“Fasting is not healthy for women who have high cortisol levels.”

For humans, cortisol begins to increase immediately after fasting commenced. Five-day fasting increases cortisol levels and shifts the peak from the morning to the afternoon. Other fasting experiments for 2.5 to 6 days dramatically elevates plasma cortisol levels. These results imply that intermittent fasting increases the level and frequency of cortisol secretion.

When analyzing the study group according to gender, we found that female participants presented alterations in the amplitude and acrophase of the cortisol rhythm. The amplitude of the rhythm on the fasting day was higher, and the acrophase occurred earlier. There were no differences in the MESOR parameter. In contrast, none of the parameters characterizing the cortisol rhythm differed in the male subjects.

Studies indicate that a 72‐h fast elevates cortisol, adrenocorticotropic hormone (ACTH), adrenaline, and noradrenaline, while simultaneously decreasing the TSH and T3. However, whether long‐term IF can modulate specific hormonal axes without overt physiological side effects, including sex‐specific effects, remains unclear.

Cortisol levels start to rise within 12 hours of fasting and remain higher than non-fasting levels for the duration of IF. In women, chronically high levels of cortisol can delay ovulation and menstruation or even cause missed periods altogether, as cortisol diverts the body's resources from reproductive functions to survival. If high cortisol or other adverse effects occur, a less restrictive fasting plan or consultation with a healthcare provider is recommended.

Since fasting is a stressor, another hormone to consider is cortisol. Cortisol is your primary stress hormone, and it also contributes to elevated blood sugar and belly fat – both already a problem for many women during menopause. While we don't necessarily have to worry about stress impacting a woman's menstrual cycle during menopause, stress is still a concern throughout our lifetime.

Fasting can trigger a stress response, increasing levels of cortisol—the body's primary stress hormone. Chronically elevated cortisol can lead to increased fat storage (especially around the abdomen), muscle loss, blood sugar imbalances, and issues with sleep and mood. These effects tend to be more pronounced in women.

Fasting triggers the body to move into survival mode and release stress hormones like cortisol. Research shows that fasting can not only elevate cortisol levels but also disrupt the cortisol awakening response, leading to a dysregulated HPA axis that may impact long-term hormone health in women, potentially worsening hormonal symptoms and weight gain for those with existing imbalances.

Hunger and changes in routine can act as stressors, and cortisol (a stress hormone) may rise, especially with long or aggressive fasts or when combined with poor sleep, intense workouts, or high job stress. Chronically high stress levels can affect mood, sleep, and appetite, and women's bodies are especially sensitive due to their fluctuating hormonal system.

Fasting raises cortisol — the wrong hormone at the wrong time. When food intake is delayed, the body must maintain blood sugar. In menopause, this is achieved less through insulin and more through cortisol and adrenaline. As a result, fasting often leads to: Elevated morning cortisol, Prolonged stress hormone activation, Difficulty winding down at night.

While intermittent fasting does increase stress hormones in animal studies, this stress response promotes longevity, unlike the response to chronic, uncontrolled stress. Dr. Mattson discusses the lack of evidence to suggest that intermittent fasting impairs fertility in women or otherwise disrupts hormone balance in human studies.

Cortisol supports fasting by mobilizing energy and enhancing alertness, but can become counterproductive when chronically high. For others—especially many women—extended or aggressive fasting can amplify stress and disrupt hormonal balance. Fasting is not equally suited to everyone. Many women, for example, may find that prolonged or aggressive fasting adds unnecessary stress to an already complex hormonal landscape.

While many people turn to fasting for weight loss and metabolic health, those with Adrenal Fatigue often experience worsening symptoms due to the added stress on their already overworked adrenal glands. Intermittent fasting forces the body to rely on cortisol for blood sugar regulation, pushing weak adrenal glands beyond their capacity.

While fasting generally elevates cortisol as a normal physiological response to mobilize energy, clinical guidelines from sources like the American College of Obstetricians and Gynecologists (ACOG) and endocrine societies caution against prolonged fasting for women with pre-existing high cortisol or adrenal dysregulation, as it may exacerbate stress responses, but short-term intermittent fasting can be beneficial if monitored.

Too much fasting and overtraining can keep your body stuck in a constant stress state. Dr. Stacy Sims explains why cortisol isn't the enemy, but excessive fasting raises it problematically, particularly advising moderation for women to avoid chronic elevation.

Intermittent fasting has been shown to be an effective way to lose weight, but critics have worried that the practice may have a negative impact on women's reproductive hormones. Among post-menopausal women, there was no change in these hormones at the end of eight weeks.