Written and medically reviewed by Nicole Grant, RD
You may not have given hormones much thought when approaching your health journey, but they are, in fact, key players. Hormones influence a wide array of body features and functions you are likely to care about, including your weight, mood, sleep, fertility, sex drive, and more. And for better or for worse, lifestyle factors can influence certain hormones. Fasting, in particular, can have a significant effect. But which hormones are affected? And are those effects desirable or should you try to avoid them?
Read on to discover how key hormones like insulin, cortisol, sex hormones, and thyroid hormones can influence your fasts, how they’re impacted by your fasts, and how you can leverage this knowledge to lose weight, gain health, and extend your healthspan and lifespan.
Hormones, Fasting, and Their Bidirectional Impact
Hormones, the body’s biochemical messengers, orchestrate a multitude of physiological processes, ranging from growth and metabolism to mood and reproduction. There are sex hormones, such as estrogen, progesterone, and testosterone, and metabolic managers, such as insulin and thyroid hormones. While each hormone plays a specialized role in your body, they also work synergistically to optimize health.
“Although over 50 hormones have been identified in the human body, the key hormones that interact with fasting discussed here are insulin, cortisol, sex hormones, and thyroid hormones.”
– Dr. Naomi Parrella, MD
Insulin
Insulin is a hormone that plays an essential role in regulating blood-glucose levels, thereby affecting glucose metabolism.
When you eat food, especially carbohydrates, it’s broken down into glucose, which enters your bloodstream, elevating blood-sugar levels. This rise in blood sugar then signals your pancreas to release insulin. Insulin helps cells absorb glucose, which they can either metabolize for immediate energy needs or store for future use. As the cells pull glucose from the blood, blood-sugar levels return to (or stay within) a safe range.
When insulin levels are elevated, the body is in an anabolic or “growth” mode — a good thing when you need to fuel your body for a hard effort, are aiming to gain muscle, or are at an age when your body is still growing. However, being in that anabolic state too long or too frequently can inhibit your fat-burning capabilities.
“In a metabolically healthy body, your body pumps out insulin in response to elevated blood sugar. And what does insulin do? It turns off fat burning to force your body to prioritize burning off the blood sugar to bring the level back down,” says Dr. Naomi Parrella, Zero’s Chief Medical Officer. “So, things that cause insulin to go up will stop fat burning until the insulin comes back down. When insulin gets back to a normal baseline, then the fat burning automatically turns back on.”
How Insulin Affects Your Fasts
Insulin blocks fat burning, and when your fat burning is blocked, you will get hungry more frequently. So, to make fasting easier and to reap the most benefits from your fast, you’ll want to get into a low-insulin state as fast as you can. (You can do this by cutting down on carbohydrates leading into your fast, exercising early in your fast, engaging in activities that reduce stress, and even simply getting adequate sleep.) Lowering your insulin will help you flip the “metabolic switch” and start burning more of your fat stores as fuel.
How Fasting Affects Your Insulin
You can directly influence your insulin levels by fasting! In the absence of food, insulin levels naturally decrease, prompting the body to shift from storing energy as fat to using stored fat for energy. Typically, once you reach a 12–18-hour fast, insulin levels have stayed low enough that your body has depleted its glycogen stores and can become a more efficient fat-burning machine. (What’s more, by reducing your insulin through fasting, you enhance your insulin sensitivity, which encourages the body to be in a low-insulin state for a longer period of time!)
Cortisol
Cortisol is another important regulator of blood glucose and energy. This hormone follows a daily circadian rhythm, peaking in the morning and returning to baseline levels by nighttime. This fluctuation helps you wake up in the morning, feel alert, and keep moving throughout the day.
Increases in cortisol also cause the liver to release glucose, which is helpful during bouts of intense exercise or when external fuel is low (both of which can occur as part of a normal day) and also when fighting infections or recovery from surgery (which are hopefully more infrequent occurrences). However, when cortisol levels are chronically elevated due to unrelenting stress or other ailments, this can also disrupt your body’s ability to function optimally and adequately partition fuel.
How Cortisol Affects Your Fasts
“In my clinics, people frequently share with me that their weight gain is due to ‘stress eating,’ which is an absolutely natural response,” says Dr. Parrella. “Cortisol that stays elevated leads to hunger and cravings for sugars and ultra-processed foods because your body wants to increase the available fuel in your bloodstream, i.e., blood sugar, so it’s prepared to fight.” However, if you’re enduring chronic stress, that “fight” never comes, so instead you may find yourself battling cravings that make fasting extra challenging. To ease this challenge, focus on cortisol-lowering activities such as meditation, walking, nature exposure, restful sleep, and spending time with people who recharge you.
On the flip side, you can also use cortisol to your advantage during fasting! As you wake up, cortisol levels begin to rise, along with energy. This is why many people find that skipping breakfast and waiting until lunch to eat is an easier fasting pattern than skipping dinner. Instead of breakfast, capitalize on that natural energy boost and get in some fasted exercise to further maximize your fat-burning capabilities.
How Fasting Affects Your Cortisol
Initially, fasting may cause a transient increase in cortisol, especially prolonged fasts that last 24+ hours. However, consistent daily fasts lasting 12–18-hours have been shown to have positive effects on natural cortisol rhythms, helping to increase morning levels (making it easier to wake up!) and decrease nighttime levels (smoothing the transition to sleep). In the long term, better cortisol patterns can result in improved glucose management, sleep, and fat loss.
Sex Hormones
The predominant sex hormones are estrogen, progesterone and testosterone. Both males and females have all three, but in different quantities.
Estrogen is essential for female development, including breast growth and regulating the menstrual cycle and reproductive system. It is also important for maintaining bone health, skin elasticity, and cardiovascular health. Estrogen also plays a role in reproductive function for males, as well as in modulating mood, cognition, and emotion.
In females, progesterone primarily prepares the body for pregnancy and sustains it during early stages of gestation. Outside of fertility purposes, progesterone helps to prevent neurological disorders and osteoporosis. In males, progesterone plays a role in reproduction, sleep, immunity, and the regulation of weight and appetite.
Testosterone, typically regarded as the primary male sex hormone, is instrumental in both males and females. In males, testosterone contributes to the development of muscle mass, body hair, and a deeper voice and is necessary for reproductive health. In females, testosterone plays a role in estrogen production, assists in the development of muscle mass, and can be protective against cardiovascular and neurodegenerative diseases.
How Sex Hormones Affect Your Fasts
Sex hormones can drive appetite and behavior around food. In females, higher levels of estrogen tend to correlate with increased satiety, while increased progesterone levels may stimulate appetite. Since estrogen tends to elevate during the first half of the menstrual cycle in premenopausal women, longer fasts, such as 16–18-hour fasts, may be more achievable due to increased satiety. On the other hand, progesterone increases during the second half of the cycle, potentially increasing appetite, so shorter 12–14-hour fasts may be more realistic during this period.
Meanwhile, in both males and females, higher testosterone levels are correlated with more muscle mass and less visceral fat. So, individuals who have naturally higher levels of testosterone might see quicker results from fasting due to the increased metabolic rate and less fat at the outset.
How Fasting Affects Your Sex Hormones
Fasting can have varying effects on sex hormones depending on the person’s sex, health status, and weight. In human trials, intermittent-fasting protocols have shown a decrease in testosterone levels (although participants on average maintained testosterone levels within the normal healthy range). In normal-weight healthy men, this effect would seem to be undesirable; however, despite the shift in testosterone, men in the study were able to maintain lean muscle mass and decrease fat mass when their fasting was coupled with resistance training. (And in fact, fasting naturally increases growth hormone, which boosts fat burn and muscle building!) It’s important to note that general fasting studies have also shown a variety of other positive health effects from fasting, such as improved blood-sugar control, cholesterol, and inflammatory markers. So, a slight shift in sex hormones may be outweighed by the other positive effects of fasting.
Premenopausal women with obesity and PCOS have increased levels of androgens like testosterone, so a decrease in testosterone is likely beneficial. Studies have shown that fasting in these populations decreases testosterone, leading to weight loss and, ideally, improving their reproductive health. In perimenopausal and postmenopausal women, sex hormones naturally decline regardless of their fasting protocol. In these populations, fasting can help maintain or lose weight and improve insulin resistance, which can potentially lessen some of the negative side effects of this particular transition in women’s lives.
Thyroid Hormones
The thyroid hormones, mainly thyroxine (T4) and triiodothyronine (T3), which are produced by the thyroid gland, are pivotal in regulating the body’s metabolic rate. They influence various bodily functions, including heart rate, body temperature, and metabolism. Imbalances in thyroid hormone levels can lead to conditions like hypothyroidism, where the body’s metabolism slows down, or hyperthyroidism, where the body’s metabolism is overactive.
How Thyroid Hormones Affect Your Fasts
Ideally, you will have healthy thyroid hormone levels and a normal metabolic rate going into your fasts. Imbalances in thyroid hormones can affect appetite and the way your body burns or stores fat. Therefore, supporting thyroid health through diet and lifestyle choices, such as optimizing sleep and managing stress, will help maintain consistent thyroid function and support your fasts.
How Fasting Affects Your Thyroid Hormones
Thyroid hormones are sensitive to both fasting and caloric intake. Fasting for 16–24 hours or longer has been shown to temporarily decrease T3 levels (a thyroid hormone) while fasting. This is a natural response to decreased caloric intake — it’s your body’s attempt to manage your energy expenditure. However, once you resume eating, your thyroid hormones return to their baseline levels.
Short fasting-related dips in thyroid hormones should not have a long-term impact on your metabolism as long as you are consuming adequate calories and nutrients. However, pairing fasting with intense caloric restriction could. Following very-low-calorie diets (400–1000 calories daily) for 8 weeks can lower thyroid levels 22% below normal baseline levels — a drop that could be made even more significant when combined with the effects of fasting. Ultimately, for healthy individuals, your thyroid levels should not be negatively impacted by a regular 12–18-hour fasting routine as long as you are consuming enough protein, nutrients, and calories for your needs.
Special Considerations for Women’s Hormones and Fasting
If you’re a woman who is contemplating or has begun fasting, there are special considerations to bear in mind due to the intricate interplay between fasting, sex hormones, and the female reproductive system. In a nutshell: Women’s hormonal balance, especially the cyclical nature of estrogen and progesterone, can be sensitive to changes in energy intake and stress, both of which can be impacted by fasting.
While short-term fasting is unlikely to disturb hormonal balance, extended periods of fasting or severe caloric restriction may not be appropriate, especially if you have preexisting hormonal conditions, such as a thyroid disorder, are trying to get pregnant, or are currently breastfeeding. On the flip side, fasting in conjunction with perimenopause could help alleviate some symptoms. “Around menopause, the ovaries stop making a type of estrogen called estradiol, and the estrogen remaining in the body is estrone, which is from the fat cells. This is the estrogen that causes hot flashes and night sweats. Studies show that maintaining weight in non-obese or weight loss of 10 pounds or more in individuals with obesity resulted in far fewer hot flashes and night sweats,” says Dr. Parrella.
Because of these nuances, it’s optimal for women to approach fasting under the guidance of healthcare professionals to ensure that your practice is aligned with your individual health needs and will not compromise your hormonal health and overall well-being.
Conclusion: Responsible Fasting Can Support Healthy Hormone Balance
Fasting, when approached mindfully and tailored to your individual needs, physiology, and goals, can support a healthy hormonal balance. This balance is crucial for both men and women, although the paths to achieving it may differ due to distinct hormonal profiles and responses. By understanding and respecting your unique hormonal needs, you can use fasting as a powerful tool to enhance well-being and promote weight loss, fat loss, and longevity.
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