Hello and welcome to PREKURE’s weekly snippet of science, where on a weekly basis we share emergent research related to extending the human healthspan.


What are the health benefits of fasting?

There are many types of fasting protocols, but they all have one thing in common – going a period of time without eating. The most popular form of fasting is time-restricted eating (TRE), which basically means reducing the number of hours you can eat each day. This is called the eating window. There is evidence that reducing this window to between 6-12 hours (without changing what you eat) positively impacts metabolic health and the gut microbiome [1,2]. TRE has been shown in human studies to reduce body weight and fat mass, improve glucose tolerance, and reduce blood pressure. Many of these benefits are particularly notable in people with overweight, obesity and with insulin resistance [3]. Isn’t that great news? By simply skipping breakfast and eating your first meal at midday you can experience some of the positive effects of fasting.

To fast, or not to fast, that seems to be the question many women find themselves asking. We find ourselves wondering (given the buzz in popular media) whether there is an ‘optimal time’ to fast in relation to our menstrual cycle or whether fasting might be an issue if we are trying to conceive. There is speculation that women may not respond as well as men to this type of restriction, or at least there are parts of a menstrual cycle that do not favour restricting energy availability like intermittent fasting does – this is the main question we are trying to tackle in this post. So, what does the research tell us?


What does the research tell us?

To date, there has been little research examining the impacts of fasting on women, with even less attention paid to newer protocols like TRE and a large proportion of the research is focused on male subjects [6].

During the late 1990s there was some research conducted examining the impacts of fasting and calorie restriction on luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These are two key hormones in maintaining a normal, regular menstrual cycle due to their role in stimulating ovulation [4,5]. The findings of these studies suggest that fasting does not effect sex hormones [4,5]. A study conducted in 1994 in normal cycling women aged 21-35 years found that when these women fasted for 72-hours during their follicular phase there were no significant effects on the pulsity of LH or FSH and their menstrual cycles continued normally [4]. This illustrates that even with extended fasts, which are more extreme than TRE, women can safely fast during the follicular phase with no adverse hormonal changes. A second study conducted in 1999 with healthy young women who fasted also engaged in an extended fast (2.5 days) during the mid-luteal phase had similar outcomes [5]. It is worth noting that both these studies were conducted with a small sample of young, otherwise healthy women; therefore, whether these findings can be extrapolated out to older women or those with other pre-existing conditions remains to be determined.

In healthy lean young women, there is some preliminary evidence that fasting during the luteal phase increases parasympathetic activity and lowers cortisol levels; this suggests a possibility to produce an anti-stress effect in the luteal phase, which may reduce menstrual symptoms [8]. This study was extremely small (7 participants) and although promising, these results would need to be replicated in large observational trials and clinical trials before they can be said to carry any real meaning.

Research doesn’t point to a particular time in the menstrual cycle that is optimal for fasting, as you can see from the research above it appears that fasting throughout the menstrual cycle incurs no negative impacts on sex hormones. But, based on what we know about the menstrual cycle, there are probably times when fasting might be easier. Fasting may be more tolerable during the follicular phase than the luteal phase. During the luteal, from a physiological perspective, everything gears up. Your body uses more energy (less efficiently) and is pretty focused on building the endometrial lining and creating the corpus luteum [10].

If you are attempting to get pregnant this is also when your body is gearing up to fertilise an ovum – being in an energy deficit from a biological perspective is probably not a great idea.

When considering conception, it is important to supplement with folic acid (regardless of whether you are fasting or not) [7]. However, a reduced eating window is likely to result in fewer calories and in turn could result in a lack of key micronutrients. If you are going to fast while trying to conceive it is important to do so with the guidance of a registered health professional to ensure you are getting all the vital nutrients you need.

A key consideration for women is relative energy deficiency. This is of particular interest if you are an active woman who exercises frequently [9].

This occurs where insufficient energy results in hormonal dysregulation and several cascading effects, of which one is the loss of a normal menstrual cycle. It is unlikely that one or two 12 or 16 hour fasts a week will cause this though, given that this condition arises from prolonged unmet energy needs (either through insufficient intake, excessive expenditure through exercise, or a combination of the two).

Speaking of exercise. Fasted cardio is common, particularly among women. Should this be something you steer clear of or jump straight into? It depends. If you are planning on doing a high-intensity group fitness class first thing in the morning with no food – probably not a great idea. Why? Well, fasting and exercise are both stressors, and, as we know when it comes to stress, finding the sweet spot (where we can adapt but are not overwhelmed is crucial). If you are going to pair fasting with exercise you probably want to consider zone 2 exercise – or very easy-moderate exercise like a walk – rather than something too strenuous. This way you are able to harness the power of both tools to improve your insulin sensitivity, something that is useful for everyone but particularly those that are insulin resistant (e.g. diabetics and those with PCOS).


The reality and what to do now?

At the end of the day, the key is to make fasting work for YOU. It is probably important to realise that it might not be easy to fast across your entire cycle and that you should approach fasting strategically, considering the other things going on in your life. Approaching fasting with the curiosity of scientists and asking yourself questions to understand why this approach may or may not be working for you will allow you to celebrate your failures along the way and learn from the experience rather than getting bogged down by the details.

What the research does tell us is that context is important and that you should consider whether fasting is right for you. Perform and n=1 experiment on yourself and record what happens. Do you notice significant changes in your mood, appetite, and weight if you fast during different times of your menstrual cycle? Keeping a journal and logging how YOU respond to different fasting protocols at different times of the month is likely the most meaningful thing you can do while science catches up.


3 things you can do right now to harness the power of fasting

  1. Reduce your eating window by skipping breakfast
  2. Pair your fasting with zone 2 exercise
  3. Record how your new fasting strategy affects the way you feel



PREKURE is all about applying cutting-edge, evidence-based science into practice and we hope that by equipping you with new knowledge on a weekly basis you can incorporate this into your own life and share it with your clients when appropriate. Each week will bring with it new and exciting research, however, if there is something you are itching to know more about please email us and we will keep it on our radar as we curate our weekly snippets. 
We will only be sharing open-access, freely available journal articles and blogs with you. However, we wanted to make you aware of the academic workaround for getting your hands on the latest research. When looking for research you might find yourself browsing PUBMED or Google Scholar and happen upon a research article that you want to read, unfortunately, the publishers want you to pay to access it. Oh well, guess you should just keep looking right? Wrong. You can use another website called ResearchGate to access journal articles. Here, researchers create a profile and upload their work. If the PDF you are after isn’t available you can simply click the ‘request PDF’ button and the researcher will email you a copy! As an example, here is our very own Prof Schofield’s ResearchGate profile.