Intermittent fasting – is it healthy for athletic women?


From a health standpoint, no.

Athletes need an adequate and balanced food and fluid intake that provides nutritional support for optimal adaption to training and optimal performance, but also for preventing illness and injuries.

The cornerstone of an endurance athlete’s diet is adequate energy intake i.e. FOOD.

In spite of what you read all over the internet, you should bear in mind that the majority of the researched population on intermittent fasting is not an athletic population, nor is it a female population. It is a male population that is either pre-diabetic or fully diabetic, or with metabolic syndrome or cardiovascular disease, or obese.

Intermittent fasting promises to give you longer life, reduce body fat and improve insulin control.

What the media do not tell you

Sex differences (male and female). Take kisspeptin for example, which is made in the hypothalamus, and is an important hormone that starts the release of several other hormones. It is also called metastin, and this interesting hormone is connected to puberty. In fasting women it is more sensitive. This neuropeptide is also responsible for the endocrine system and is the feedback mechanism for the gonadotropin-releasing hormone which is of the utmost importance for the hypothalamic response which allow us to have an endocrine system and a menstrual cycle.

Kisspeptin is more sensitive to a perturbance in that endocrine system and the lack of calories and hence is a precursor to menstrual cycle dysfunction that we see when women are in low energy states.

Intermittent fasting has shown and is advertised to bring clarity of mind and focus  and reduce brain fog.  This has been seen in men and is a parasympathetic response. In women it is not a parasympathetic response. It is sympathetic drive or response and women get anxious when fasted, depressed, they get elevated heart rate, and brain fog. 

Are you convinced yet?

It has also been found that women have no improvement in blood glucose control through Intermittent Fasting.  This is well documented for men but no result seen in women, if minimal.  Autophagy is prolific in fasting men, which is essentially a metabolic clean out where the cells clean out damaged cells, in order to regenerate newer, healthier cells.  This effect is minimal in women.

The biggest thing about Intermittent Fasting that the increase in visceral fat in women, which is really difficult to get rid of and increases cardio vascular disease.  As more research women comes out it is evident that a lot of negative aspects are prevalent.

You (women) could try it by eating early dinner and then fasting until breakfast, (short fast for the autophagy) but it will not give you a huge stimulus to disrupt everything else going on in the body.

Note, that the research spoken of by Dr. Stacy Sims says that the longer women stay in a catabolic state, the more they disrupt their endocrine system and their resting metabolic rate.

There was a study on female endurance athletes who ate before training and then fasted all day and didn’t eat again until night.

Even though they ate the total amount of calories required to keep up the right energy levels (avoiding Low Energy Availability aka LEA) they spent most of their day without food in a catabolic state.  They ended up with reproductive abnormalities.  (Irregular menstrual cycles, anovulatory cycles, amenorrheic, all due to this perturbance in the kisspeptin and not being able to take care of the endocrine cycle, EVEN though they thought they were eating enough. 

So, if you do not eat on a regular basis, and are active, you can create this thyroid dysfunction.  This high catabolic state that women stay in purposefully or due to poor lifestyle of not eating regularly  can create this endocrine dysfunction of which the symptoms can be dead-end fatigue, increase in body fat, and in spite of taking time off and trying to rest they are not getting in that proper recovery that they need. 

From a health point of view only, if you are looking at exercise, the very long line of research says that exercise is actually in itself a fasted state, because you are using fuel, and creating stress to which your body is responding including a little autophagy (cell regeneration).  You are also getting more blood sugar control and insulin responses.  You are losing body fat and building lean muscle mass.  SO, everything that Intermittent Fasting is purporting to do in the literature, is also the same thing that is purported in the exercise literature!

Short summary:

If you are not exercising, and you’re a man, then Intermittent Fasting is right for you.

If you are not exercising and you’re a woman, do not go for Intermittent Fasting, GET OFF YOUR BUTT!

What is the implication of Intermittent Fasting with purposeful training?

For example, a female athlete that does a 12-hour fast, and then gets up to do a training session fasted, and doesn’t eat afterwards immediately, then the large catabolic state that she was in when waking up plus the additional state during exercise, will not be beneficial for her adaptation.   See the studies done on men vs women training in fasted states.

The outcomes of muscle protein synthesis, body composition, aerobic adaption, anaerobic adaption, as so much better when a woman goes into training in a fed state.  This is boiling down to the neuropeptide response as mentioned earlier and the sensitivity to the female endocrine system to the non-fed state.

If you’re looking to change your body composition you need to fuel up and make sure you’re getting in the appropriate fruit and veg, carbohydrates and protein to match what you’re consuming to keep up your resting metabolic rate.

It comes back to that energy availability and reduction of the catabolic state to allow your body to adapt to the stress that you are putting it under.

Moral of the Story? Eat, train, and adapt.


Dr Stacy T. Sims , Global Expert on Female Physiology and Endurance Training, Senior Research Scientist at the University of Waikato.

Fahrenholtz, Ida & Sjödin, Anders & Benardot, Dan & Tornberg, Asa & Skouby, Sven & Faber, Jens & Sundgot-Borgen, Jorunn & Melin, Anna. (2017). WITHIN-DAY ENERGY DEFICIENCY AND REPRODUCTIVE FUNCTION IN FEMALE ENDURANCE ATHLETES. Scandinavian Journal of Medicine and Science in Sports. ER  – ( Study on whether within-day energy deficiency is associated with resting metabolic rate (RMR), body-composition, S-cortisol, estradiol, T3, and fasting blood glucose.)