Interested in the health benefits of fasting, but worried it might mess with your hormones and health?

Mirthe Precision Health
9 min readJan 8, 2024


Fasting has gained a large amount of popularity in recent years. But as we got excited about the many health benefits, and many started implementing it at lengths — reports of excessive fasting messing with hormones and other health markers started surfacing, making the whole fasting business a rather confusing topic.

Research shows wonderful health benefits to fasting, including:

  • Cellular renewal (getting rid of old and sick ‘zombie’ cells (senescent cells) and mutated ones, making space for new and healthy cells instead via a process called autophagy), which helps to slow down ageing and to prevent disease occurrence like cancers and autoimmune disease
  • Stem cell activation and NAD+ recycling via sirtuin production — more anti-ageing support (1).
  • Weight loss (2).
  • Better health profiles such as reduced cholesterol, blood sugar levels and increased insulin sensitivity (which can help with issues like PCOS (polycystic ovarian syndrome) and the dreaded apple shape body with associated health concerns) (3).
  • Gut microbiome reset
  • Immune system strengthening
  • Reduced hunger
  • More focus and energy, in part due to its benefits for mitochondria
  • Better sleep
  • Reduced inflammation
  • And many more.

So the more we fast, the better? Not so fast.

While fasting has wonderful health benefits, more is not always better. Some of the common issues people started to experience as fasting became more mainstream, were

  • Irregular cycles in women — or even amenorrhea (a stopping in a monthly period altogether)
  • Burnout
  • Trouble sleeping
  • Hairloss
  • Bloating and weight loss plateau
  • Blood sugar swings
  • ‘Looking old’ and frail
  • Feeling weak.

Oh dear, none of them are fun! Luckily, there’s been a ton of research done in the past years on fasting, so we do know at least in some parts what works and what doesn’t (with a few grey zones thrown in). I will dive into them for you below and hopefully you’ll feel empowered to add some (safe) fasting into your daily biohacking and health optimisation routine!

To fast or not to fast? And if so, does it matter where in your cycle you are? Let’s dive into what research has to say.

What we know about the optimal length of a fast

Research suggests that anything up to 24 hours, the body goes through a range of wonderful health benefits. After that, we do ramp up cellular renewal and some of the processes even more, but often at the risk of more downsides too. In a study done at the University Hospital in Tubingen, Germany, researchers found that once fasting went longer than 24 hours, health measures like HRV, which is a measure of whether the body is overly stressed (whether the body is more in the healing mode (parasympathetic nervous system) vs fight-or-flight(sympathetic nervous system (4)), went down (5).

Another study showed that after 24 hours of fasting, the subcutaneous fat tissue changed to acquire key properties of visceral fat. Visceral fat is the fat that is often linked to detrimental health concerns like cardiovascular disease, inflammation and neurodegeneration (6).

While I put some of my patients on regular longer fasts, such as the 5 day fasting-mimicking diet developed by researcher Valter Longo at the University of Southern California, in particular if there are more severe issues at play like cancers or autoimmune disease, for most people that are simply looking to optimise and extend health span, this more rigorous fast might only be a good idea once in a while.

Time restricted eating

One of the most popular ways to practise intermittent fasting is time restricted eating. This strategy confines ones’ daily eating window to a shorter time frame, leaving more time per day to fast. (There are other forms, such as the 5/2 diet, which I won’t get into in today’s post.)

This can range anywhere for a 10/14 split (10 hours eating and 14 hours fasting window) to 1/23 (1 hour eating with the remaining 23 hours fasting), without necessarily changing quantity and quality of food.

Now the problem is that there is a big difference between these types of time restricted eating, which can add to the confusion. With dinner often being the most social part of the day, many started fasting all day, then gorge themselves at night.

However, spoiler alert, this might in fact not be a good way to do it. Research shows that if individuals were eating all their calories at night (any time after 4pm) compared to a ‘normal’ 3 meal per day schedule, their metabolic health markers were way worse and reversed the health benefits of fasting. Not great.

On the contrary, recent research highlights the importance of aligning our eating with the natural daily daylight, which to many of us is quite a foreign concept.

The timing of your meal might matter as much as what you eat

However, research repeatedly showed that early time restricted eating, where the eating window was confined to the earlier part of the day, and fasting started in the afternoon, had all of the above mentioned great health benefits as a result. Typical protocols would often range from around 8am or 10am to 2 to 4pm. They linked these benefits to being in alignment with our circadian rhythm.

Much of our body is fine-tuned to the natural rhythm of the light. Our bodies have natural rhythms, such as when we are best able to process food, sugars, are most hungry, will burn or store more fat, etc. (7).

And while light cues help keep our circadian rhythm healthy, food cues can either assist or mess with it.

It appears that our body is in fact best primed to have the largest meal of the day around 11am to noon. That’s when it can utilise the fuels best, without storing the calories as fat. The same meal, if eaten at night, has an entirely different effect, and will in fact be much more likely to be stored as fat around the waistline, plus mess with cholesterol, inflammation and other metabolic health markers (8, 9, 10, 11, 12, 13).

Another interesting player in this topic is melatonin. Melatonin is our main sleep hormone. Its production gets stimulated by light (or the lack thereof). It peaks at night, and once cortisol rises in the morning, it fades to negligible levels. If we eat while melatonin is elevated, it not only messes with our sleep, but also gets stored as fat rather than used as a fuel.

Take away message: To reap most benefits of time-restricted eating, best to eat during daylight hours — the time frame of which can be slightly longer during the summer, and shorter during the winter.

But what if finishing your meals by 3pm doesn’t suit your work or social schedule? Then you are not alone. In our society dinners are largely a nice time to connect with family members or friends, and bond. Taking that away may have detrimental effects on their own.

As a result, a popular way to practise time restricted eating is one where we eat from noon until dinner. This is a much more convenient way to incorporate for many people.

But is that as good as if we were to eat between, say 10am and 4pm?

Sadly this is one of the grey zones, and more research is needed.

While most of the studies that show benefits of time restricted eating have been done on the earlier form, research is not conclusive if delayed time restricted eating (starting to eat at 12pm and finishing around 7/8pm) has the same benefits. Yet research does show that it is much easier for people to stick to in the long run, reducing their overall calorie intake without having to constantly count calories, which has health benefits in its own right (14, 15, 16).

What seems to be sure, is that waiting until past 4pm to initiate eating likely is not the way to go, and one would be better off with a more lax eating schedule of say 10am to 6pm.

Time restricted eating — early vs late

But what about your hormones and monthly cycle?

While there is quite a lot of different information on when in your cycle best to fast, feast, etc on the net, this is what research says:

1. Fasting and its effect on ovulation

Longer term fasting and caloric restrictions will temporarily inhibit ovulation (and with it a healthy cycle and hormones) by lowering a neuropeptide called kisspeptin, yet once normal eating resumes, our reproductive organs might be better able to respond to hormones (17, 18, 19, 20).

2. Fasting and its long term effect on reproductive health

Long-term caloric restriction (as long as we get the nutrients we need, and aren’t starving) has also been shown to extend reproductive years and improve ovarian (egg) reserve, as compared to an overfed state. (21).

3. Fasting and its effect on testosterone and PCOS

Fasting has been shown to lower androgens (like testosterone), which can be a good thing in cases like PCOS where high androgen levels are contributing to many of the unpleasant symptoms like facial hair, acne, male pattern hairloss and infertility (22, 23).

4. Fasting and its effect on progesterone

Fasting excessively (and with a drop in kisspeptin) has also been shown to lower progesterone, which we need in the second half of the cycle to feel good.

This is perpetuated indirectly also by a weak or absent ovulation, as if the body doesn’t ovulate properly, we cannot make adequate progesterone in the second phase of the cycle. In addition to that, being stressed (and anything more than a 24 hour fast, or extending the daily fasting window beyond noon) has been shown to increase stress biology, which in itself can mess with the body’s progesterone production in the second phase.

Fasting has also been shown to lower thyroid hormones, and we need thyroid hormones in particular in the second half of the cycle to allow for progesterone to do its thing. Some of the wonderful benefits of progesterone include:

  • Balanced and calmer mood via its effect on GABA production — less PMS
  • Better sleep
  • Less bloating, painful and swollen breasts
  • While allows for the endometrial lining to mature and prepare for a potential baby, it also makes sure it does not grow excessively, reducing the occurrence of overly heavy periods

5. Fasting and its effect on estrogen

A study published in showed that excessive fasting can increase estrogen levels in rodents. In this experiment, rats fasted for 24 hours every other day for 12 weeks. In the fasting group, rats experienced an increase in estradiol along with a decrease in luteinizing hormone compared to controls, as well as menstrual cycle irregularities. It should be noted that these rats were only three months old, which would correlate to nine years of age in humans (24).

Other research does suggest that excessive fasting does upregulate an enzyme called CYP17a1, that is also needed to make ketones — which the body needs as an energy source in a fasted state. While we want estrogen levels to increase in the initial half of the cycle to trigger ovulation, we often run into trouble if we have too much estrogen in the second half of the cycle (25, 26, 27).

However in another study estrogen levels declined as a result of caloric restriction. More studies need to be done to come up with definite conclusions on fasting’s effect on estrogen levels (28).

Another study showed that if time restricted eating was in favour of eating most of the daily calories for dinner rather than early in the day, estrogen levels increased (29).

6. Time restricted eating and its effect on postmenopausal hormones.

In a recent study (30, 2023), time restricted eating did not change testosterone nor estrogen or progesterone levels.

So what does that mean for you? It depends…

→ If you are a woman with a monthly cycle, it appears it might be best to restrict longer fasts than the daily time restricted eating to around day 3–10 and of the cycle.

→ For the rest of the cycle, if you practise safe time — restricted eating as mentioned above, you should be fine, and even benefit.

→ The days leading up to ovulation (around day 11–14) you should make sure to get enough calories so your brain won’t get signals of starvation.

→ If you are a woman postmenopause, or you are on the pill without a cycle nor natural hormones, you may choose to incorporate longer fasts occasionally, where your stress levels permit.

If you’d like to learn more on how best to fast, what to eat and how to live to support your hormones, metabolism and health span to feel and look your best, for longer, then you might want to join my 8 week reset. In addition helping you implement the best fasting window for you, we also discuss other important topics like what to eat when you eat, gut health, inflammation, how to exercise, supplement, sleep and more to live your best life.

I’d love for you to join!

In Health,




Mirthe Precision Health

Functional Medicine Practitioner. Hormones, epigenetics, health optimisation. IG/ TikTok/ Youtube/ Fb: @mirthe_precisionhealth,,