Intermittent fasting is not just a diet trend; it is a lifestyle change that can have a profound impact on your hormones and overall health. By triggering changes in insulin, growth hormone, cortisol, and sleep, fasting can help balance your hormones and prevent potential health issues. But remember, don’t jump into it blindly; always consult with a healthcare professional before starting an intermittent fasting plan. With the right mindset and guidance, intermittent fasting can be the perfect weapon in your hormonal health arsenal, helping you achieve your goals of being in control of your body and hormones.
HEALTH & LIFESTYLE
WHAT HAPPENS TO OUR HORMONES DURING INTERMITTENT FASTING?
May 13, 2023
Quick Answer
Intermittent fasting affects several hormones at once, including insulin, growth hormone, thyroid hormone, and the hunger hormones leptin and ghrelin. Changes to your thyroid can start within a single fast, while reproductive hormone shifts typically take 5 to 24 weeks depending on your fasting schedule. If you have a history of disordered eating, are pregnant, breastfeeding, or take medication for diabetes, I want you to talk with your doctor before you start.
Key Takeaways
- Intermittent fasting can improve insulin sensitivity and raise growth hormone levels.
- Your thyroid responds to fasting within hours, not weeks, through a normal energy-conserving shift.
- Leptin and ghrelin, your hunger hormones, tend to drop with regular fasting, though the research on ghrelin specifically is still mixed.
- Reproductive hormone changes are usually small and take 5 to 24 weeks to show up.
- Skip intermittent fasting, or check with your doctor first, if you have a history of disordered eating, are pregnant or breastfeeding, or take diabetes medication.
Intermittent fasting (IF) – the trending lifestyle that has taken the world by storm in the past decade. This popular way of life has been getting a lot of press recently due to its potential benefits, such as weight loss, improved metabolism, and increased energy levels. The truth is that this lifestyle can be challenging, and thus, is not for the faint-hearted.
Intermittent fasting is an eating plan that cycles between periods of voluntary fasting (or reduced caloric intake) and periods of eating. Unlike other eating plans which might focus on what you can eat, intermittent fasting focuses on when you can eat and when you cannot eat.
Intermittent fasting can also be referred to as Alternate Day Fasting (ADF), the 5:2 Diet (two fast days and five feast days per week), and Time-Restricted Eating (TRE); however, for the most part, most people think of time-restricted eating (TRE) when they talk about intermittent fasting.
Some people who are practicing intermittent fasting might choose to eat only during an eight-hour period each day and fast the rest of the day while others might choose to eat one meal a day for three days a week and regular meals the rest of the week. Some might choose to fast up to 20 hours a day while others might choose fewer fasting hours. There is no wrong or right schedule as long as you have a defined period of fasting and feasting.
During the extended fasting period, the body exhausts its sugar reserves from the last few meals you ate and then switches to burning stored fat to survive after hours without food. This is called metabolic switching and is one of the keys to the weight loss many people experience while doing intermittent fasting. Basically, intermittent fasting gives the body a chance to use up fat storage rather than derive energy from multiple meals eaten throughout waking hours.
So, how does intermittent fasting fit into our hormonal goals and our need to keep our hormones balance for an overall healthy lifestyle?
Hormones are critical to the way our entire body functions including metabolism, hunger, energy levels, and overall health and well-being. It is no secret that the desirable effect of this lifestyle is from the hormonal changes our body experiences during the periods of cycling of eating and fasting over time. Our endocrine system, responsible for producing and regulating hormones, is significantly affected during intermittent fasting. Since intermittent fasting affects the natural rhythm of our hormones, we should expect some changes in our body – both positive and negative while intermittent fasting.
Intermittent Fasting and Reproductive Health/Fertility
It is understandable that any female wanting to start intermittent fasting, or any sort of time-restricted eating (TRE), would be worried about the long-term effects of the lifestyle on their hormones, especial their sex hormones. Although studies are conflicting on the effects of intermittent fasting on our reproductive hormones, recent research shows that in pre-and post-menopausal obese women, intermittent fasting may slightly reduce the levels of dehydroepiandrosterone (DHEA), although within normal ranges. Over a short period of time, intermittent fasting does not seem to affect the levels of most other female reproductive hormones such as testosterone, androstenedione, and sex hormone binding globulin (SHBG) within this same population.
DHEA is a steroid hormone that is important for creating other hormones including estrogen and testosterone. High levels of DHEA is a risk factor for breast cancer so a reduction in DHEA level might have the benefit of reducing the risk of breast cancer in pre- and post-menopausal women. On the other hand, really low levels of DHEA can be associated with a decrease in libido and fertility, as well as vaginal dryness and sexual dysfunction especially in postmenopausal women. Fertility clinics prescribe DHEA to improve egg quality and ovarian function. This minor drop in DHEA levels can be weighed against the huge benefits of weight loss to fertility.
Fasting improves insulin sensitivity and decreases insulin resistance. This hormonal change may be beneficial for women with hormonal imbalances, such as polycystic ovary syndrome (PCOS) where the underlying driver of PCOS infertility is insulin resistance. PCOS is a hormonal disorder that affects about 7-10% of all women of childbearing age. It is one of the most common causes of infertility and about 5-6 million women in the United States suffer from PCOS. Sadly, most women do not even know they have PCOS. PCOS is characterized by high levels of androgens (male hormones) as well as insulin resistance or high levels of insulin. This hormonal imbalance leads to irregular periods, acne, and fertility problems. Improved insulin regulation can improve fertility in women with PCOS. If PCOS is part of your story, understanding how PCOS affects your hormones can help you decide whether intermittent fasting fits your treatment plan.
It is important to note that the effects of fasting on reproduction can vary between individuals. A potential decrease in estrogen levels during long-term fasting may improve insulin sensitivity and reduce androgen production, ultimately leading to better menstrual cycle regulation and fertility in some women, yet, in others, fasting for extended periods may disrupt menstrual cycles or increase the risk of infertility and other hormonal imbalances.
Our body is complex, and science is always evolving. Different populations of females (obese vs. non-obese, premenopausal vs. postmenopausal) will have different outcomes with intermittent fasting. Since research on the effects of intermittent fasting on our reproductive health is mixed and ongoing, it is important to pay attention to your body and speak with your healthcare provider before making drastic changes to your diet. If you are already noticing signs of infertility or increasingly irregular cycles, that is exactly when I want to see you in my office, not just have you researching on your own.
INTERMITTENT FASTING AND INSULIN
By cutting out food for an extended period, we can significantly lower insulin levels leading to improved insulin sensitivity and better glucose metabolism.
Insulin is the main driver for fat storage. When we eat food, our body converts the food into dietary sugars. Insulin is then secreted by the beta cells of the pancreas to convert the sugars into energy and transport glucose from the blood into the cells.
When we are constantly eating, our body is constantly making insulin and when we make too much insulin, the cells in our muscles, fat, and liver are overwhelmed and do not respond well to insulin and so cannot use up the glucose in our blood. This is known as insulin resistance.
Insulin resistance is a metabolic disorder that affects a significant portion of the population, and it is characterized by the body’s inability to respond to insulin as it should. When glucose cannot get into the cells, blood glucose levels keep rising, leading to severe health consequences like type 2 diabetes. In type 2 diabetes, the pancreas may still produce insulin, but the body cannot use it effectively. The beta cells of the pancreas will keep trying to do their job of making insulin to regulate the excess blood sugar; however, over time the cells will wear out and may stop producing insulin altogether.
Fasting gives the body a chance to rest from overproduction of insulin and allows insulin levels to be used correctly, thus improving insulin sensitivity. This is particularly important for individuals with insulin resistance such as pre-diabetic and diabetic patients. Intermittent fasting has been shown to prevent type 2 diabetes as well as have a positive impact on individuals with type 2 diabetes. It can also lead to weight loss and improved glycemic control, two key factors in managing type 2 diabetes.
Fasting also triggers a process called autophagy, which can help repair damaged cells and improve insulin sensitivity. These effects can lead to better glucose metabolism, improved blood sugar control, and decreased insulin resistance. If you are working on resetting your hormones for sustainable weight loss, insulin sensitivity is usually the first place I have patients start.
Intermittent Fasting and Thyroid Function
Your thyroid gland controls your metabolism, so it makes sense that fasting affects it. Even a single 24-hour fast can lower your active thyroid hormone (T3, by about 6 percent, and raise reverse T3), an inactive form your body makes when it wants to conserve energy. Your thyroid-stimulating hormone (TSH) and thyroid-releasing hormone (TRH) tend to stay low rather than rise during this process.
This is not thyroid dysfunction. It is your body’s normal way of slowing your metabolism slightly to protect you during a period without food. In my practice, I still ask patients with hypothyroidism or Hashimoto’s disease to talk with me before starting intermittent fasting, since we do not yet have enough research on how a diagnosed thyroid condition changes this response.
INTERMITTENT FASTING AND GROWTH HORMONES
By cycling between periods of eating and fasting, you can increase levels of human growth hormone (HGH) also known as growth hormone (GH), a vital hormone involved in tissue repair, muscle growth, and bone density.
Growth hormone, which is considered the “fountain of youth” hormone, plays a crucial role in our body’s functions. It stimulates cell growth and regeneration, promotes muscle growth and repair, supports metabolism, improves strength, and helps maintain bone density. As we age, our levels of growth hormone naturally decline, which can lead to decreased muscle mass, decreased bone density, lower quality of life, faster weight gain, and increased risk of certain diseases.
Intermittent fasting can help boost growth hormone levels through two ways. First, intermittent fasting gets rid of body fat which has a direct effect on the production of HGH. Second, intermittent fasting suppresses insulin production, and high insulin is disruptive to HGH production.
Growth hormone (GH) preserves lean muscle mass and promotes tissue repair. This can be particularly beneficial for individuals looking to build muscle or maintain muscle mass as they age.
Also, increased growth hormone levels have been linked to improved bone density, which can help prevent osteoporosis, a condition characterized by weak and brittle bones. Although intermittent fasting seems to offer a lot of benefits, it is important to do it right. Fasting for extended periods or without proper guidance can lead to muscle loss and other negative health consequences.
Intermittent Fasting and Hunger Hormones: Leptin and Ghrelin
Leptin tells your brain you are full. Ghrelin tells your brain you are hungry. Both shift when you fast regularly, though the research on exactly how is still evolving.
In one study of adults who fasted daily during Ramadan for a month, both ghrelin and leptin levels dropped by the end of the month compared to where they started. A separate, smaller study found leptin dropped overnight during fasting periods, but ghrelin did not change significantly. The two studies do not agree on ghrelin, which tells us your hunger hormones may respond differently depending on your body, your fasting schedule, and how long you have been fasting.
What this means for you day to day: many people notice their hunger cues change within the first two to four weeks of consistent fasting. If your hunger feels unmanageable, or you cannot get through your eating window without feeling out of control around food, that is worth mentioning at your next appointment.
INTERMITTENT FASTING AND THE STRESS HORMONE
Cortisol is an essential hormone that helps us deal with stress. It is released in response to a stressful situation and helps our body cope by increasing blood sugar levels and suppressing the immune system. However, when cortisol levels are elevated for prolonged periods, it can have negative effects on our health, including weight gain, sleep disturbance, fertility problems, libido, hair growth, and immune system suppression.
Intermittent fasting can initially cause cortisol levels to rise as scarcity of food is a stressor. Our bodies cannot determine the origin of the scarcity, and thus, reacts by producing more stress hormones. If your stress levels are normal, the stress from fasting is not necessarily a bad thing. Good stress can motivate the body to adapt and thrive during challenging times like when you adopt a new fasting schedule. Under normal stress levels, your body may gradually adapt to the stress of intermittent fasting and become more efficient at using its energy stores, thus leading to better cortisol regulation. However, if you are already under a lot of stress, and therefore, are most likely already producing excess cortisol in your body, you should consider waiting to start intermittent fasting when your stress levels decrease to healthy levels. If chronic stress is already part of your picture, it is worth reading about how cortisol shapes your body before adding a new stressor like a fasting schedule.
INTERMITTENT FASTING AND SLEEP
Although studies are mixed on how much sleep is affected by intermittent fasting, certain fasting schedules has been shown to improve sleep quality by regulating our body’s internal clock which can be disrupted by irregular eating patterns. By restricting food intake to specific periods, we can help our body’s circadian rhythm work more efficiently, leading to better sleep, improved hormone balance, better mood, more daytime alertness, and more overall focus. People who practice intermittent fasting are less likely to wake up during the night or suffer from insomnia.
Fasting has also been linked to a reduction in the risk of sleep apnea, a condition often associated with hormonal imbalances. Sleep apnea is a serious sleep disorder in which breathing involuntarily stops and starts throughout the sleep cycle. Sleep apnea can be a result of obesity, large tonsils, changes in hormone level, or the brain simply not sending the body a signal to breathe. Over time, sleep apnea increases the risk of heart abnormalities as well as causes significant disruptions to sleep quality and quantity. Fasting improves insulin sensitivity and glucose metabolism which are essential in regulating hormone levels and reducing the risk of sleep apnea and other sleep disorders.
How Soon Do These Hormonal Changes Happen?
How quickly you will notice a hormonal change depends on which hormone we are talking about.
Your thyroid responds first, sometimes within a single fasting window, through the energy-conserving shift described above. Insulin sensitivity improves noticeably within the first one to two weeks for most people. Growth hormone increases can happen within days of consistent fasting. Reproductive hormones move the slowest. In controlled human trials, changes to hormones like DHEA and SHBG showed up somewhere between 5 and 24 weeks, depending on the fasting protocol and the person.
There is no single timeline that fits everyone. Your hormones, your baseline health, and the fasting schedule you choose all play a role.
Common Intermittent Fasting Schedules
Not all intermittent fasting looks the same. Here is how the most common schedules compare.
| Schedule | How It Works | Good to Know |
|---|---|---|
| 16:8 (time-restricted eating) | Eat within an 8-hour window each day, fast the other 16 hours | The most flexible option, and usually where I recommend starting |
| 5:2 | Eat normally 5 days a week, eat about 500 to 600 calories on 2 non-consecutive days | Easier to maintain long term since most days are unrestricted |
| Alternate-day fasting (ADF) | Eat roughly 25 percent of your normal calories every other day, eat normally in between | More intense, and harder for most people to sustain |
| Eat-Stop-Eat | A full 24-hour fast once or twice a week | The most restrictive option, and not usually where I recommend starting |
If you are new to intermittent fasting, starting with a shorter fasting window and gradually extending it gives your hormones time to adjust.
When to Avoid Intermittent Fasting
Intermittent fasting is not right for everyone. I want you to check with your doctor first, or skip it altogether, in these situations.
- You have a history of disordered eating. Restricting when you eat can trigger old patterns, even years into recovery. Fasting is not worth that risk.
- You are pregnant, breastfeeding, or trying to conceive. Your body needs steady, reliable fuel during this time, not a restricted eating window.
- You take medication for diabetes, especially insulin or a sulfonylurea. Fasting alongside these medications raises your risk of dangerously low blood sugar. This needs to be managed with your prescriber, not adjusted on your own.
- You notice your period becomes irregular or stops. A missed period while intermittent fasting is your body telling you something changed. Come see me before you keep going.
- You have a diagnosed thyroid or adrenal condition. We do not have enough research yet on how fasting interacts with a diagnosed thyroid or adrenal disorder specifically, so I want to weigh in on your particular situation first.
Sis, this list is not here to scare you out of trying intermittent fasting. It is here so you try it safely, with the right person in your corner.
Conclusion:
Frequently Asked Questions
Is intermittent fasting safe for women?
Generally, yes, for healthy women without the risk factors described above. Intermittent fasting affects several hormones at once, including insulin, growth hormone, thyroid hormone, leptin, ghrelin, and your reproductive hormones, and the effects vary by individual. If you have a history of disordered eating, are pregnant or breastfeeding, take diabetes medication, or notice your period becoming irregular, talk with your doctor before continuing.
Can intermittent fasting affect your period?
It can. Research on intermittent fasting and reproductive hormones is still mixed, but changes to hormones like DHEA and SHBG have been measured 5 to 24 weeks into a fasting protocol, and some women experience menstrual cycle disruption during extended or intense fasting. If you notice your period becoming irregular or stopping while you are intermittent fasting, that is your signal to check in with your OB/GYN rather than push through.
What is the best intermittent fasting schedule for hormone health?
Most people do best starting with 16:8, eating within an 8-hour window and fasting the other 16 hours. It is the most flexible schedule and gives your hormones a gentler adjustment period than 5:2, alternate-day fasting, or Eat-Stop-Eat. The right schedule ultimately depends on your health history, your goals, and how your body responds, so I recommend starting gently and adjusting from there rather than jumping straight into the most intense option.
Does intermittent fasting affect thyroid function?
Yes, but not in a way that signals a problem. Even a single 24-hour fast can lower your active thyroid hormone (T3) and raise reverse T3 slightly, while your TSH and TRH stay low rather than rise. This is a normal, energy-conserving response, not thyroid dysfunction. If you have hypothyroidism or Hashimoto’s disease, talk with your doctor before starting, since research on how a diagnosed thyroid condition changes this response is still limited.
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