Why I don't recommend fasting (intermittent or otherwise) for most women

I get questions about fasted workouts or intermittent fasting all the time. It has become super popular in the media and social media world, with perceived benefits ranging from weight loss and increased energy levels to longevity, better athletic performance, blood sugar management and even prevention of cancer cell growth or chemotherapy side effects.* There is sort of a strange intersection here because while there is some research on intermittent fasting (IF) as it relates to these various topics, I also very much consider it a diet in the way it’s being used in the mainstream. It checks all of the diet boxes, in that it has rigid rules around meal timing, is usually not sustainable for the long term and therefore, neither are any of its “results.”

one of my favorites: hearty chicken and rice soup

For women specifically, I have a lot of concerns about IF because of the lack of peer reviewed research on women (compared to men) and the increased risk of negative hormonal side effects. Let’s get into it.

Women’s bodies are extremely sensitive to times of stress (i.e., fasting)
Fertility and reproduction are an important function of the female body, and these systems will shut down in times of prolonged stress. Consistent fasting is a type of stress, and can increase stress hormones like cortisol, decrease the hormones estrogen and progesterone, sending the body a message that it’s not a good time for reproduction. Even if fertility is not an immediate concern for you, there are other downsides to these hormonal changes, like poor bone health, altered or slowed metabolism and mood changes, to name a few. To add more complications to the mix, every woman’s body is different in how it reacts to stress. One woman may handle stress very well (and therefore, some types of IF), and another may extremely sensitive to stress or fasting in any form. Most are probably somewhere in the middle, but can be sensitive to even the most common (at least that I see) type of intermittent fasting - 16:8 or similar.

You may lose connection with hunger and fullness cues
Since IF has rules about when you can and can’t eat, it’s common to ignore hunger cues until you’re “allowed” to eat. That itself acts as a stressor on the body, and can also affect how well you trust your body, and vice versa, to let you know when it’s time to eat. If you finally get to your eating window and you’re starving, it can also be hard to hone in on your satiety cues and stop eating before you feel extremely full.

Research on IF in women is extremely lacking
As I mentioned above, there are many more studies on IF in men compared to women. Part of the reason is that it’s so hard to control for the hormonal changes we experience on a monthly basis, and the hormonal changes I described above when some women are fasting. A lot of the studies I did find on women have small samples (less than 100 participants) and were done over short time periods (less than a year, many only 6-8 weeks). It’s hard to translate something like that to real life, when you have to account for normal life circumstances, don’t have a strict study protocol or guidance and need to continue what you’re doing for years to see long term benefit.

It’s hard to meet energy and protein needs
If you’re essentially skipping one meal per day, or having a mini meal, it’s going to be harder to meet your body’s energy and protein needs. And, it’s almost impossible to have at least 5 servings (~1 cup raw, 1/2 cup cooked = a serving) of vegetables and fruits per day, which is what I typically recommend. For female runners, this can be even more detrimental because they often have a hard time meeting increased energy needs even when not fasting at all. Performance can also also be affected if you’re not refueling after morning runs or workouts because you’re waiting for your IF eating “window” to start.

No benefits to athletic performance
While there is some research indicating fasted workouts can help the body adapt to use more fat for energy (vs. carbohydrates), there are no conclusive benefits to performance when these adaptations are made. Which, to me, makes it pretty pointless to try them in the first place.

In conclusion
While there are clearly many reasons why I don’t recommend IF to my (mostly female) clients, there are always exceptions to the rule and I recognize that too. I wouldn’t stop a woman from doing IF if they were happy, felt it was sustainable for the long term, had a positive relationship with food and felt great. In my experience though, this would be somewhat of a unicorn scenario. I also totally recognize that the research could get better or change at some point, and I always try to do my best and stay on top of that too. As of right now though, IF gets a thumbs down from me and I hope I did a decent job explaining why.

*for the record, I never recommended fasting to any of my breast cancer patients. The research absolutely is not supportive and there are risks.

as always, this post is for educational purposes only and not intended to serve as individual nutrition or medical advice. For more information on working with me, please visit my website