How does fasting affect your growth hormone?

Fasting triggers a well-known physical response: when your blood sugar drops or you stop eating, the pituitary gland releases more growth hormone. This has long been described as the body's stress response to a lack of energy, similar to what happens during intense exercise or sleep. This is a recognised mechanism, not a measured clinical benefit.

In practice, however, the picture is more nuanced. In twenty female dance athletes who fasted for sixteen hours every day for six weeks (eating only between 11:00 and 19:00), growth hormone levels in the blood did not change in any measurable way. The growth factor that is largely driven by growth hormone and is often used as a measure of growth hormone activity also remained unchanged. This was without any calorie restriction, so the effect of the eating window alone.

The same limited response was seen with weight loss through a strongly reduced calorie intake over four weeks in people with obesity: despite an average weight loss of 7.4%, that growth factor remained unchanged. Short-term fasting may therefore raise growth hormone release at the level of brief peaks, but that does not automatically translate into higher blood levels of growth hormone over the longer term with regular fasting.

What you can realistically expect: the classic rise in growth hormone from fasting is an acute, short-lived effect. If you eat within a daily time window, you will probably not see that reflected in higher hormone levels over the longer term. The available studies are small and were carried out in women only, so little can be said here about men or other fasting schedules.