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The Effects of Fasting on Hormones


I’ve seen hundreds of patients who have incorporated some fasting regimen as a part of their health journey. Fasting has the potential to be a powerful tool in achieving better health. However, it’s essential to approach it with proper care and individualization. In this article, I will share what I’ve observed in my practice. I will share this experience to help you make the most of fasting in your quest for better health. 

Fasting is a natural metabolic process that is vital for the regulation of a number of physiological systems. It affects nearly all physiological processes. This includes the functioning of the endocrine system. 

In a very general sense, fasting tends to retard physiological processes. With respect to the endocrine system, fasting both retards and stimulates hormonal response. The ways in which fasting affects physical systems depend on several factors, especially the duration of the fast. 

What is Fasting? 

Fasting is a natural process, one component of a two-cycle system that every human body engages in every day. The two components of this cycle are feeding and fasting. When we are eating, and during the period of time after we eat, when we are digesting our food, we are in the feeding cycle. As soon as all the food we have consumed is fully digested, the body immediately switches to the fasting cycle. 

The fasting cycle is equally important as the feeding cycle in maintaining optimal health. When we eat, we consume more caloric energy than we need right away. This energy is stored for later use. The hormone, insulin, breaks down this food energy and stores it in the liver and muscles in the form of glycogen. Since the ability to store glycogen is limited, once the muscles and liver are saturated with glycogen, the body converts the excess glycogen to fat. The human body has an unlimited capacity to store fat. 

When the body is in its fasting cycle, this allows the body to tap the stored glycogen and use it for energy. If the duration of the fast is long enough, once the glycogen is depleted, the body begins to access stored fat for energy. 

The fasting cycle is highly beneficial for the body in many ways. Researchers have linked the process of ‘caloric restriction’ to enhanced cellular function and longevity. 

There are essentially two forms of fasting: 

  1. Prolonged fasting 
  2. Intermittent fasting (IF) 

A prolonged fast has a duration of two days (48 hours) or longer. Prolonged fasts can include some caloric intake, such as fruit juices, or no caloric intake at all, which would be a water-only fast. 

An intermittent fast (IF) is a fast that is structured to prolong the normal daily periods of fasting. We fast every night for eight or more hours as we sleep. An intermittent fast is designed to extend that daily period of fasting to as much as sixteen hours, thereby limiting the intake of food to eight hours per day. It can also be one day of the week (36 continuous hours) of dedicated fasting. 

Fasting and Hormones 

Both prolonged and intermittent fasting affects the endocrine system. They affect both men and women but in slightly different ways. 

Women have different hormone profiles, which are in a constant state of flux. Men’s hormone profiles tend to be steadier, day to day. Women are also more sensitive to changes in nutritional intake due to a molecule called kisspeptin, which is present in higher levels in women compared to men. Kisspeptin is related to both the reproductive pathway and to hormones like insulin and leptin, which regulate hunger signals. 

Research has demonstrated fasting health benefits to women in the areas of; 

  • Cancer risk reduction 
  • Chronic pain reduction 
  • Lowering risks and symptoms of metabolic syndrome 
  • Supporting mental health 
  • In the treatment of PCOS (polycystic ovary syndrome) 

Fasting has a number of hormone-related effects affecting females. It is important to consider the length of the fast when evaluating these effects. Most intermittent fasting protocols will have a lesser effect than a prolonged fast. 

Intermittent Fasting Effect On

1. Reproductive Hormones

A literature review in 2022 suggested that intermittent fasting decreases testosterone while increasing sex hormone-binding globulin (SHBG) levels in premenopausal females with obesity. It doesn’t affect SHBG in men but can decrease testosterone levels in men too. This could negatively affect metabolic health, energy levels, bone health, muscle mass, and libido.

One symptom for women to watch, who are engaging in any type of fasting is their menstrual cycle. Any changes in their cycle may be caused by the hypothalamus reducing the production of reproductive hormones. Fasting can reduce the number of days of a woman’s cycle or, in some cases, eliminate it altogether. These effects are usually short-term and will disappear when the fast is completed

2. PCOS 

Intermittent fasting, specifically a 16-hour daily time-restricted fast, has been shown to have favorable effects in women experiencing polycystic ovary syndrome. 

Research has shown a reduction in levels of androgens, like testosterone. The same study also demonstrated an improvement in insulin resistance, resulting in a reduction of body fat. 

3. DHEA 

According to research conducted in 2023, time-restricted eating can reduce the levels of DHEA, in both men and women. 

Lower DHEA levels can lead to reduced production of downstream hormones, mainly testosterone and oestrogen,  impacting mood, energy levels, and overall hormonal balance.

DHEA stimulates egg production, so it plays a role in fertility. A woman who is trying to become pregnant will have a lower probability during a prolonged fast. Please note that individual responses to fasting on hormones can vary.

4. Thyroid Function 

A study published in 2006 suggested that calorie restriction due to prolonged fasting was linked to a decrease in thyroid function. Once the fast was completed, and the test subjects began eating food again, the thyroid immediately returned to its normal function. The relationship between fasting and decreased thyroid function was not observed in subjects engaged in intermittent fasting. 

Reduced thyroid function is also associated with fertility issues. Any woman who is trying to get pregnant will have better odds once her period of fasting is concluded. 

Please note that it’s essential to consult with a functional medicine practitioner when considering fasting, especially if there are pre-existing thyroid issues. 

5. Insulin Levels 

During periods of fasting, insulin levels fall. This decline in insulin is accompanied by a surge of other hormones, including adrenaline, human growth hormone, and cortisol. These three hormones assist the body in releasing its stored glycogen and increase the body’s metabolic rate. 

These hormonal surges are the result of stress signals stimulating the hypothalamus. The stress that is registered in the brain during a professionally monitored fast is a good type of stress, which results in physiological benefits, such as a higher tolerance to stress in general and in fat-burning.


6. Ghrelin and Leptin 

Fasting also affects the secretion of the hormones leptin and especially ghrelin. Ghrelin is the hunger-signaling or appetite hormone. As soon as we have completed a meal, the body secretes leptin, informing us that we are satiated. However, as soon as the meal is completely digested, the body will begin to secrete ghrelin, which causes us to feel hungry again. Ghrelin is subject to a circadian rhythm, with 8:00 PM in the evening being its peak. 

Anyone who engages in intermittent fasting will be challenged by the secretion of ghrelin in the evening hours before bed. During a prolonged fast, the body shuts down the secretion of ghrelin after 48-60 hours, and the faster no longer feels hungry. 

Monitor Your Fast 

The first consideration when engaging in any type of fast is that men and women react differently to fasting. While fasting can be a valuable tool for weight management, including for those who are obese, a more personalized and cautious approach is critical. 

When you begin your fast, it is always advisable to consult with your licensed health professional. There are many excellent tools that can be utilized in order to monitor the effects of the fast, including: 

  • Comprehensive Female or Male Hormone Panel 
  • Fasting Blood Glucose Test 
  • Fasting Insulin Test 
  • Continuous Glucose Monitoring 
  • Comprehensive Stool Analysis Test 

These are just a few of the means that your health professional can utilize to help you make the most of your fast. Always inform them when considering any type of fasting regimen. 

A Personalized Functional Medicine Approach to Fasting

Each individual has unique health conditions, goals, and responses to fasting. 

  • If you are under a great deal of stress, this is probably not the right time to embrace a fasting strategy.
  • People with hormonal dysfunction may want to check their hormones via the DUTCH test and see if fasting is suitable for them.
  • Fasting can affect different health conditions such as diabetes, heart disease, or gastrointestinal issues differently. Customizing a fasting regimen ensures that it aligns with an individual’s specific health needs and minimizes potential risks.
  • Personalization enhances the likelihood of achieving desired results. It allows individuals to choose the fasting method that aligns best with their health goals. For weight loss, some may prefer intermittent fasting, while others may find time-restricted eating more suitable. Those aiming for metabolic health improvements might opt for alternate-day fasting. 
  • People with certain medical conditions, pregnant or breastfeeding individuals, and those with a history of eating disorders may need to avoid fasting or pursue highly tailored approaches. Consulting a healthcare professional is essential to determine the suitability of intermittent fasting.
  • Metabolic rates differ among individuals, influencing how they respond to fasting. Some people can tolerate longer fasts, while others may become hypoglycemic or experience excessive stress. 
  • Some may find fasting empowering, while others may feel stressed or develop disordered eating patterns. Personalized guidance can address these psychological aspects.
  • To reap the benefits of fasting over time, long-term adherence is the key. Choosing a fasting pattern that suits an individual’s lifestyle, preferences, and cultural factors ensures they are more likely to stick with it in the long term. 
  • It is important to consider overall well-being, including individual nutritional needs, physical activity, stress levels, and sleep quality. It ensures that fasting integrates seamlessly into their life and supports their overall health.

If you would like to explore more about a fasting regimen customized for your optimal health, please contact us.

Frequently Asked Questions

When should a woman fast during her cycle?

Fasting for women, especially around their menstrual cycle, should be approached with care to avoid disrupting hormonal balance. For example, during perimenopause, hormone levels, particularly estrogen and progesterone, start to fluctuate and decline. I discuss this more comprehensively in my article, Everything You Need to Know About Cycle Syncing. Dr Mindy Pelz, author of “Fast like a girl”, also discusses benefits of tailoring fasting practices to one’s age, cycle, and overall health.

Does fasting release stress hormones?

Fasting is a stress on the body and research shows that it increases cortisol levels. I find that women and those already dealing with chronic stress are more likely to have stress and cortisol problems. For these people, the existing situation of elevated cortisol can be potentially aggravated.

Is fasting good for female hormones?

Fasting influences female hormones in various ways and requires a personalised approach. For some women, an extremely restrictive feeding window could disrupt leptin signalling, which is necessary to maintain ovulation for menstruating women. Intermittent fasting may alter reproductive hormone balance, which are sensitive to stress induced by calorie restriction.

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Dr Menka Gupta


Dr. Menka Gupta, an IFM certified Functional Medicine Doctor, is now a member of IFM Certified Practitioners. IFM, the global leader in Functional Medicine, offers the gold standard IFM Certification Program (IFMCP).

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