Fasting is a dietary practice that involves cycling between periods of eating and periods of voluntary food restriction. It takes various forms, ranging from intermittent fasting (IF), such as daily time-restricted eating, to prolonged fasting (PF), which can last for several days. These practices directly interact with the endocrine system, the body’s network of glands that produce chemical messengers called hormones. Hormones regulate nearly every physiological process, from metabolism and growth to mood and reproduction. The question of whether fasting negatively impacts this delicate hormonal balance depends entirely on the duration, frequency, and intensity of the fast.
The Initial Metabolic Hormonal Response
When food is withheld, the body quickly shifts from storing energy (fed state) to mobilizing stored energy (fasting state). The most immediate and significant change involves insulin, which falls within the first 12 to 24 hours of a fast. This decline is a normal, adaptive signal that allows the body to access stored glucose and fat for fuel, a process known as metabolic switching.
As insulin decreases, the counter-regulatory hormone glucagon rises, signaling the liver to release stored glucose into the bloodstream to maintain blood sugar stability. This early hormonal change is beneficial for improving insulin sensitivity over time. Growth hormone (GH) also surges, increasing several-fold during a short fast. This release helps to preserve muscle mass and promotes the breakdown of fat for energy.
Stress Signaling and Cortisol Levels
The body’s perception of fasting, particularly when prolonged or combined with other stressors, can trigger the Hypothalamic-Pituitary-Adrenal (HPA) axis, often referred to as the stress axis. This activation results in the release of Corticotropin-Releasing Hormone (CRH), which prompts the pituitary gland to secrete Adrenocorticotropic Hormone (ACTH). ACTH then stimulates the adrenal glands to release cortisol.
Cortisol’s function during fasting is to promote gluconeogenesis—the creation of new glucose from non-carbohydrate sources like amino acids—to prevent hypoglycemia. While a temporary rise is normal, chronic, high-intensity fasting can lead to persistently elevated cortisol levels. Chronically high cortisol can disrupt the normal sleep-wake cycle, promote anxiety, and contribute to the breakdown of muscle tissue for fuel.
Fasting’s Influence on Thyroid Regulation
The thyroid gland regulates the body’s metabolic rate and is highly sensitive to changes in energy intake. Prolonged energy restriction signals food scarcity, prompting the body to conserve energy by modulating thyroid hormone production. The thyroid primarily releases thyroxine (T4), which is converted into the active form, triiodothyronine (T3), in peripheral tissues.
During extended fasting, the conversion of T4 to T3 is suppressed, leading to a drop in circulating T3 levels. Simultaneously, the body may increase the production of reverse T3 (rT3), an inactive metabolite that further blocks T3 action. This temporary metabolic slowdown can manifest as symptoms like fatigue, feeling cold, or a slight drop in body temperature.
Effects on Reproductive Hormone Balance
The Hypothalamic-Pituitary-Gonadal (HPG) axis, which governs reproductive function, is sensitive to perceived energy availability. This sensitivity is a protective biological mechanism that prevents reproduction during times of severe energy deficit. When fasting is too aggressive or prolonged, the body senses a significant energy imbalance, directly impacting the HPG axis.
The hypothalamus reduces the release of Gonadotropin-Releasing Hormone (GnRH), which suppresses the pituitary’s release of Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH). This suppression leads to a reduction in the production of sex hormones, including estrogen and testosterone. Women are particularly susceptible to this disruption, experiencing menstrual cycle irregularities or the complete loss of menstruation (amenorrhea). In both sexes, prolonged fasting reduces Dehydroepiandrosterone (DHEA) levels, a precursor to sex hormones that affects mood, energy, and bone health.
Practical Thresholds for Hormonal Disruption
Hormonal disruption occurs when fasting transitions from a brief metabolic challenge to a state of chronic nutritional deficiency. Fasting durations that regularly exceed 48 to 72 hours, especially when combined with intense physical activity or existing psychological stress, are most likely to trigger hormonal disruption. Individuals who are already underweight, have a low body fat percentage, or have a history of disordered eating are at higher risk for adverse effects.
Signs that fasting is negatively impacting hormonal health include:
- Chronic fatigue.
- Persistent irritability or mood swings.
- Hair thinning or loss.
- Unexpected changes to the regularity or duration of the menstrual cycle (for women).
Pregnant or nursing women, those with Type 1 diabetes, or anyone with a known adrenal or thyroid condition should avoid fasting until they have consulted with a healthcare provider.