Health & Fitness

Fasting: Why and how (from a female perspective)

31/10/2025 /

The emergency cereal bar

“90% of Americans are metabolically sick…A metabolically well human adult should be able to fast for 24 hours”1 

– Dr Ben Bikman (Chatterjee, 2025)

I recently overheard the following exchange in a what-will-remain-nameless cafe:

Mother to child (toddler): “Do you want a cinnamon bun?”

Mother to friend: “He’s just had lunch, but it keeps him quiet.”

Don’t compare yourself with people around you – it’s the sick leading the sick. “A classic sign of a failing metabolism that is rarely addressed is a person’s inability to go without food.”2 according to Mindy Peltz (Peltz, 2022, p.3). How many people do you know like this? Is this you? I was one of these people.

Ten years ago my metabolism was screwed, a legacy of diet/binge behaviour, and poor food choices. My mother insisted that I always carry an “emergency cereal bar” to mitigate against my frequent “hangry” attacks.

In 2014, I was 9kg heavier yet I struggled more with hunger than I do today. Surprisingly, excess fat is not protective of hunger cues. Dr Ben Bikman explains: “The liver only stores about 2000 calories of glucose in fuel and then will promote hunger to try to correct that [once depleted], even if there is abundant fuel stored in fat.”1 (Chatterjee, 2025)

As the old adage goes: The more you eat, the hungrier you become. 

Today, I adopt a cyclical fasting lifestyle. I am not at the mercy of feelings of hunger. I choose what food I put in my mouth, and when. And I definitely wouldn’t promote cereal bars as solution for poor metabolic health! (See section “Bags and Boxes with Barcodes”, below)

Here you will learn some of the benefits and biomechanics of fasting, why feast-fasting in harmony with your hormones is better for women, and how combining a fasting lifestyle with nourishing and varied food choices is best of all.

Traditional “diets” that focus on what (not when) to eat are doomed to failure:

The “Minnesota Starvation Experiment”11 (Keys et al, 1950) aimed to define famine-recovery best practice during the Second World War. Thirty-six male conscientious objectors had their food intake restricted to 1,500 calories per day for 6 months, before food was reintroduced. Not only did participants become, “depressed, anxious, listless and hypochondriacal, and [begin] to withdraw socially” their experience also describes that of the “yo-yo” dieter: “When the experiment was over, [they] quickly gained back the weight they had lost – and packed on an extra 10% of their body weight.”2 (Pelz, 2022, p.13-14).

So calorie restrictive diets are ineffective. But why?

Pelz explains: “Every time you eat less and exercise more, you lower your metabolic set point.”2 (Pelz, 2022, p. 13). This is the point at which your body maintains its weight within a preferred range of calories. Old-school thinking said your genetics determined your set point. In reality, “you train your set point, and when you eat less and exercise more you lower your set point threshold…when you go back to eating more calories or working out less, the pounds come back on much easier because you are above your set point threshold.”2 (Pelz, 2022, p. 13). Unless you are willing to adopt a reduced calorie intake as a permanent lifestyle choice, traditional dieting will fail you, if weight loss is one of your goals.

On the other hand, all forms of fasting are proven to support weight loss compared to calorie restrictive diets.7 (Chen et al, 2024), and there are innumerable additional benefits to fasting.

Why fast?

“fasting is the quickest path back to better health…I had never seen the body heal so quickly just by tweaking something as simple as when a person eats”2 (Pelz, 2022, p.4)

As mentioned previously, fasting in conjunction with dietary improvements is the gold standard, yet incredibly consuming the same diet in a shorter window (aka increasing your fasting window) offers numerous metabolic health benefits in and of itself, such as: reduced total body fat percentage, reduced visceral fat, reduced waist circumference, lower blood pressure, decreased LDL cholesterol, decreased haemoglobin A1c.3 & 4 (Gabel et al, 2018; & Wilkinson et al, 2020)

You may be surprised to learn that, “In the absence of food our cells get stronger, not weaker”2 (Pelz, 2022, p.25), as demonstrated by Nobel-prize winning scientist Dr Yoshinori Ohsumi.5 (Tsukada et al, 1993)

The upshot is, as Pelz states, “Changing the time period in which we eat is more important than the actual quality of the food we eat…The best outcomes to our metabolic health happen not when we change what we eat but by the simple act of compressing our food intake into a smaller eating period of 8-10 hours.”2 (Pelz, 2022, p.23 and p.28-29). This is hugely exciting since expensive nutritious food choices are no longer a singular barrier for anyone wanting to improve their metabolic health – the tool of fasting is free.

Further benefits of fasting were highlighted in a 2019 paper published in the New England Journal of Medicine6, a review of over 85 studies, that concluded  that intermittent fasting should be the “first line of treatment for obesity, diabetes, cardiovascular disease, neurogenerative brain conditions and cancer”2 (Pelz, 2022, p. 22). Fasting was also found to have anti-aging effects and to support pre- and post-operative surgery healing6 (de Cabo et al, 2019).

The biology and benefits of fasting 

So how does this all work? Why does fasting boost our health?

First, we must understand that the human body has two energy generating processes:

1. Sugar Burner (not fasting): Ingesting food increases blood glucose that cells convert to energy. In sugar burning mode, insulin is elevated.

2. Fat Burner (fasting): In a fasted state, the decline of blood glucose triggers the switch to the ketogenic (“Fat Burner”) energy system, at approximately 8 hours post food (with individual variation). In fat burning mode, insulin is lowered.

The 2019 paper published in the New England Journal of Medicine6 identified different cellular changes that occur in the ketogenic (fasted or “fat burning”) state. I describe some such changes here, alongside their related health benefit(s):

1. Increased ketones

Naturally produced in absence of glucose (i.e. when fasting). Ketones have a reparative effect, specifically supporting neuro-regeneration in the brain, thereby improving memory. Ketones are the preferred fuel of mitochondria, providing consistent, powered-up energy, and mental clarity. Ketone release also triggers GABA release – an anti-anxiety neurotransmitter.

2. Increased autophagy

Autophagy or “self-eating” occurs when there is nothing to consume other than that within the cell, and malfunctioning cell parts are thus detoxed out of the cell. Of particular note, “mitophagy” is the specific term coined for the clearing of faulty mitochondria (our energy generators), which offers protection from illness associated with damaged or dysfunctional mitochondria i.e. cognitive disabilities, muscle weakness, chronic fatigue, visual-auditory impairments, liver and gastro issues.8 (Green et al, 2011). Although autophagy is unable to detox synthetic chemicals or heavy metals out of cells, it will trigger cellular death (or “apoptosis”) of toxin-ridden cells that may be precancerous / cancerous.

3. Decreased glycogen

Excess sugar (aka eating too many sugars/carbs) is stored as glycogen in muscle, liver and fat. Muscle glycogen is released during intense exercise. Fasting releases excess liver and fat glycogen, promoting improved liver function, and weight loss/maintenance. In this way, fasting is protective against diabetes and fatty liver disease.2 (paraphrased after Pelz, 2022, p.27)

4. Decreased MTOR

Excess mTOR has been associated with accelerated aging, cancer, autism and Alzheimer’s. In an observational study of more than 2,000 women of varied ages, those that fasted 13 hours plus had a 64% less chance of recurrence of breast cancer.9 (Marinac et al, 2016). Although we need mTOR to grow cells, build skeletal muscles, and produce hormones, constant eating, constantly stimulates mTOR, putting cells into a growth state too often, shortening cell lifespan and accelerating aging.10 (Zoncu et al, 2011)

5. Reset dopamine pathway (easier to feel content)

Eating, even just thinking about eating, your phone pinging, and other such stimulants, give you a “dopamine hit“. Modern populations are dopamine saturated. Dopamine resistance and decline of dopamine receptors has been linked with obesity since more food is required to get a normal dopamine response (to feel sated and content).  Fasting can actually “make your dopamine receptors more sensitive. In some cases new dopamine receptor sites are formed, increasing your overall feelings of contentment.”2 (Pelz, 2022, p.30)

6. New/replaced white blood cells (72+ hour fast)

Dr Valter Longo’s famous 3-day water fast confirms that fasting provides an immune system reboot, via a mechanism of stem cell release at around 72 hours. Those stem cells become new white blood cells. The fasting group had better outcomes in patients undergoing chemotherapy.11 (Longo et al, 2016)

7. Improved microbiome (more Bacteroidetes)

Strikingly, one round of antibiotics destroys 90% of gut bacteria, and we need those microbes to help pull vitamins and minerals out of foods, and to produce the happy hormone serotonin, amongst other things. The most abundant microbes in our gut are Bacteroidetes and Firmicutes. An excess of Firmicutes is linked with weight gain.2 (Pelz, 2022, p. 31). Fasting can re-balance the gut microbiome and impacts microbes that will change white fat into brown fat. Brown fat is preferable since it has more mitochondria, providing more power and keeping you warmer. It is also easier to burn (if weight loss is one of your goals). 

8. Improved insulin sensitivity

In normal operation, insulin opens cellular gates to shunt glucose from the blood to the cell interior, where it is used for fuel and provides energy. Constantly ingesting high-sugar loads causes insulin spikes, and eventually insulin receptors on the cell surface can become congested, so that the gates cease to function. This is insulin resistance and is a precursor to diabetes. Fasting lowers insulin and can re-sensitise insulin receptors.

This is not an exhaustive list of the benefits of fasting and Pelz goes into greater detail in her book, “Fast Like a Girl”2 (Pelz, 2022)

“Sign me up!” I hear you cry. But it’s not as simple as only fasting, and especially not if you are biologically female: Cycling between “feasting” and fasting is key.

Fasting is not a magic bullet, the key is metabolic switching 

When you fast too much, constantly stimulating autophagy, you can cause too much breakdown of the skeletal muscle, which gets broken down to release stored glucose, diminishing your opportunity for improved strength (paraphrased after Pelz, 2022, p.49). 

Considering our ancestry, metabolic switching between sugar- and fat-burning makes evolutionary sense. Our hunter-gatherer ancestors necessarily cycled between involuntary fasting (famine) and feasting. The ‘thrifty gene’ hypothesis posits that we retain genetic coding that requires feast-famine cycling for health optimisation. (Pelz, 2022, p.21)

It is of particular importance to women to avoid fasting and stress at specific times in our menstrual cycle.

Women – Honour your Menstrual Cycle

The whole premise of Fast Like a Girl by Mindy Peltz is that women need to fast differently than men, and that our fasting schedule needs to be mapped onto our menstrual cycle. Our cycling sex hormones have different nutritional demands at different points in our cycle.

It makes evolutionary sense that women’s metabolism is more nuanced since women carry the metabolic burden of fertility. Dr Ben Bikman explains: “Progesterone has a central nervous system effect to promote hunger…Progesterone wants to set the stage [for pregnancy]…[therefore it’s] harder to fast during the luteal phase*…Fasting would not be favourable to pregnancy if it happened too much.”1 (Chaterjee, 2025). Simply put, a pregnant woman (or a woman with the potentiality for pregnancy) needs fuel to grow a baby – and the body provides hunger cues at the time in her cycle when pregnancy could occur.

[*Luteal Phase = week before menstruation.]


It’s also important to consider that even with the powerful tool of fasting, where stress is a key and constant player in one’s life, it is not possible to optimise health. Stress causes the hormone cortisol to spike, including the ‘stress’ of over-fasting or over-exercising. As a woman with a finely orchestrated balance of sex hormones, cortisol spikes crash sex hormones, and spike insulin. Chronic stress can cause insulin resistance and its associated poor health outcomes and fertility challenges. (After Pelz, 2022, p.15-16)

The key takeaway of Fast Like a Girl is not to attempt fasting in the week before your period (luteal phase), and to allow a higher ratio of carbohydrates in your diet at this time, and to keep stress low. I encourage interested readers of this post to buy Peltz’s book (I have no affiliation with the author). It contains an indispensable fasting-cycle diagram that maps your menstrual phases to fasting and dietary advice. 

For me, the book was revelatory and matched my lived experience of craving carbs before my period. Having an understanding of the biological imperative behind my cravings, and giving myself permission to lean into more whole-food carbohydrates at that time, has supported me to eat intuitively and in-tune with my needs. 

I find it easy to fast at other times in my cycle so that, rather than feeling like a failure for not having enough “will power” to continuously intermittently fast, I allow myself to fast only at times when it will be supportive to my body and hormonal health, and it feels effortless.

Bags and boxes with bar codes


If your goal is diminished physical and mental health, consume foods from: “bags and boxes with barcodes”1

– Dr Ben Bikman (Chaterjee, 2025)

Alongside cyclical fasting, enjoying nutritious food will further optimise your health. As Pelz states, “Fasting part of the day will clean up your cells and following that up with good healthy food will provide necessary nutrients to help your cells grow strong”.2 (Pelz, 2022, p.49). Indeed pages 174-257 of her book Fast Like a Girl are dedicated to recipes that offer pre-menstrual support (“hormone feasting recipes”), and what Pelz terms “ketobiotic recipes” that aim to keep you in your fat-burner energy system. Surprisingly, more than 60% of your food should come from good fat, when your aim is to remain in your fat-burner/ketogenic energy system (see section “The biology and benefits of fasting”, above).2 (Pelz, 2022, p.111).

So what is good, healthy, nutritious food?

The answer is both universal and individual. It is almost universally accepted that eating whole foods with plenty of plants will optimise health. Beyond this, there are numerous personal and circumstantial influences on individual food choices that will not be discussed here, but some key principles can help us all make better food choices:

1. Ingredients matter – Whole foods are best. Where buying processed, avoid anything that contains ingredients that are artificial or that you don’t recognise, or where sugar is included in the top 4 listed ingredients.2 (Pelz, 2022, p.88-91)

2. Glyceamic load (carbs/sugars) matters – Overconsumption of carbohydrates and sugars causes excessive insulin release, which can lead to insulin resistance – very bad news for metabolic and overall health.13 (Liu et al, 2019). As Dr Ben Bikman states, “To some degree insulin resistance is a common root cause of most chronic diseases: type 2 diabetes, obesity, cardiovascular disease, Alzheimer’s, fatty liver disease, infertility.”14 (Patrick, 2025)

3. Variety matters – “The more variety you have in your [whole] food choices, the more your microbes can thrive.”2  (Pelz, 2022, p.108). Choose not to succumb to convenience or habit, and instead maximise the diversity of your whole food intake (spices count!).

In summary 

Fasting is a freely accessible tool that can support metabolic, hormonal and overall health, and mitigate against illness and disease. The key is switching in and out of fasting, to promote metabolic flexibility. For women, mapping fasting to your menstrual cycle, being sure to avoid fasting premenstrually, will be most supportive to hormonal and overall health. To further maximise health, nutritious food is key.

Freya’s Fasting Cycle:

  • 17+ hour fasting (days 1-9)
  • 15+ hour fasting (days 10-15)
  • 17+ hour fasting (days 16-18)
  • NO FASTING (days 19-25)

Additional fasts to boost health outcomes*:

  • 24-hour fast monthly
  • 48-hour fast 2 x yearly
  • 72-hour fast 1 x yearly 

Discover how Freya can help you to optimise your health and wellbeing, view the services page.

[*The specific health benefits of longer fasts are discussed in detail in Pelz’s book Fast Like a Girl2. (Pelz, 2022, p.38-45)]

References

  1. Chatterjee, R. (Host). (2025, October 1). How food, fasting & lifestyle can transform your metabolic health & reduce your risk of disease with Dr Ben Bikman (No. 582) [Audio podcast episode]. In Feel Better, Live More with Dr Rangan Chatterjee. https://drchatterjee.com/58
  2. Pelz, M. H. (2022). Fast like a girl: A woman’s guide to using the healing power of fasting to burn fat, boost energy, and balance hormones (1st ed.). Hay House.
  3. Gabel, K., Hoddy, K. K., Haggerty, N., Song, J., Kroeger, C. M., Trepanowski, J. F., Panda, S., & Varady, K. A. (2018). Nutrition and Healthy Aging, 4(4), 345-353. https://doi.org/10.3233/NHA-170036
  4. Wilkinson, M. J., Manoogian, E. N., Zadourian, A., Lo, H. H., Fakhouri, S. I., Shoghi, A., … & Panda, S. (2020). Ten-hour time-restricted eating reduces weight, blood pressure, and atherogenic lipids in patients with metabolic syndrome. Cell Metabolism, 31(1), 92–104.e5. https://doi.org/10.1016/j.cmet.2019.11.004
  5. Tsukada, M., & Ohsumi, Y. (1993). Isolation and characterization of autophagy-defective mutants of Saccharomyces cerevisiae. FEBS Letters, 333(1–2), 169–174. https://doi.org/10.1016/0014-5793(93)80398-E
  6. de Cabo, R., & Mattson, M. P. (2019). Effects of intermittent fasting on health, aging, and disease. The New England Journal of Medicine, 381(26), 2541–2551. https://doi.org/10.1056/NEJMra1905136
  7. Chen, YE., Tsai, HL., Tu, YK. et al. (2024) Effects of different types of intermittent fasting on metabolic outcomes: an umbrella review and network meta-analysis. BMC Med 22, 529. https://doi.org/10.1186/s12916-024-03716-1
  8. Green, D. R., Galluzzi, L., & Kroemer, G. (2011). Mitochondria and the autophagy-inflammation-cell death axis in organismal aging. Science, 333(6046), 1109-1112. https://doi.org/10.1126/science.1201940
  9. Marinac, C. R., Nelson, S. H., Breen, C. I., Hartman, S. J., Natarajan, L., Pierce, J. P., Flatt, S. W., Sears, D. D., & Patterson, R. E. (2016). Prolonged nightly fasting and breast cancer prognosis. JAMA Oncology, 2(8), 1049-1055. https://doi.org/10.1001/jamaoncol.2016.0164
  10. Zoncu, R., Efeyan, A., & Sabatini, D. M. (2011). mTOR: from growth signal integration to cancer, diabetes and ageing. Nature Reviews Molecular Cell Biology, 12(1), 21–35. https://doi.org/10.1038/nrm3025
  11. Longo, V. D., Dorff, T. B., & Quinn, D. (2016). Fasting and cancer treatment: A pilot clinical trial. BMC Cancer, 16, 1–9. https://doi.org/10.1186/s12885-016-2684-0
  12. Keys, A., Brožek, J., Henschel, A., Mickelsen, O., & Taylor, H. L. (1950). The biology of human starvation (Vols. 1–2). University of Minnesota Press.
  13. Liu, Y. S., Wu, Q. J., Xia, Y., Zhang, J. Y., Jiang, Y. T., Chang, Q., & Zhao, Y. H. (2019). Carbohydrate intake and risk of metabolic syndrome: A dose-response meta-analysis of observational studies. Nutrition, Metabolism & Cardiovascular Diseases, 29(12), 1288–1298. https://doi.org/10.1016/j.numecd.2019.09.003
  14. Patrick, R. (Host). (2025, July 14). Dr. Ben Bikman: How to reverse insulin resistance through diet, exercise, & sleep [Audio podcast episode]. In FoundMyFitness Podcast. https://www.foundmyfitness.com/episodes/ben-bikman.com

Disclaimer: Freya Freely is not affiliated with the author or publisher of Fast Like a Girl in any way, nor does Freya Freely have any affiliation with any other author, researcher, broadcaster  or anyone else mentioned or referenced herein. Freya Freely has not received any compensation, monetary or otherwise, for writing or sharing this blog. All opinions expressed here are entirely those of Freya Freely.

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