When NOT to fast

Fasting, particularly the type known as intermittent fasting, has been popular in many health communities for awhile. Many people learn about it through Mark Sisson, Leangains, or Eat Stop Eat. Some people do it for weight loss, other people do it for its other potential benefits such as boosting cellular cleanup known as autophagy.

But it has been getting some negative press. The latest I saw was this horror story: How Intermittent Fasting Saved Me…while Slowly Killing Me:

By week 8, my chin was breaking out more. By week 9, more, by week 10, I had legitimate acne; large cist-like monsters just hanging out under my skin. A bumpy, unhealthy face, tired eyes, no energy, what my mom called a “depressed” state of mind. My hormones were ALL out of whack.


Yikes! But many people report that fasting has great benefits. That was my own experience. At first at least. I liked the idea of IF because I've always hated breakfast and it jived with my natural tendency to want to work without thinking about food. And often it was pretty awesome for me. I felt energetic and focused.

But sometimes I just felt terrible. Fatigued, lacking focus, light-headed, distracted by gnawing hunger. Was it time to ditch the IF habit?

No, because this was only happening sometimes. Clearly there were times my body was not up for fasting. And others when it provided a boost.

So over time I've figured out a few rules that keep me from fasting when it's going to back-fire. Of course this requires you not follow a strict regimen, that you be willing to skip fasting when it's not the right time. Don't worry you can always go back to Leangains or whatever later. And you'll be better prepared to do it if your body isn't a mess. Because fasting in the wrong context can stress your body, telling your biological systems that you are in a very bad place. And when that happens, it can respond dysfunctionally.

When to consider skipping fasting rather than skipping meals


  1. When your sleep is disrupted: Oh, you accidentally stayed up all night because you were watching a movie? Or your sleep was bad because a dog was barking outside your window all night? This might not be the best time to fast. Wait until you get a good night's sleep before you fast.
  2. When you are psychologically stressed: Have a huge project due at work? Just broke up with your boyfriend or girlfriend? Have an important speaking engagement? This not be the best time to skip meals.
  3. When you are not getting good nutrition: if the meals you are eating when you aren't fasting aren't nutritionally dense and nourishing, you aren't in much of a good state for fasting.
  4. When you are using it to reach unrealistic or unhealthy goals: Are you a woman trying to get six-pack abs? Maybe you should consider that at the body fat percentage required to have those, many women experience amenorrhea. My own opinion is…well, why bother risking it? Do you want to trigger the dreaded female athlete triad? I don't know, I've always been perfectly happy with a normal flat stomach with vague muscles though. I guess if you are trying for the bodybuilder look, go slow and be careful. You might want to try many small meals instead of IF because it might be less likely to trigger starvation adaptations.
  5. On that note, IF might not be for you at all if you have serious hormonal dysfunction: I'd recommend not trying IF if you have amenorrhea, because the last thing you want to do is trigger more stress in those hormonal systems. Also, if you start to experience menstrual irregularities, maybe it's time to reconsider your fasting habits. I've only experienced this once in my fasting and it was when I did my vegan paleo experiment. And I discontinued it immediately because I've encountered so many women in this community who lost their periods on IF…and never got them back. I'm not going to say that IF is dangerous for women, but be aware that when done dysfunctionally it absolutely has the potential to make you a hormonal wreck.
  6. When you have issues that can be exacerbated by caloric disturbances like hypothyroidism, low blood pressure, and feeling cold all the time. I've had issues with low blood in the past and I suspect that IF led to an episode in which I fainted. Over time I was able to recover by eating a higher carb diet, but until my blood pressure was up, I fasted from fasting.
  7. When you are suffering from certain digestive issues is another bad time to do it. Fasting might help bloating/gas, but if you are constipated, it's probably going to make it worse by decreasing motility and moisture.
  8. When you are suffering from constant hunger pangs. Some people are going to disagree with me, but my own experience and the experience of many others I know is that if your body is in an appropriate state for fasting, you will not experience hunger during your fasts. That is one of the reasons I do not ever time my fasts. I eat when I am hungry. For me, to constantly have to obsess about food and suffer from hunger pangs totally defeats the purpose of fasting. I don't know if it's a sign of physiological stress,but it's definitely a source of psychological stress.
  9. When you are trying to adapt to a new diet. Whether it be a paleo diet, a ketogenic diet, a gluten-free diet, an elimination diet… or any other major change in dietary habits, let your body adapt to your new diet, which often is a source of stress, without adding fuel to the stress fire by not actually eating your diet.
  10. When you are trying to adapt to a new exercise regimen. Another dumb thing I did…keeping up my fasting at the same time I was taking an Olympic lifting class. Yeah, and then I wondered why I felt crappy. Changes are hard and especially when you are ramping up your activity level it can take your body time to adjust. It's probably a good idea to provide consistent nourishment at the beginning of this change.


A borderline case is when you feel like you are getting sick. Often I will fast in this case and end up not getting sick. But the research on this matter is mixed, with few studies in humans. One study showed that it might be a good idea to feed a cold and starve a fever. Instead of fasting, it might be a good idea to stick with gentle easy to digest immune nourishing foods like soup or stews. There is even some scientific evidence that chicken soup might help fight colds

edit: and as someone pointed out in the comments on Facebook, it's probably not the greatest idea to fast when you are pregnant or nursing. I'd hope that would be obvious...but you never know. And it seems to be a problem in some cultures.

I'll fully admit that sometimes this is hard to follow. When I'm stressed and have a big project due, the last thing I want to think about is breakfast. But to be honest, I've found it's better to eat something "bad" in this kind of situation than to fast. I wish I could just skip a meal if there are no good choices, but sometimes I have to bite the bullet and know that while what I'm eating isn't optimal, at least I am not pushing my body into a bad place.

And as always: if something makes you feel bad and isn't working for you, it's probably a good idea to stop doing it.