What does fasting do to growth hormone and testosterone?

Fasting can raise growth hormone, sometimes substantially, but time-restricted eating may lower free testosterone. The effects depend on the person and the fasting pattern.

• Growth hormone: Fasting can increase growth hormone—one account describes roughly a doubling while awake—and ghrelin, the hunger hormone, can stimulate its release. However, the rise does not necessarily translate into stronger downstream growth-hormone or IGF-1 signaling; for normal signaling, fasting specifically to raise growth hormone may not be helpful. Avoiding food for about two hours before bed is described as a useful way to support the normal sleep-related growth-hormone pulse. 1 2
• What growth hormone supports: It is associated with metabolism, fat burning, and tissue repair, and higher levels can benefit muscle and general tissue health. 3
• Testosterone: In a study of elite cyclists eating the same calories, time-restricted eating significantly reduced free testosterone. How consequential that is may depend on starting levels; caloric restriction can also affect testosterone differently depending on whether someone is obese or metabolically healthy. 4 5
• Why the drop may matter: Free testosterone is described as the form responsible for most actions in the brain and body. Testosterone and estrogen also contribute to libido and muscle growth, and fasting or excessive food restriction can affect reproductive function. 6 7 8