GHRP-2 and GHRH efficacy in men driven by visceral fat and IGF-I, not testosterone
Background
Pulsatile GH secretion declines with aging, but disentangling sex-steroid effects from other modulators has been methodologically difficult. Testosterone (T) and estradiol (E₂) are known regulators of the GH/IGF-1 axis, making it hard to isolate nonsteroidal contributors to somatotropic dysfunction in older men. GHRH acts directly on pituitary somatotrophs via GHRHR, while GHRP-2 engages the GHSR (ghrelin receptor), providing two mechanistically distinct probes of GH reserve. Prior studies could not adequately control for age-related differences in sex-steroid milieu, leaving nonsteroidal determinants — such as abdominal visceral fat (AVF) and circulating IGF-1 — confounded.
Study Design
Twenty-two men — 11 young (age 24 ± 0.99 yr) and 11 older (64 ± 2.4 yr) — underwent GnRH-agonist (leuprolide) down-regulation of the gonadal axis followed by fixed-dose testosterone replacement (leuprolide/T clamp) to equalize T and E₂ across age groups. Under this controlled milieu, subjects received consecutive infusion of l-arginine + GHRH or l-arginine + GHRP-2 to quantify peptide-secretagogue efficacies. The primary endpoints were GH release magnitude and multivariate predictors including AVF and IGF-I concentrations.
Results
The leuprolide/T clamp produced statistically age-comparable total, bioavailable, and free T and E₂ concentrations across young and older men. Despite equalized sex steroids, sequential l-arginine/GHRH infusion stimulated 1.4-fold more GH release in young than older men (P = 0.021), and l-arginine/GHRP-2 stimulated 1.3-fold more GH in young men (P = 0.045). AVF correlated negatively with both GHRH (P = 0.0006; R² = 0.39) and GHRP-2 (R² = 0.29) efficacy, while IGF-I positively predicted the same endpoints (R² = 0.25–0.30).
In multivariate analysis, AVF emerged as the dominant negative determinant of GHRH efficacy (P = 0.002; R² = 0.41), and IGF-I as the primary positive determinant of GHRP-2 efficacy (P = 0.007; R² = 0.31) — together accounting for large fractions of GH-response variability independent of sex-steroid status.
Key Findings
- l-arginine/GHRH stimulated 1.4-fold more GH in young vs older men (P=0.021) under T/E₂ clamp
- l-arginine/GHRP-2 stimulated 1.3-fold more GH in young vs older men (P=0.045)
- AVF was dominant negative predictor of GHRH efficacy: P=0.002, R²=0.41
- IGF-I was primary positive predictor of GHRP-2 efficacy: P=0.007, R²=0.31
- AVF negatively correlated with GHRP-2 efficacy as well (R²=0.29)
Why It Matters
Clinicians and peptide users targeting GH optimization cannot rely on testosterone normalization alone to restore somatotropic function — AVF and IGF-I levels are independent, dominant modulators. For practitioners using GHRP-2 or GHRH-based protocols, this means body composition (specifically visceral adiposity) and baseline IGF-I should be assessed before and during secretagogue regimens, as they predict response magnitude. Reducing abdominal visceral fat may be a prerequisite for maximizing GHRH efficacy, while optimizing IGF-I status appears most relevant for GHRP-2 response. Translation to clinical practice is relatively proximal, as the probes and clamp methodology used here are established human tools.
