Arginine is an amino acid that is known as a growth hormone releaser and a precursor of the vital substance nitric oxide. The decrease of growth hormone in the human body with aging is a major reason why muscle mass tends to decrease with age and body fat tends to increase with age. Decreases in growth hormone also are partially responsible for the slower rate of skin growth with aging, which results in thinner and less flexible skin. Injections of growth hormone can help to reverse these problems. Even though the cost of growth hormone injections came down in price considerably during the first 15 years since the injections became available, the prices have gone back up in recent years, making them more costly than any time in the past decade. Growth hormone injections introduce growth hormone in a manner that is very different from the way it is produced by the pituitary gland. Growth hormone cannot be taken orally, because it is broken down in the digestive tract.