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NIH: Fasting and Hormone Regulation

Fasting can change how our hormones work in our bodies. When we fast, our insulin levels go down and our glucagon levels go up, which helps break down stored energy. Growth hormone increases, helping our body repair tissues and use fat for energy. Fasting also produces ketones, another type of energy our body can use. Hormones like cortisol, leptin, ghrelin, and norepinephrine can also be affected by fasting, which can influence how we handle stress, feel hungry, and use energy. However, the effects of fasting on hormones can vary based on factors like fasting duration and individual differences.

  1. Increased Growth Hormone (GH): Fasting triggers an increase in growth hormone secretion. Growth hormone plays a crucial role in promoting tissue growth, repair, and maintenance. It can enhance fat burning, preserve muscle mass, and support overall body composition.

  2. Increased Adrenocorticotropic Hormone (ACTH): ACTH levels may rise during fasting as part of the body's stress response. ACTH stimulates the release of cortisol, a hormone involved in the regulation of stress, metabolism, and immune function.

  3. Regulation of Cortisol: Fasting can have complex effects on cortisol levels. Initially, cortisol levels may rise during fasting as part of the body's stress response. This increase in cortisol helps mobilize stored energy sources and maintain blood glucose levels. However, studies have also shown that prolonged fasting or severe caloric restriction can lead to a decrease in cortisol levels.

  4. Increased Glucagon: Fasting stimulates the release of glucagon from the pancreas. Glucagon acts in opposition to insulin and promotes the breakdown of stored glycogen in the liver into glucose, which is released into the bloodstream to provide energy.

  5. Decreased Insulin: Fasting reduces insulin levels since there is less need for insulin to manage blood sugar levels when no food is consumed. Lower insulin levels facilitate the breakdown of stored glycogen and promote the use of alternative fuel sources like fat.

  6. Increased Ketones: During fasting, the body enters a state of ketosis, where it shifts to using fat as the primary fuel source. The liver produces ketones, such as beta-hydroxybutyrate (BHB), which are released into the bloodstream and serve as an alternative energy source for the brain and other tissues.

  7. Increased Norepinephrine: Fasting can lead to increased norepinephrine levels. Norepinephrine is a neurotransmitter and hormone that helps regulate alertness, attention, and energy expenditure. Higher norepinephrine levels during fasting may contribute to increased mental clarity and focus.

  8. Increased Leptin: Short-term fasting may cause an initial increase in leptin, a hormone produced by fat cells. Leptin helps regulate appetite and energy balance by signaling satiety to the brain. However, prolonged fasting may lead to decreased leptin levels, which can contribute to increased hunger.

  9. Increased Ghrelin: Ghrelin, often called the hunger hormone, tends to increase during fasting. Ghrelin stimulates appetite and promotes food intake. Rising ghrelin levels can contribute to feelings of hunger during fasting periods.

  10. Changes in Sex Hormones: Fasting can affect sex hormone levels, including testosterone and estrogen, although the specific effects may vary. Some studies suggest that intermittent fasting or caloric restriction can affect sex hormone levels, potentially influencing fertility, libido, and reproductive health. However, further research is needed to fully understand the extent and implications of these changes.


  1. Hartman ML, Veldhuis JD, Johnson ML, et al. Augmented growth hormone (GH) secretory burst frequency and amplitude mediate enhanced GH secretion during a two-day fast in normal men. J Clin Endocrinol Metab. 1992;74(4):757-765.

  2. Zauner C, Schneeweiss B, Kranz A, et al. Resting energy expenditure in short-term starvation is increased as a result of an increase in serum norepinephrine. Am J Clin Nutr. 2000;71(6):1511-1515.

  3. Heilbronn LK, Smith SR, Martin CK, Anton SD, Ravussin E. Alternate-day fasting in nonobese subjects: effects on body weight, body composition, and energy metabolism. Am J Clin Nutr. 2005;81(1):69-73.


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