Several studies have found an increase in BODY mass after a cycle of rHGH, but not an increase in Muscle mass (no steroids, non-weight trained individuals).
This is true, but studies have indirectly shown (by measuring protein synthesis) that GH does cause an increase in muscle protein synthesis but not whole body (organ) protein synthesis (see below). These studies were short term, done over a period of a few hours.
This contradicts longer term studies (14 days) in humans that have shown no change in protein synthesis due to GH treatment (1).
Rats that overexpress either GH or IGF-1 show enhanced growth of the intestines (2). The intestines are particularly rich in GH receptors. "GH belly" may be due to increased intestinal growth more than growth of other organs.
Metabolism 1993 Sep;42(9):1223-7
Growth hormone acutely stimulates skeletal muscle but not whole-body protein synthesis in humans.
Fryburg DA, Barrett EJ.
In a previous study, a 6-hour local infusion of growth hormone (GH) into the brachial artery of normal subjects stimulated net muscle protein anabolism by augmenting skeletal muscle protein synthesis. In the present study, we examined whether systemically infused GH affects forearm and whole-body protein metabolism. Normal volunteers aged 18 to 24 years (n = 8) were given an 8-hour systemic infusion of 3H-phenylalanine and 14C-leucine. Between 90 and 120 minutes of tracer infusion, basal samples for determination of forearm and whole-body amino acid kinetics were taken. GH was then infused at 0.06 micrograms/kg/min, increasing GH concentration from 2.4 +/- 0.3 to 32 +/- 3 ng/mL. Systemic insulin-like growth factor 1 (IGF-1) level increased from 224 +/- 20 to 262 +/- 21 ng/mL (basal v 6-hour, P < .01). By 6 hours, GH suppressed forearm phenylalanine and leucine net release (each P < .05) by increasing 3H-phenylalanine (66%, P < .05) and 14C-leucine (13%, P < .05) extraction or disposal (Rd). Whole-body leucine rate of appearance ([Ra] an index of whole-body proteolysis) and nonoxidative leucine Rd (whole-body protein synthesis) did not change over the course of the GH infusion, whereas oxidative leucine Rd decreased (20%, P < .03). Acute stimulation of muscle but not whole-body protein synthesis by systemically infused GH suggests that muscle protein is acutely and specifically regulated by GH.
(1) J Appl Physiol 1993 Jun;74(6):3073-6
Short-term growth hormone treatment does not increase muscle protein synthesis in experienced weight lifters.
Yarasheski KE, Zachweija JJ, Angelopoulos TJ, Bier DM.
(2) Gastroenterology 1997 Feb;112(2):444-54
Enhanced growth of small bowel in transgenic mice expressing human insulin-like growth factor I.
Ohneda K, Ulshen MH, Fuller CR, D'Ercole AJ, Lund PK.
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