Increasing Growth hormone levels in the pituitary gland leads quite quickly to an increase in muscle mass and a decrease in body fat. When Growth hormone is released or increased the hormone stimulates your liver to release another key muscle building hormone called IGF-1.
Multiple factors cause your body to make less and less GH & IGF-1 ( age is the largest factor ) as we get older our bodies produce less and less, being able to stimulate the hormones to increase GH & IGF-1 no matter what age is vital for achieving a lean muscular physique.