Peptides: Growth Hormone Physiology Basics
GH pulse amplitude declines ~14% per decade after age 30. Somatostatin and GHRH compete continuously; fasting and deep NREM sleep maximize pulse amplitude.
| Measure | Value | Unit | Notes |
|---|---|---|---|
| Evidence Grade | A | grade | GH physiology — established by decades of human RCTs and controlled mechanistic studies (PMID 7744886; 8491152; 9861545) |
| Daily GH Pulses | 8–10 | pulses/day | Pulsatile secretion in healthy adults; amplitude varies; largest pulse ~1 hour after sleep onset during deep NREM |
| GH Pulse Decline with Age | ~14 | % per decade | GH pulse amplitude declines ~14% per decade after age 30 (PMID 8491152); IGF-1 declines ~15% per decade after age 35 |
| Peak GH Pulse Timing | ~60 | min after sleep onset | Largest daily GH pulse occurs approximately 1 hour after sleep onset during slow-wave (deep NREM) sleep |
| IGF-1 Source | Liver (primary) | organ | GH stimulates hepatic IGF-1 production; IGF-1 mediates most anabolic effects and provides negative feedback to GH |
| Somatostatin Effect | Inhibits GH | mechanism | Released from hypothalamus; competes with GHRH; elevated during fed state and high glucose; key timing consideration for GH peptide protocols |
| GHRH Effect | Stimulates GH | mechanism | Released from hypothalamus; binds GHRH-R on pituitary somatotrophs; primary physiological GH secretagogue |
Growth hormone (GH) physiology is the essential foundation for understanding why GH-stimulating peptides (GHRPs, GHRH analogues) are used, how they work, and why timing is critical. Unlike most hormones, GH is released in discrete pulses — not continuously — meaning the pattern of secretion is as important as the total amount.
The Hypothalamic-Pituitary-GH-IGF-1 Axis
GH is produced and released by somatotroph cells in the anterior pituitary, controlled by two competing hypothalamic signals:
- GHRH (Growth Hormone Releasing Hormone): Stimulates GH synthesis and release. Released in pulses from hypothalamic neurons.
- Somatostatin (SRIF): Inhibits GH release. Also hypothalamic origin. Somatostatin dominates during the fed, high-insulin, high-glucose state — which is why GH peptides work poorly after eating.
A third input — ghrelin, released from the stomach — acts via the GHSR receptor on pituitary somatotrophs and amplifies GH release beyond what GHRH alone achieves. Synthetic GHRP compounds (GHRP-2, GHRP-6, ipamorelin) are ghrelin receptor agonists that exploit this pathway.
Downstream, GH stimulates hepatic production of IGF-1 (Insulin-like Growth Factor 1), which mediates most of GH’s anabolic effects. IGF-1 also feeds back negatively on GH secretion, creating an autoregulatory loop.
GH Secretion: Key Regulatory Factors
| Factor | Effect on GH Pulse | Practical Implication |
|---|---|---|
| GHRH | Strongly stimulates GH release | GHRH analogues (CJC-1295) exploit this pathway |
| Somatostatin | Inhibits GH release; blunts GHRH effect | Fasted state reduces somatostatin; inject peptides fasted |
| Ghrelin (endogenous) | Amplifies GH via GHSR; additive with GHRH | GHRPs mimic this signal; combining GHRH + GHRP is synergistic |
| Sleep (deep NREM) | Largest daily GH pulse ~60 min after sleep onset | Pre-sleep peptide injection aligns with this natural peak |
| Fasting/low glucose | Reduces somatostatin tone; enhances GH pulse | Inject at least 2 hours post-meal; pre-sleep is ideal fasted state |
| Age | Pulse amplitude declines ~14%/decade after age 30 | Older individuals have blunted baseline GH; peptide effect may differ |
| Exogenous GH use | Suppresses endogenous GH via IGF-1 negative feedback | Exogenous GH and GH peptides should not be combined |
| Intense exercise | Acute GH spike; returns to baseline within ~1 hour | Post-exercise fasted window can also be a useful injection timing |
Age-Related GH Decline
GH secretion peaks during puberty and early adulthood, then declines progressively. By age 60, total daily GH output is approximately 5–10× lower than at age 20. Key data from Corpas et al. 1993 (PMID 8491152):
- GH pulse amplitude: ~14% decline per decade after age 30
- IGF-1: ~15% decline per decade after age 35
- Pulse frequency: relatively preserved; amplitude drives most of the decline
- Slow-wave sleep: also decreases with age, reducing the size of the sleep-associated GH peak
This age-related somatotropic axis decline (sometimes called “somatopause”) is the biological rationale for GH secretagogue peptide use in older adults.
Why Pulsatility Matters
Continuous GH elevation (as produced by exogenous recombinant GH or CJC-1295 with DAC) produces different tissue effects than pulsatile GH. Physiological pulsatile GH preferentially promotes lean mass and fat mobilization. Continuous GH exposure is associated with greater insulin resistance, edema, and IGF-1 dysregulation. This distinction is why most GH peptide protocols aim to preserve or amplify pulsatile secretion rather than maintain constant elevation (PMID 9861545).
Related Pages
Sources
- Veldhuis JD et al. Somatostatin and growth hormone secretion. J Clin Endocrinol Metab. 1995. PMID 7744886
- Corpas E et al. Human growth hormone and human aging. Endocr Rev. 1993;14(1):20-39. PMID 8491152
- Giustina A, Veldhuis JD. Pathophysiology of the neuroregulation of GH secretion. Endocr Rev. 1998;19(6):717-97. PMID 9861545
Frequently Asked Questions
Why does timing matter so much for GH peptide injections?
GH peptides work by amplifying naturally occurring GH pulses rather than replacing them. During periods of somatostatin dominance — which occurs in the fed, high-glucose state — injecting GH peptides produces a blunted response. Fasted state and low blood glucose suppress somatostatin, allowing GHRH and GHRP signals to drive larger GH pulses. Injecting before sleep aligns with the body's largest natural GH pulse window.
What is the difference between GHRH and ghrelin in GH regulation?
GHRH (Growth Hormone Releasing Hormone) is the primary hypothalamic signal for GH release, acting directly on pituitary somatotrophs via the GHRH receptor. Ghrelin, produced in the stomach, acts via the separate GHSR (growth hormone secretagogue receptor) and amplifies GH release synergistically with GHRH. Importantly, ghrelin also stimulates appetite — a side effect that synthetic GHRP mimetics partially replicate.
How much does IGF-1 decline with age, and why does this matter?
IGF-1 declines approximately 15% per decade after age 35 (PMID 8491152). Since IGF-1 is the primary downstream mediator of GH's anabolic effects — including muscle protein synthesis, fat mobilization, and bone turnover — this decline contributes to age-related sarcopenia and metabolic changes. GH secretagogue peptides are often used in an attempt to partially restore this axis, though long-term human data on efficacy are limited.