of the type of diet those of us blessed with abundant food 24/7 follow—be it low fat, low carb, Paleo, keto, vegetarian—many of us probably do eat too often. We’re wired for feast and famine, not feast, feast, feast. We eat when we’re happy, sad, bored, excited, stressed out, lonely, watching television, celebrating, mourning, and just about anything and everything else. Because we are blessed with abundant food all year long, we now have to make a special effort to impose food scarcity upon ourselves for therapeutic purposes. I get it. I do.

Plenty of people fast, and plenty of people do great with it.
But let’s talk about who should not fast.

And I’m not talking about the obvious ones—the ones Dr. Fung, himself, is clear should not fast: pregnant women, breastfeeding women, children, and people with medical conditions or on certain medications for which fasting might be contraindicated.

I’m talking about healthy, young, active people who think they need to fast, but who actually seem to fare poorly in doing so. The people who seem to do poorly with fasting are not Dr. Fung’s typical patient base. Let’s remember, Dr. Fung is a nephrologist. Almost by definition, anyone who gets referred to him probably has some degree of kidney damage or dysfunction, and the vast majority of those with kidney damage or dysfunction not due to congenital problems or physical trauma most likely have insulin and blood glucose issues. For those of you who are fans of Dr. Bill Lagakos, I’ll emphasize his favorite phrase: context matters.

I have no doubt that Dr. Fung has had great success with his female patients. But what he doesn’t say—and I wish he would—is that the people he supervises on fasts are in his “Intensive Dietary Management” clinic specifically for diabetes, obesity, kidney disease, and the like. His patient population is NOT young, athletic, fit, healthy people who are already chronically undereating and not getting enough sleep, particularly when combined with intense workouts multiple times a week. This is a totally different ballgame! (Please take this with a grain of salt, because unlike Dr. Fung, I have not supervised thousands of people through fasts. But I do deal with the aftermath of what happens when people who should not fast can’t seem to shake the idea that they “should.”)

Based on what I see, hear, and read in the “Paleosphere,” the people who report the most trouble with fasting—the ones whose adrenals crashed,” or whose thyroid got killed”—are women. Usually younger women. Usually younger women who do a great deal of intense exercise. Usually younger women who do a great deal of intense exercise on insufficient calories (from whatever combination of macronutrients you like; doesn’t matter – the bottom line is, it’s just too damn little food). They also usually have full-time jobs and wake up at the crack of dawn to get in a spin class, boot camp, or CrossFit WOD, and sometimes do another one after work. These women are already in metabolic and hormonal danger. Add fasting to that mix and, yes, of course they don’t fare well. Why is this a surprise to anyone?!  (*Tap, tap.* Is this thing on?!)

(Case in point: Elle Russ, author of The Paleo Thyroid Solution. According to her profile as a “success story” on Mark’s Daily Apple: “I am 5’2”, I was 110–115 lbs, 16% to 17% body fat, and before hypothyroidism hit me, I exemplified the image of health and fitness. […] My doctor tested my thyroid incorrectly by only testing my TSH. He said the TSH was within range, I did not have a thyroid problem, and I just needed to exercise more and eat less, which I thought was impossible because I had been working out two hours a day and eating 1,200 calories or less…” To be clear, Elle wrote nothing about fasting. I am simply sharing her story as an example of who should not fast, but the kind of person I regularly hear from who is doing so. And I find it inexcusable that none of the doctors she ever saw for her thyroid suggested that she EAT MORE FOOD and EXERCISE A LITTLE LESS. [Also, perhaps 110 pounds and 16% body fat maybe did not “exemplify the image of health and fitness.” And please know I mean absolutely no disrespect toward Ms. Russ. As someone who has struggled with my own thyroid issues, I think her story is an important one, and I’m sure her book will be an incredibly valuable resource for other women, whose doctors are complete morons. I am simply speculating that her thyroid troubles might have—might have—been directly related to working out 2 hours a day on 1200 calories.])

So yes, fasting is great for women. Except for the women it’s not great for. I can almost guarantee you that none of the women claiming that fasting “wrecked their adrenals” or “trashed their thyroid” were 250 pounds, type 2 diabetic, and on five prescription medications. They were probably already following a low carb or Paleo diet, highly active, type-A personalities, severely under-fueling their activity, and already stressed to the max. These people are not suitable candidates for fasting.

So what’s up with men, then? Why do women seem to run into these issues far more often than men?

I do think there’s something to the argument that the female endocrine milieu is more “delicate” than that of males, but let’s get real: we delicate flowers can go without eating just fine. We’re not going to stop menstruating, or lose all our hair if we skip a couple meals now and then. The problems come when women combine meal skipping with unrelenting (and largely self-imposed) physiological stress, particularly if their normal, habitual food intake is already a long-term caloric deficit. It’s fasting on top of fasting and stress on top of stress. Robb Wolf and Chris Kresser often talk about the “allostatic load.” English translation: there’s only so much the body can take before (as Robb would say) the wheels come off the wagon.

The difference between the inappropriately fasting women and men of just about any stripe—stressed out, cool as a cucumber, lean, heavy, and anything in between—is that when men fast and do a hard workout, at some point, they will eat a huge meal. Maybe it will be immediately after the workout, or maybe they’ll wait 2, 4, 8, or 15 hours. But eventually, They. Will. Chow. Down. Not so with the women! They’ll fast and do a hard workout, and then “refuel” with three ounces of skinless chicken breast, a slice of avocado, and four cherry tomatoes. For all intents and purposes, they are already fasting. Compared to the demands they are placing on their bodies, they are already not eating anywhere near enough food. If we’re wired for feast and famine, then we’re wired for feast and famine, not famine and famine, or famine and eat a tiny bit of food, which, in the case of the women I’m talking about, are basically the same thing.

(To be clear, I do understand that, biochemically, eating a small amount of food is actually very different from full-on fasting. I’m just trying to make a point.) The larger point I’m trying to make here: if you’re gonna work hard, you gotta eat hard! This is the part most women are just not psychologically prepared to do. We have let magazines brainwash us into thinking that an effing ridiculous “deck of cards”-sized piece of virtually fat-free tilapia is enough to fuel a healthy adult human being. I can’t even… I seriously don’t even know what to say anymore. I get clients like this all the time. (And I will be writing quite the related rant about keto zealotry. Hang onto your hats for that one, folks.)

Okay, since I don’t know what to say anymore, even though there’s lots more I want to say, I’ll make this a two-parter. (Click here for part 2.)

In closing, I haven’t verified this with Dr. Fung, but I would venture to guess that he’s never had a 240-pound, type-2 diabetic female patient on six different medications claim that one of his recommended fasting regimens landed her in “adrenal fatigue” or induced hypothyroidism in her. Honestly, I kind of have to roll my eyes at the mere thought of that.

Like I said: fasting is great. Except when it’s not.

(Also: one more thing before I go: regarding the women who run into hormonal mayhem from fasting, I don't think it’s the fasting that’s the culprit. I think it’s the long-term caloric deficit combined with too-frequent intense exercise and unrelenting psychological stress. What the fasting does is exacerbate and bring to light problems that were already there. So it’s wrong of me to say that women who fit this description “shouldn’t fast.” What they more likely should avoid is exercising themselves stupid on 1200 calories a day and being insanely micronutrient depleted.)

P.S. If you would like to explore intermittent fasting, I highly recommend Dr. Ted Naiman’s guide, which you can find here. (Or the printable pdf version here. A great resource!)

And here are some good podcast interviews with Dr. Fung:

·        What Really Causes Obesity? (With Vinnie Tortorich, 3/4/2016)
·        Unlocking the Secrets of Weight Loss (With Vinnie Tortorich, 10/23/2015)
·        The Obesity Code (on the 2 Keto Dudes show)

And don’t forget one of my all-time favorites: The Two Big Lies of Type 2 Diabetes (Hint: It’s the Insulin, Stupid!)

Disclaimer: Amy Berger, MS, CNS, NTP, is not a physician and Tuit Nutrition, LLC, is not a medical practice. The information contained on this site is not intended to diagnose, treat, cure, or prevent any medical condition and is not to be used as a substitute for the care and guidance of a physician. Links in this post and all others may direct you to, where I will receive a small amount of the purchase price of any items you buy through my affiliate links.


  1. Thanks for the shout-out to the 2KD Podcast.

    We had a few women among our group with around 25 lbs of body fat who were frustrated that they couldn't comfortably fast, but their boyfriends with 60 lbs of body fat had taken to it like ducks to water.

    So I went looking for some science on maximum body fat mobilization rates and did the math ... and you're right for some people fasting is just not great.

    1. Thanks, Richard! Love your podcast! :)

    2. Also, this calculator does not take into consideration the degree of insulin resistance that can reduce the amount of fat mobilized per lb per day… I suspect this is a major contributor

    3. I don't quite understand (2) How many calories you will be short and (3) What % of energy savings you would have to find. Do you mind clarifying please?

    4. I think of it like an ATM daily transaction amount. You might have 35k in your bank account, but your daily ATM transaction limit is $300.

      It's similar with body fat. There is a limit of how much energy it can deliver up. The only study to calculate that is the Alpert study which puts it at 31.5 kCal/day for every pound of body fat. I'm not sure that the data underlying this is correct (it's from Ancel Keyes after all) but the math calculating the rate from that data checks out. It's roughly 1% of your total stored energy that you can use (if you call upon it) every day.

      So let's go with the ATM analogy. Let's say you have 35K in the bank (10lbs of body fat = roughly 35,000 kCal) and a $300 daily transaction limit (daily limit of body fat draw down on 10lbs = 315 kCal). Let's say in our analogy you wanted to rent a fancy townhouse at $5600 a week, or $800 a day (your today daily energy expenditure is 800 kCal).

      So here's the problem; you can only withdraw $2100 (7x$300) from your ATM, so now you are short $500 every day.

      You can't print money, so you have 2 options;

      * negotiate the rent down (your body reduces your metabolic rate and/or consumes lean mass to the tune of 500 kCal/day)
      * Get a second job for cash (eat about 56g of fat every day)

      How many calories you are short depends on how many you need (TDEE) and how many your body fat is willing to release.

      BTW: Izzy is correct, insulin will also limit access to body fat even further. At the risk of further straining my analogy ... maybe your teenager has your ATM card so some days you can withdraw a *lot* less than $300.

    5. Thanks! This succinctly explains the HF in LCHF. We still need dietary fat, even when we have a lot of adipose cells to draw from. Frankly, I've never understood that or seen it cogently explained before.

  2. Amazing as always! I started reading Dr. Fung after reading you, and I love how fasting has helped me break my stubborn set point. I thought Dr. Fung's point about how fasting and feasting balance each other is important (and often overlooked). And I know several of the type of woman you are talking about. They exercise hard, think a sliver of avocado and drizzle of olive oil on their salad is high enough fat, and limit the protein too.

  3. People with lots of adipose tissue can fast easily and for longer fasting periods than people with limited adipose tissue. Yup.

    Certainly not a woman problem compared to a man. The only difference is that males have a much lower essential body fat %. Take a bodybuilder doing contest prep already sub 6% BF and get them to total fast for a week. That would be a disaster.

    1. Yes, good point! It has a lot to do with body fat percentage. Someone with a lot of body fat ought to be able to go a while without food with no catastrophic effects on health or emotions. But like the example bodybuilder you mentioned, the women I talked about are *already* lean and maybe don't have the reserves to draw on. OR, actually, sometimes they are *not* lean, but their overall total stress load is already too high and fasting makes that worse. I have gotten inquiry after inquiry from females who are just plain too scared to eat an appropriate amount of food to properly nourish an adult human. Period. And I am so sick of it.

    2. Amy, if I'm understanding you correctly, you're not talking about daily IF regimens that are not calorie restricted. Provided we eat a sufficient quantity of protein and fat during our eating window, do you expect even lean persons without high body fat to suffer from intermittent fasting?

    3. Correct. As long as someone is eating enough, it should be fine. Especially if we're talking about intermittent fasting, as opposed to extended, multi-day fasts.

  4. Great post, Amy! You "split the arrow which was previously in the bullseye" yet again!

    1. Thanks Tess! I knew you would like this one. ;-) We seem to be on the same page with a lot of this stuff.

  5. Great post! Made me wonder if you had been a member of too many health groups on Facebook.

    1. I am. (But I also get people emailing me directly with questions related to things like this.)

  6. Hi Amy,

    I think your points are quite valid for intermittent fasting (IF). Intermittent fasting has... become... a... kinda... pop-medical... thingy? And yet with sincerely valid value for many.

    That Dr. Fung and Dr. Furman et al allopathic physicians have become hip to the value of intermittent fasting, and invoke IF as a practical adjunct therapy to accomplish caloric restriction among their patients that need it, is truly a great thing.

    However, IF is NOT the 1,000s of years traditional therapeutic practice of successive days without food and only water amidst enduring convalescence that is fasting.

    Fasting is fasting: It’s transformational. It’s curative. It’s complete: it’s done once for what ails you. And you’re done.

    Fasting would in fact likely be the best antidote for those hardwired, obsessive compulsive, ledge lingering folks you find suffering and freaking out from IF–or any other aberrant dietary behavioral problems. Fasting cures physiologic maladaptive behaviors as well as obsessive emotional dispositions handily in its transformational manner. It’s a long, deep journey to the other side of you.

    IF is best understood as caloric restriction on this side of the mountain. And, as you point out, it’s good for an appropriate segment of folks that can most benefit from their calories being restricted–not those already living close to their bones or overly obsessed by their next workout, or any other contraindications.

    IF can be a positive practice, but–like any dietary madness–is also prone to easily leading any good number of folks into modes of obsessive excess, self-abuse and extremism in abstinence from eating. This is a shame, but hardly surprising.

    My one request is to not call “intermittent fasting” actual “fasting”. Let’s call IF what it is–dieting, caloric restriction and/or intermittent fasting. IF will never be fasting, or achieve the therapeutic effects of fasting, anymore than walking up a hill each day would add up enough miles to actually get you on the moon–with IF you’re always right back where you started each day.

    Well, at least you’re not rolling that same heavy rock you started off with at the bottom of that hill–it’s getting a lot lighter day by day... Sisyphus or not.

    1. THIS, 100%, YES --
      "IF can be a positive practice, but–like any dietary madness–is also prone to easily leading any good number of folks into modes of obsessive excess, self-abuse and extremism in abstinence from eating. This is a shame, but hardly surprising."

      I could delete my blog post and just post this line. :)

  7. I think this is why "safe starches" became all the rage a few years ago. People felt better eating those safe starches because they were running themselves ragged and eating like a bird. Funny how more food made them feel better!

  8. I can do a 16/8 or 18/6 fast pretty comfortably on most days but have been pretty wary of longer fasts, almost fearful actually. I sometimes fear becoming too hungry thinking it might lead me down the slippery slope of eating whatever the hell I see in front of me - even though I typically have keto snacks squirreled away for just such an occasion. I remember reading on that people with past or present eating disorders should be careful with fasting and I fall into that camp. For years I would binge eat at night, waiting for the family to go to bed before breaking out my stash of cookies and chocolate bars then destroying the evidence. While I am pretty happy and satiated on Keto I still always feel like I am just one super hungry/emotional day away from a Oreo and Lindt eating regularly works better for me.

  9. Which medications would rule out IF?

    1. To be honest, I'm not sure. Maybe there aren't any. I was just trying to cover the bases and not get in trouble, hehheh.

  10. Very good thoughts, thank you for this. I am a busy mama to nine who has taken her health back thru keto (optimal protein, moderate fat) and feel fantastic. I also practice TRF (time restricted feeding) mostly because it works so easily with my schedule and I do love the benefits I get from it. But, I do eat well when I am eating!! My kids think I am a piggy at times. I also engage in 2-3 long fasts (14 days) a year and find them extremely beneficial, but they are for a purpose and I am sure to feast well before and after and in between, something Raymund Edwards taught me.

    I am at goal weight and 'goal' blood work ;-) I love to exercise, as for many years I was simply too unhealthy to do so. I have to be vigilant to NOT over-train (under recover, if you will) and under eat, because as you point out, it's just what we women tend to do, under eat and work ourselves into the ground! I slip up now and again and find myself pulling back with the food or increasing my workouts and have to snap back to wisdom and eat more and exercise less! Not sure how long the battle will be, but I am so thankful for posts like this to help me remember. I chased health and fat loss for over 27 years, might be a few years til I am able to realize I don't need to do that anymore. Anyway, bless you and your work!

  11. Well balanced article. Listen to your bodies peeps!!

  12. " But what he doesn’t say—and I wish he would—is that the people he supervises on fasts are in his “Intensive Dietary Management” clinic specifically for diabetes, obesity, kidney disease, and the like." Dr Fung does say this, I've seen it on multiple occasions in his writings. He is talking to those of us who are overweight and have metabolic syndrome or diabetes. But I agree with your diagnosis, fasting is not a bandwagon for everyone to climb on.

  13. I'm "serial-reading" all your posts. I'm addicted. Question:

    1. What are your thoughts on fasting (36+ hours) + keto?

    2. During the non-fast days (macronutrients and calories being spot-on) do we need to be concerned about total weekly caloric deficits?

    1. It depends on what the goal is with fasting. Fasting is great for some people, and a total disaster for others. I can't comment without knowing more. Feel free to email me privately, but I can't give any recommendations without knowing what the goal is, current health status, etc.

      As for "serial reading," start with these: ;-)

    2. Yeah, I realized that after I clicked "Publish". Sorry. I'll zap you a short message. Thanks