Human growth hormone (HGH) is naturally produced by your body in sufficient quantities until you reach the age of 21. After this, production starts to drop.
HGH is the master hormone controlling growth. It helps build muscles, bones, organs and the basic links between the cells. Lower levels of HGH also make it more difficult to lose weight -- HGH increases your metabolism, which helps to burn more fat.
According to Fitness Health Zone:
“Thus a person loses weight but not their body muscle. It also helps to increase the levels of HDL the good cholesterol, thus reducing cardiovascular risks too.”