Promote human growth hormone production (HGH). Fasting can raise HGH by as much as 1,300 percent in women and 2,000 percent in men.12 HGH plays an important part in health, fitness, and slowing the aging process. It's also a fat-burning hormone Lower triglyceride levels and improve other biomarkers of disease Boost production of brain-derived neurotrophic factor (BDNF), stimulating the release of new brain cells and triggering brain chemicals that protect against changes associated with Alzheimer's and Parkinson's disease. (Alternate-day fasting — restricting your meals on fasting days to about 600 calories — can boost BDNF by 50 to 400 percent, depending on the region of the brain.13
In a physiological context, fasting may refer to the metabolic status of a person who has not eaten overnight, or to the metabolic state achieved after complete digestion and absorption of a meal. Several metabolic adjustments occur during fasting. Some diagnostic tests are used to determine a fasting state. For example, a person is assumed to be fasting once 8–12 hours have elapsed since the last meal. Metabolic changes of the fasting state begin after absorption of a meal (typically 3–5 hours after eating).