When Does Fasting Cross the Line?

A little while back, Mark posted an article about 14 scenarios in which intermittent fasting (IF) might be just the ticket. We got some requests for a follow-up about times when IF might not be advised.

Mark has already written about cautions for women and athletes specifically. I’ll link those at the bottom. More generally, it’s important that anyone considering IF make sure that they are in a good place physically and mentally to handle the additional stress of IF.

As Mark said:

If you haven’t satisfied the usual IF “pre-reqs,” like being fat-adapted, getting good and sufficient sleep, minimizing or mitigating stress, and exercising well (not too much and not too little), you should not fast. These pre-reqs are absolutely crucial and non-negotiable, in my opinion—especially the fat-adaptation. In fact, I suspect that if an IF study was performed on sugar-burning women versus fat-adapted women, you’d see that the fat-burning beasts would perform better and suffer fewer (if any) maladaptations.

Fasting is generally healthy when done properly. The same goes for exercise and carb restriction. These behaviors help many people; but they can also be unhealthy for certain people or when used too much or in the wrong circumstances.

Today I’d like to wade into potentially controversial waters and talk about when fasting potentially crosses the line into disordered eating territory.

Before You Get Defensive…

If you find yourself feeling defensive already because you enjoy fasting and think that I’m going to suggest that you have a problem, please hold off commenting until the end.

To be clear: I do not think that fasting is inherently disordered. However, its popularity has skyrocketed so rapidly, and its proponents are so enthusiastic, that the potential downsides have been overshadowed. Actually, that’s not entirely fair. I have seen plenty of people in mental health and disordered eating circles voice concerns. The ancestral community tends to be very rah-rah about fasting.

The goal for today is to shine a light on this issue so that we don’t go into fasting with a blind spot.

Eating Disorders vs. Disordered Eating

The prevalence of full-blown eating disorders (ED) is fairly low according to NEDA, the National Eating Disorders Association. Experts believe biological and/or psychological predispositions make certain individuals vulnerable. These predispositions probably interact with environmental factors to trigger ED.

That said, many more people engage in disordered eating behaviors. The behaviors and their effects are not problematic enough to qualify for diagnosis with ED, but they still negatively affect physical and/or mental health and quality of life. Depending on their severity, they might be considered suboptimal to truly unhealthy.

Before going any further, let me be clear: I’m not in any way trying to diagnose readers with ED or disordered eating. That is way beyond my pay grade. Nor should you self-diagnose. If you want to learn more or get help, the best place to start is with NEDA’s Help & Support center. They have an online screening tool and a helpline there.

As I said, I don’t think fasting is inherently bad. However, NEDA lists fasting among the behaviors that can be indicative of ED. Of course, professionals in the ED and mental health worlds are looking at fasting through the lens of restricting and exercising tight control over food intake. We in the ancestral health space tend to look at it through the lens of optimizing health.

Neither perspective is more correct. It’s all about context:

  • why you’re fasting,
  • how fasting makes you feel physically and mentally,
  • whether it takes over your life,
  • if you are using it in a way that is actually undermining your health,
  • whether you are taking it “too far” (admittedly a nebulous metric)

As with so many things, deciding if you’ve crossed the line is highly personal. This applies to much more than food. When does worry become anxiety? When does fear become a phobia?

One criteria might be whether fasting causes you distress. Another is whether it interferes with your quality of life, social relationships, and work. This is called the criteria of clinical significance in the DSM-V, the diagnostic tool used by mental health professionals.

It’s often apparent, though not always easy to accept, when a behavior is no longer serving us. Unfortunately, though, it’s not always that simple. When it comes to fasting, it can cross over into problematic territory and still feel good.

Wouldn’t I Know If My Fasting Regime Was Too Much?

Maybe, maybe not. Restricting calories and fasting can lead to elevated mood, increased energy and motivation, and even feelings of euphoria. (Fasting that doesn’t result in a caloric deficit might be less prone to this.)

There are physiological reasons why this might be the case. First, scientists hypothesize that this is an adaptive response. If you were truly starving, this increased energy, drive, and focus would help you redouble your efforts to find food.

There is also some evidence that food restriction affects serotonin and dopamine pathways and can, for some individuals, reduce anxiety. Researchers believe that this is one way anorexic behaviors are reinforced in the brain. Again, fasting and anorexia nervosa are NOT the same animal, but they can share common features. The “high” associated with anorexia nervosa sounds not unlike the profound energy and cognitive benefits that some people report with fasting.

All this is to say, food restriction can feel good. For some people—or more accurately, for some brains—the withholding of food becomes inherently rewarding. In that case, you wouldn’t necessarily recognize when enough is enough.

How Do You Know If You’ve Crossed the Line?

I’m not going to be able to provide a definitive answer here. That said, these seem like potential red flags:

  • Fasting makes you feel euphoric. That doesn’t necessarily mean it’s harmful by any means. Still, take a step back and evaluate whether you are always making healthy choices.
  • Your fasting regimen causes distress or significantly interferes with your life.
  • You feel like you have to fast or something bad will happen.
  • When you eat after a fast, you feel out of control with your food intake or binge.
  • You’re ignoring symptoms that might be related to too much fasting: adrenal or thyroid issues, fatigue, hair loss, weakness or inability to perform usual workouts, sleep disturbances, too much weight loss, etc.

Again, I am in no way diagnosing anyone here. If you think that perhaps your behavior has crossed into unhealthy territory, you should seek the counsel of someone trained in these issues.

The Bottom Line

Fasting, though it confers certain health benefits, is not good for every person. Experts recommend that individuals at risk for developing ED or with a history of ED refrain from fasting, including intermittent fasting, altogether. Even if you don’t think you’re at risk, it still might not be in your best interest.

Because fasting is so popular right now, some people think they have to fast for optimal health. They worry that they are missing out on tremendous health benefits if they can’t or don’t want to fast. The main reasons people fast are autophagy, glycemic control, and fat loss. You can achieve all of those goals through means other than fasting.

Remember, too, that you need not rigidly adhere to the same eating window every single day. It is fine to fast stochastically, to borrow from Paleo OG Art De Vany. You might also consider doing one longer fast a few times per year instead of daily intermittent fasting.

In any case, if fasting is wrong for you, it’s wrong for you, period. It doesn’t really matter if your neighbor loves it or the lady in the next cubicle lost 60 pounds with IF. If it negatively impacts your physical and/or mental well-being, it’s not worth it.

And in case this isn’t perfectly clear: Enjoying fasting DOES NOT MEAN that you are doing anything wrong. On the contrary, I hope you are enjoying fasting if it’s a tool you’re using regularly. Feeling great is the goal. If it’s not negatively affecting your physical health, psychological well-being, or social relationships, it’s not problematic. Likewise, if you are consuming enough calories when you do eat, and you’re able to be flexible with your fasting schedule when the situation calls for it, you likely don’t have to worry.

This issue can be hard to talk about because so many people feel passionately about defending their way of eating. Those who question whether fasting is always healthy, or who point out that a desire to eat healthfully can sometimes slip into orthorexia, are accused of “fit shaming.”

However, not talking about it isn’t the answer. Our goal should be to bring these issues into the light and try to better understand them. Then we can make informed decisions about which behaviors best serve us now and in the future.


Related Posts from MDA



Kaye, WH, Fudge, JL, Paulus, M. New insights into symptoms and neurocircuit function of anorexia nervosa. Nat Rev Neurosci. 2009;10: 573–584. 

Kaye WH, Wierenga CE, Bailer UF, Simmons AN, Bischoff-Grethe A. Nothing tastes as good as skinny feels: the neurobiology of anorexia nervosa. Trends Neurosci. 2013 Feb;36(2):110-20. 

About the Author

Lindsay Taylor headshot

Lindsay Taylor, Ph.D., is a senior writer and community manager for Primal Nutrition, a certified Primal Health Coach, and the co-author of three keto cookbooks.

As a writer for Mark’s Daily Apple and the leader of the thriving Keto Reset and Primal Endurance communities, Lindsay’s job is to help people learn the whats, whys, and hows of leading a health-focused life. Before joining the Primal team, she earned her master’s and Ph.D. in Social and Personality Psychology from the University of California, Berkeley, where she also worked as a researcher and instructor.

Lindsay lives in Northern California with her husband and two sports-obsessed sons. In her free time, she enjoys ultra running, triathlon, camping, and game nights. Follow along on Instagram @theusefuldish as Lindsay attempts to juggle work, family, and endurance training, all while maintaining a healthy balance and, most of all, having fun in life.

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