Fasting: A Cautionary Tale?

by | 0 comments

There is a dangerous idea circulating that intermittent fasting is good for everyone, all the time. 

That just isn’t true. If you’re pregnant, have chronic health issues, an eating disorder, brittle diabetes, or a low body mass index, it may not be a good choice.

When it comes to fasting  for women, there are special considerations.  Very few studies have been done on how intermittent fasting (IF) impacts female hormones, particularly reproductive hormones.  So if you’re a woman who practices any form of fasting, it’s important to begin slowly and listen to your body. 

During my ReInvent Healthcare podcast interview with Cynthia Thurlow, nurse practitioner and TEDx celebrity with over 10.5 million views, we discussed the special considerations for women, and the influence of fasting  on female health and hormones.  

I would have loved to talk to Cynthia for hours.  There is so much to be learned from these  conversations with leaders in the field of functional healthcare.  Although we covered a lot, there are a couple of things we touched on that I feel could use some more attention, including  fasting’s potential impact on the female reproductive hormones and thyroid health.

Fasting and Female Reproductive Hormones

Whether you still have your period, are pre-menopausal, or post-menopausal, as a woman, you don’t need a study to confirm that you  often feel ruled by your hormones, but it sure feels good to be validated! 

 Research published in the National Library of Medicine confirms the undeniable importance of reproductive hormone health to a woman’s well-being. 

Optimizing your diet and practicing intermittent fasting can go a long way towards improving  your hormonal health at every stage of life. But if you’re in your reproductive years, even if you aren’t thinking about pregnancy, your body is.  How it responds to fasting is key to figuring out how to sustain a healthy hormonal balance as you incorporate IF into your health regime.



If you’re stressed, whether it’s physiological or psychological, and your calorie intake is low, your body questions your ability to carry a pregnancy and your brain will shut off the reproductive cycle. Even if you don’t want to get pregnant, this situation is not ideal and indicates imbalance.

HPG axis  

Reproduction and fertility are regulated by hormones produced by the hypothalamic-pituitary-gonadal axis (HPG axis). Cortisol, a stress hormone, can affect gonadotropin-releasing hormone (GnRH), the first step in the ovulation process.


During the follicular phase, prior to ovulation, GnRH is released by the hypothalamus. It signals the anterior pituitary gland to secrete luteinizing hormone (LH) and follicle stimulating hormone (FSH). The FSH stimulates the ovarian follicle to grow and the LH causes the release of the egg. If stress disrupts the production of GnRH, chances are you won’t ovulate.


Ten days prior to the start of your menses, known as the luteal phase, your body’s production of progesterone peaks. It’s a time that is particularly difficult to fast because your body is needing and wanting more nutrition in case it needs to support a pregnancy. If you’re struggling to stick to your IF schedule, it might be a great time to listen to your body and take a break from fasting.


I suggest the following modifications for women as they learn what IF is best for their unique physiology:

  • Fast 12-16 hours, 2-3 days a week, on non-consecutive days.
  • Do alternate day fasting, but consume 20-25% or your usual calorie intake (about 500 calories) on your “fasting” days.
  • Eat well on non-fasting days.

 “Cycle syncing” is a relatively new term that refers to modifying your diet according to your menstrual cycle. Some are finding it incredibly helpful in restoring and sustaining balance. 

Fasting and Thyroid Health

Thyroid dysfunction has reached nearly epidemic levels. I especially see it in women. 

In my Thyroid Revive and Thrive program, included as part of my  Empowered Self-Care Lab,  I teach more about how to feed and rebalance your thyroid than most doctors know. 

I train practitioners in our Nutritional Endocrinology Practitioner Training how to recognize the complicated symptoms that may result from thyroid dysfunction and effectively help clients restore balance.

And, as much as I tout the benefits of intermittent fasting for overall metabolic health, I can’t ignore the controversy about whether or not fasting runs counter to thyroid health, including the balance between the hormones T4, T3, and reverse T3. 

TSH, T4, T3

The pituitary gland produces thyroid stimulating hormone (TSH). TSH tells the thyroid to produce the hormone T4 and a little T3.  Your body needs T3 but must first employ enzymes called deiodinases to convert T4 to T3, thereby turning it into a usable form. T3 is considered the “active” hormone. If the conversion from T4 to T3 is poor, you can present with symptoms of hypothyroidism like brain fog, depression, fatigue, and sensitivity to cold temperatures.


Under certain circumstances T4 can also produce “reverse” T3. Reverse T3 is the “inactive” form of T3. Your body will try to use it in the same way it uses active T3, but when it does, nothing happens. This can also lead to all the signs of hypothyroidism.

So, what does this have to do with fasting? Here are some possibilities:

Lowering your caloric intake can cause your body to go into ketosis which has many positive benefits. But if you’re just starting out and not acclimated to restricted eating, this can cause stress that may temporarily increase your cortisol levels. An increase in cortisol levels can result in an increase in the production of reverse T3 leading to symptoms of hypothyroidism.

This increase in reverse T3 can also happen with low-calorie diets in which you are severely and consistently restricting your calorie intake. This increase in reverse T3 causes a drop in your metabolism, a protective mechanism your body uses to survive in times of famine.

Fasting Strategies to support thyroid health.

If your thyroid health is otherwise optimal, the temporary impact of fasting on thyroid hormone conversion can be minimized by using different fasting techniques.

Low and slow is my motto with fasting (as well as supplementation). 

When you first try intermittent fasting, start with a shorter fasting window a few days a week. Be sure to go at a pace you’re comfortable with, gradually increasing your fasting interval when you feel ready. This will minimize the stress involved. 

You may never be someone who can or wants to fast for extended periods of time and that’s OK.  IF still has great health benefits.

If you’re severely restricting your calories and start to show signs of hypothyroidism, this can also be a function of malnourishment. If you’re confident that you’re getting enough macro and micronutrients (the cronometer app is fantastic for tracking this), one solution is to add healthy “feast days”. By adding occasional healthy high calorie days, your body will be reassured that food is available and help your metabolism normalize. This is referred to as “cycling” feast and famine days.

There is still much to be learned about how fasting impacts  thyroid health and hormones. It’s not surprising to read about the controversy it can stir up.  What I urge you to do is pay attention to your own body and make decisions based on how your body is responding and what you are feeling.

Intermittent fasting is an amazing tool you can use to help achieve your optimal health. Never let anyone tell you there is only one way to do IF. Experiment with it until you find what works best for you.

Did you know ?

There is the misconception that in order to benefit from fasting you need to starve yourself.  I’m not saying you won’t possibly have some minor discomfort as your body adjusts, but you can achieve amazing results without feeling famished.

I love it when people participate in my Fasting While Feasting program and experience all the benefits of achieving ketosis without the pain. Weight loss, clear head, more energy, better mood, all while eating satisfying, plant-based, vegan meals. 

Yes, it’s possible!


The Hypothalamic-Pituitary-Gonadal Axis Effects of Thyroid Hormone Enzyme Blocking on HypothyroidismDeiodinase – ScienceDirect Topics Cognitive, sensory, and emotional changes associated with the menstrual cycleWhy Can Insulin Resistance Be a Natural Consequence of Thyroid Dysfunction? National Institute on Aging: Research on intermittent fasting shows health benefitsCynthia ThurlowBrittle DiabetesThyroid Disease: Causes, Symptoms, Risk Factors, Testing & Treatment Mayo Clinic: Hypothyroidism – Symptoms and causes Mayo Clinic Minute: The difference between micronutrients and macronutrients Science Daily: Feast-and-famine diet could help extend life

Related Posts


Submit a Comment

Your email address will not be published. Required fields are marked *

A flyer with the words health crisis solutions.
A flyer with the words health crisis solutions.


Pin It on Pinterest