Ibutamoren (MK-677)

Ibutamoren (mesylate salt: ibutamoren mesylate; research codes MK-677, MK-0677, L-163,191) is an orally active, non-peptide small molecule that mimics ghrelin and acts as an agonist at the growth hormone secretagogue receptor 1a (GHSR1a). It is not a peptide despite frequent misclassification: it belongs to the spiroindoline scaffold class of synthetic GHS-R1a ligands. It stimulates pulsatile GH release and secondary IGF-1 elevation without directly administering either hormone. Multiple Phase 2 human RCTs have demonstrated effects on lean body mass, bone turnover, and sleep architecture in older adults. Merck developed it through Phase 2/3 before discontinuing; it is now under re-evaluation as a compounded drug under FDA 503A.

Identity

PubChem CID: 178024 (confirmed; note: the CID 9912462 sometimes cited in grey literature is incorrect)
InChIKey: UMUPQWIGCOZEOY-JOCHJYFZSA-N
ChEMBL: CHEMBL13817 (freebase); CHEMBL2105872 (mesylate salt)
Molecular formula: C27H36N4O5S (freebase)
Molecular weight: 528.7 Da (freebase); 624.8 Da (mesylate salt)
IUPAC name: 2-amino-2-methyl-N-[(2R)-1-(1-methylsulfonylspiro[2H-indole-3,4’-piperidine]-1’-yl)-1-oxo-3-phenylmethoxypropan-2-yl]propanamide
Class: Non-peptide growth hormone secretagogue (spiroindoline scaffold)
USAN designation: Ibutamoren (1997); mesilate variant recognised
Salt form used clinically: Ibutamoren mesylate (methanesulfonate salt)

Mechanism

Primary: GHSR1a agonism

MK-677 binds the growth hormone secretagogue receptor (GHSR1a, also called the ghrelin receptor) in the hypothalamus and anterior pituitary ¹. GHSR1a is a Gq-coupled GPCR whose endogenous ligand is ghrelin (the “hunger hormone” produced by gastric X/A-like cells). MK-677 is a full agonist at GHSR1a and produces:

Increased amplitude of pulsatile GH secretion from the anterior pituitary somatotrophs — preserving the hypothalamic feedback arc (distinct from exogenous recombinant GH, which suppresses endogenous GH release).
Secondary elevation of circulating IGF-1, predominantly liver-derived via GHR-JAK2-STAT5 signalling.
Modest elevation of GH-binding protein (GHBP) and IGF-binding protein-3 (IGFBP-3).

MK-677 does NOT directly stimulate GHRH receptors (GHRHR); its mechanism is receptor-distinct from CJC-1295 or tesamorelin. At pharmacological doses in elderly subjects, it restores GH pulsatile amplitude toward young-adult levels ².

Secondary: Sleep-architecture modulation

GHSR1a is expressed centrally in hypothalamic nuclei and brainstem regions governing sleep regulation. MK-677 administration at 25 mg/day produces increases in both REM sleep and slow-wave (stage IV) sleep in young adults, and a marked increase in REM sleep in older adults ³. The predominant sleep benefit differs by age group: slow-wave sleep is more strongly affected in young adults (~50% increase stage IV; >20% increase REM), whereas in older adults the primary documented effect is REM increase (~50%). The sleep effect appears partially independent of GH itself, consistent with direct central GHSR1a action.

Pharmacokinetics

Route: Oral (tablet/capsule); this is the primary differentiating feature — all GH-releasing peptide analogues (GHRP-2/6, hexarelin, ipamorelin) require injection.
Bioavailability: ~60-70% oral bioavailability in animal models; human bioavailability not precisely published but consistent with its clinical efficacy at 25 mg/day.
Half-life: ~5-6 hours (human PK data from early Merck studies; allows once-daily dosing with sustained GH pulsatile elevation over 24 hours at steady state).
Metabolism: Primarily hepatic (CYP3A4 substrate); urinary excretion as metabolites. Dose escalation beyond 25 mg/day does not proportionally increase GH response (ceiling effect at GHSR1a saturation).
Typical research doses: 10 mg/day, 25 mg/day (most common in trials); 2 mg and 25 mg studied by Copinschi 1997 in older adults.

dose-response-unclear — Head-to-head PK comparison across older-adult subpopulations (frail vs robust) has not been published. Whether dose reduction is required in hepatic impairment is not established in published literature.

Human evidence

Lean body mass and body composition

Nass 2008 (Annals of Internal Medicine, 2-year RCT) ²: The most robust aging-context human trial. Double-blind randomized modified-crossover trial in 65 healthy older adults (60–81 years; men, women on HRT, and women not on HRT). MK-677 25 mg/day for 2 years significantly increased fat-free mass (MK-677: +1.1 kg [95% CI 0.7–1.5 kg]; placebo: −0.5 kg [95% CI −1.1 to 0.2 kg]; p<0.001). Body cell mass (intracellular water) also increased in MK-677 group vs placebo (p=0.021). No significant difference in abdominal visceral fat or total fat mass; limb fat was greater in MK-677 group (1.1 kg vs 0.24 kg; p=0.001). Body weight increased 2.7 kg in MK-677 vs 0.8 kg in placebo (p=0.003). GH pulsatile amplitude and IGF-1 levels were restored toward young-adult levels. Fasting blood glucose increased an average of 0.3 mmol/L (5 mg/dL) in MK-677 group (p=0.015) and insulin sensitivity decreased. Cortisol increased 47 nmol/L (p=0.020). LDL cholesterol decreased in MK-677 group (−0.14 mmol/L; p=0.026). Most frequent side effects: increased appetite (subsided in months), transient mild lower-extremity edema, and muscle pain. No change in functional performance measures or quality of life at this dose and duration. Conclusion: lean-mass benefit established; functional-outcome benefit not demonstrated.

Svensson 1998 (JCEM, 8-week parallel) ⁴: 24 obese males (18–50 years), MK-677 25 mg (n=12) vs placebo (n=12), parallel group. Serum IGF-I increased ~40%; fat-free mass increased (DEXA p<0.01; four-compartment model p<0.05); basal metabolic rate increased transiently at 2 weeks but not at 8 weeks. Fasting glucose and insulin were unchanged but oral glucose tolerance test showed impaired glucose homeostasis at both 2 and 8 weeks. Cortisol: NOT elevated at 2 and 8 weeks (p=NS vs placebo). Provides early confirmation of the anabolic signal in non-elderly obese subjects.

Murphy 1999 (JBMR, elderly cohort) ⁵: 187 elderly adults (≥65 yr) pooled across three separate randomized double-blind placebo-controlled sub-studies (dose-response, healthy elderly, functionally impaired elderly). In the primary functionally impaired arm (n=91 completing 9 weeks): osteocalcin increased 29.4% (p<0.001), bone-specific alkaline phosphatase (BSAP) increased 10.4% (p<0.001), urinary NTX resorption markers increased 22.6% (p<0.05). IGF-I elevated 55–94% across study groups. Bone turnover stimulation confirmed; long-term BMD benefit not assessed in this study.

Murphy 2001 (JCEM, 18-month RCT with alendronate) ⁶: 292 postmenopausal women (64–85 years) with low femoral neck BMD; four-arm factorial design (3:3:1:1 allocation: MK-677+alendronate, alendronate alone, MK-677 alone, double-dummy placebo). MK-677 + alendronate increased femoral neck BMD 4.2% vs 2.5% for alendronate alone (p<0.05). No significant enhancement with combination vs alendronate alone at lumbar spine, total hip, or total body. IGF-I elevated 39–45% across MK-677 groups. MK-677 monotherapy did not significantly improve BMD at most sites but increased bone formation markers (osteocalcin +22%, NTx +41% vs placebo). Suggests adjunctive benefit at femoral neck in combination with antiresorptive therapy, but clinical significance limited by lack of generalization to other BMD sites.

Hip fracture recovery

Adunsky 2011 (Arch Gerontol Geriatr, Phase IIb) ⁷: Multicenter, randomized, double-blind, placebo-controlled trial; n=123 (MK-677 n=62, placebo n=61) older adults recovering from hip fracture; MK-677 25 mg/day vs placebo during rehabilitation. Primary outcomes: rank analysis of change in objective functional performance measures and IGF-1. At 24 weeks: stair-climbing power increased 12.5 W in MK-677 group (95% CI −10.95 to 35.88; p=0.292 — not significant). Gait speed improved (0.7-score difference; 95% CI 0.17–1.28; p=0.011). No improvement in several other functional performance measures. Fewer falls in MK-677 group (p=0.096, not significant). IGF-1 increased +51.4 ng/mL (95% CI 34.42–68.44; p<0.001). Critical safety finding: trial was terminated early due to a safety signal of congestive heart failure in a limited number of patients. The abstract concludes MK-677 has “an unfavorable safety profile in this patient population.” Illustrates the gap between biomarker effects (IGF-1) and functional outcomes, and raises a clinically significant safety concern in frail elderly. needs-replication

Sleep

Copinschi 1997 (Neuroendocrinology) ³: Double-blind crossover in young adults (18–30 yr, n=8) and older adults (65–71 yr, n=6). Study design: young adults underwent three 7-day treatment periods (5 mg MK-677, 25 mg MK-677, placebo; Latin-square crossover); older adults underwent two 14-day periods (2 mg MK-677 in period 1, 25 mg MK-677 in period 2, with baseline data on the 2 days preceding period 1). In young adults at 25 mg (“high-dose”):

~50% increase in duration of stage IV (slow-wave) sleep vs placebo (p<0.05)
20% increase in REM sleep duration vs placebo (p<0.05)
Frequency of sleep deviation episodes fell from 42% (placebo) to 8% (high-dose MK-677; p<0.03)

In older adults at 25 mg: nearly 50% increase in REM sleep (p<0.05) and decreased REM latency (p<0.02); frequency of sleep deviations also decreased (p<0.02). Important precision note confirmed against abstract: the “20–50% REM increase” framing in secondary literature conflates two distinct findings from this paper — the ~50% figure is the stage IV (slow-wave) effect in young adults; the >20% figure is the REM effect in young adults. The nearly 50% figure for older adults is specifically a REM sleep increase (not stage IV), which is the opposite pattern from young adults where the slow-wave effect dominated. The stage IV effect in older adults is not specifically quantified in the abstract. needs-replication — study sample sizes are very small (n=6–8 per group).

Aging-relevance paradox: GH stimulation vs longevity

The GH-IGF-1 axis sits at the center of a profound tension in aging biology:

Evidence favouring IGF-1 reduction for longevity:

Ames and Snell GH-deficient dwarf mice live substantially longer than wild-type littermates (specific % in primary sources cited by Barzilai 2012) ⁸
Heterozygous Igf1r knockout mice have extended lifespan (specific % in primary sources cited by Barzilai 2012) ⁸
C. elegans daf-2 (IIS receptor, ortholog of IGF-1R) loss-of-function mutants have >2-fold lifespan extension
Human centenarian studies identify IGF-1 pathway variants associated with longevity
Caloric restriction, the most reproducible lifespan-extending intervention, lowers IGF-1

Evidence favouring GH stimulation for healthspan in older humans:

Somatopause (age-related GH/IGF-1 decline) is associated with sarcopenia, adiposity, reduced bone density, and sleep disruption
Short-term GH supplementation and GH secretagogues partially reverse these phenotypes in clinical trials
Fat-free mass preservation may indirectly support multiple functional outcomes

Current resolution: The paradox is not resolved. GH stimulation may provide short-term anabolic benefit (musculoskeletal, sleep) while accelerating cellular aging processes (IGF-1 drives mTOR, suppresses FOXO, reduces autophagy). The human lifespan effect of chronic GH-axis elevation is unknown. MK-677’s net aging effect is genuinely uncertain — it may improve healthspan metrics while having neutral or adverse effects on lifespan. The preclinical longevity literature argues strongly against chronic use; the human sarcopenia/frailty literature argues for short-term use in specific populations. See deregulated-nutrient-sensing for the pathway-level framing. contradictory-evidence

Side effects and safety

Established adverse effects (from human trials):

Effect	Frequency	Significance
Increased fasting glucose	Common	Mean +0.3 mmol/L (+5 mg/dL) in healthy older adults ²; insulin sensitivity decreases
Peripheral edema (lower extremity)	Common	Mild; consistent with fluid retention
Increased appetite / hyperphagia	Common	GHSR1a in the hypothalamus drives appetite signalling
Increased cortisol	Reported in healthy older adults (Nass 2008: +47 nmol/L, p=0.020); not seen in obese young males (Svensson 1998)	Magnitude modest; population-dependent; clinical significance unclear
Joint pain / arthralgia	Occasional	Consistent with IGF-1 effects on cartilage and synovium
Fatigue	Occasional	Mechanism unclear

Insulin resistance is the most clinically significant concern. Nass 2008 documented increased fasting glucose (+0.3 mmol/L) and decreased insulin sensitivity in healthy older adults over 2 years ². For older adults with pre-diabetes, metabolic syndrome, or type 2 diabetes, this is likely a contraindication. This is the primary reason Merck discontinued development: the lean-mass benefit did not outweigh the metabolic risk in a broad frailty/sarcopenia population. long-term-unknown — Long-term safety (>2 years) in older adults has not been characterized.

Congestive heart failure signal in frail elderly. The Adunsky 2011 hip fracture rehabilitation trial (n=123) was terminated early due to a safety signal of congestive heart failure in a limited number of MK-677-treated patients ⁷. The absolute event count was not reported in the abstract. This substantially restricts the eligible population for MK-677 use in aged, frail individuals who are the primary target demographic for a sarcopenia/frailty indication. needs-replication

Cancer risk: IGF-1 is a mitogenic signal. Chronic IGF-1 elevation raises theoretical carcinogenesis concern. No clinical trial has demonstrated elevated cancer incidence with MK-677, but existing trials are underpowered and too short to detect this signal.

Regulatory and compounding status

Merck development history

Merck Research Laboratories developed MK-677 through Phase 2/3 trials in the 1990s–2000s for multiple indications (sarcopenia, frailty, hip fracture rehabilitation, osteoporosis). Development was discontinued — reasons not formally published, but the combination of (1) modest functional-outcome benefit despite strong biomarker effects, (2) insulin resistance adverse effects in the elderly target population, and (3) Merck’s portfolio prioritisation account for abandonment. MK-677 was never submitted for FDA approval and has no FDA-approved indication. unsourced — Merck’s internal discontinuation rationale has not been published; the above reconstruction is inferred from trial results and conference presentations.

PCAC 503A bulks list review

The FDA Pharmacy Compounding Advisory Committee (PCAC) reviewed ibutamoren (MK-677) for potential inclusion on the 503A bulks list (the list of substances that can be compounded by traditional compounding pharmacies for individual patients). At its October 2024 meeting, the committee voted against inclusion on the 503A bulks list. The vote reflects concerns about clinical necessity (given no FDA-approved compounded GHS exists and the compound has no approved indication), evidence sufficiency, and safety signals. 2026 reconsideration: MK-677 is reportedly under re-evaluation in the 2026 PCAC review cycle, potentially reflecting changed agency priorities. The 2026 outcome is pending as of 2026-05-09. unsourced — The PCAC October 2024 meeting minutes and vote record are the authoritative source for the October 2024 vote; this entry is based on the user-supplied context and has not been independently verified against the published meeting transcript.

Current active clinical trials

ClinicalTrials.gov (queried 2026-05-09): 1 active recruiting trial — NCT06948214, Phase 3 study of LUM-201 (ibutamoren mesylate) in children with growth hormone deficiency (pediatric GHD indication, not aging). No active aging-indication MK-677 trial identified on ClinicalTrials.gov as of 2026-05-09.

LUM-201 context: Lumos Networks is developing ibutamoren mesylate under the trade name LUM-201 for pediatric GHD using a predictive enrichment marker (PEM) strategy to identify children likely to respond. This is a repurposing away from the adult aging context.

Extrapolation assessment

Dimension	Status
Pathway conserved in humans?	Yes — GHSR1a is human-expressed; GH-IGF-1 axis is fully conserved
Phenotype conserved in humans?	Yes — somatopause (GH/IGF-1 decline with age) is well-established in humans
Replicated in humans?	Lean-mass effect: yes (multiple RCTs). Functional outcome: not demonstrated. Sleep: limited replication.

Model-to-human gap note: the longevity data is from model organisms (mice, worms) where GH/IGF-1 reduction extends lifespan. The human aging benefit from MK-677 is for symptom-level endpoints (lean mass, sleep) not lifespan. These are not in conflict if MK-677 is framed as treating somatopause symptoms rather than extending lifespan.

Classification

SENS strategy: Not directly applicable — does not map to any SENS damage-removal category. Indirect relevance to stem-cell-exhaustion via lean-mass maintenance.
Hallmark targets: deregulated-nutrient-sensing (via GH-IGF-1 axis), stem-cell-exhaustion (via lean mass / satellite cell context)
Mechanism class: growth-hormone-secretagogue (primary), ghrelin-receptor-agonist, gh-igf1-axis-stimulation — see intervention-classes § growth-hormone-secretagogue
Clinical category: Investigational drug (discontinued by Merck); not FDA-approved; available via research supply and compounding (pending regulatory status)
DrugAge entry: Absent as of 2026-05-09 needs-canonical-id

Limitations and knowledge gaps

Functional outcomes not demonstrated: The most powered trial (Nass 2008, 2 years, n=65) showed lean-mass gain but no improvement in functional performance or quality-of-life measures. The lean mass signal is a biomarker; clinical translation requires functional endpoints.
Insulin resistance is a barrier: Consistent across trials; makes MK-677 unsuitable for the high-prevalence metabolic-syndrome/pre-diabetes elderly subpopulation.
Congestive heart failure signal in frail elderly: Adunsky 2011 (n=123, hip fracture) was terminated early for CHF; authors concluded “unfavorable safety profile in this patient population.” This substantially narrows the eligible population.
No frailty endpoint RCT: No completed trial has used frailty composite scores (Fried criteria, Clinical Frailty Scale) as primary endpoints.
Longevity direction paradox unresolved: see § Aging-relevance paradox above. contradictory-evidence
PCAC vote outcome not independently verified: The October 2024 vote-against has not been confirmed against FDA published meeting transcripts in this page’s creation. unsourced
No DrugAge lifespan extension data: MK-677 has no DrugAge entry; no model-organism lifespan extension published.
Long-term safety beyond 2 years: Unknown. long-term-unknown
Cancer risk: Theoretically elevated with chronic IGF-1 stimulation; not demonstrated in clinical trials but trials are underpowered for this outcome. long-term-unknown

Footnotes

doi:10.1196/annals.1404.023 · Smith RG, Sun Y, Jiang H et al. · Ann N Y Acad Sci 2007 Nov;1119:127–140 · review · model: review of preclinical + clinical GHS-R1a biology · GHS-R1a agonism in elderly subjects restores GH episodic release to young-adult amplitude; functional benefits include increased lean mass and bone density; in old mice, GHS-R1a agonist partially restores thymic function and reduces tumor growth; mechanism in liver: inhibition of cyclin D3:cdk4/cdk6 activity and increased PP2A activity, stabilising dephosphorylated C/EBPalpha; dopamine-GHS-R1a heterodimer found in hippocampus, cortex, substantia nigra, VTA · archive: doi confirmed, no local PDF (closed access); abstract verified via Europe PMC ↩
doi:10.7326/0003-4819-149-9-200811040-00003 · Nass R, Pezzoli SS, Oliveri MC, Patrie JT, Harrell FE Jr, Clasey JL, Heymsfield SB, Bach MA, Vance ML, Thorner MO · Annals of Internal Medicine 2008 Nov;149(9):601–611 · rct (double-blind randomized placebo-controlled modified-crossover) · n=65 healthy older adults (60–81 yr; men, women on HRT, women not on HRT) · p<0.001 (fat-free mass) · model: healthy older humans · MK-677 25 mg/day × 2 yr; fat-free mass: MK-677 +1.1 kg (CI 0.7–1.5), placebo −0.5 kg (CI −1.1 to 0.2); fasting glucose +0.3 mmol/L (p=0.015); cortisol +47 nmol/L (p=0.020); LDL −0.14 mmol/L (p=0.026); no functional performance benefit; PMC: PMC2757071 · archive: doi confirmed; PMC OA but DOI lookup failed; abstract verified via PubMed efetch ↩ ↩² ↩³ ↩⁴
doi:10.1159/000127249 · Copinschi G, Leproult R, Van Onderbergen A et al. · Neuroendocrinology 1997 Oct;66(4):278–286 · rct (crossover) · young adults n=8 (3-period Latin-square: 5 mg, 25 mg, placebo × 7 days each); older adults n=6 (2-period: 2 mg × 14 days then 25 mg × 14 days) · p<0.05 (sleep stages) · model: healthy humans 18–30 yr and 65–71 yr · Young adults at 25 mg: ~50% increase stage IV sleep (p<0.05), >20% increase REM (p<0.05), sleep deviations 42%→8% (p<0.03). Older adults at 25 mg: nearly 50% increase REM sleep (p<0.05), decreased REM latency (p<0.02). Note: the ~50% figure in older adults is REM (not stage IV); stage IV effect in older adults not separately quantified in abstract · archive: doi confirmed, no local PDF (closed access); abstract verified via Europe PMC ↩ ↩²
doi:10.1210/jcem.83.2.4539 · Svensson J, Lönn L, Jansson JO et al. · J Clin Endocrinol Metab 1998 Feb;83(2):362–369 · rct (parallel group, not crossover) · n=24 obese males (18–50 yr; BMI >30, waist/hip >0.95) · model: obese young human males · MK-677 25 mg (n=12) vs placebo (n=12) × 8 weeks; IGF-I +40% (p<0.001); fat-free mass increased (DEXA p<0.01); BMR increased at 2 wk (p=0.01) but not 8 wk; cortisol NOT increased (p=NS); oral GTT impaired at 2 and 8 wk · archive: doi confirmed (bronze OA), PDF download failed; abstract verified via Europe PMC ↩
doi:10.1359/jbmr.1999.14.7.1182 · Murphy MG, Bach MA, Plotkin D et al. · J Bone Miner Res 1999 Jul;14(7):1182–1188 · rct (three sub-studies pooled) · n=187 elderly adults (≥65 yr) across three randomized, double-blind, placebo-controlled studies · model: human elderly (healthy + functionally impaired) · Three sub-studies: (1) dose-response n=10–12/group, 2-week, 10 mg/25 mg/placebo; (2) n=50 healthy elderly (20 placebo, 30 MK-677 at 25→50 mg), 4 weeks; (3) n=105 functionally impaired elderly (63 MK-677, 28 placebo), 9 weeks. Functionally impaired arm: osteocalcin +29.4% (p<0.001), BSAP +10.4% (p<0.001), urinary NTX +22.6% (p<0.05); IGF-I +55–94% across studies (p<0.05) · archive: doi confirmed (bronze OA), PDF download failed; abstract verified via Europe PMC ↩
doi:10.1210/jcem.86.3.7294 · Murphy MG, Weiss S, McClung M et al. · J Clin Endocrinol Metab 2001 Mar;86(3):1116–1125 · rct (4-arm factorial, multicenter, double-blind, placebo-controlled) · n=292 postmenopausal women (64–85 yr) with low femoral neck BMD · 3:3:1:1 randomisation to: MK-677 25 mg + alendronate 10 mg; alendronate 10 mg; MK-677 25 mg; or double-dummy placebo · p<0.05 (femoral neck BMD: MK-677+alendronate vs alendronate alone) · 18-month trial (plus crossover extension from month 12–18); femoral neck BMD: MK-677+alendronate +4.2% vs alendronate +2.5% (p<0.05); no significant enhancement at lumbar spine, total hip, or total body vs alendronate alone; IGF-I increased 39–45% with MK-677 · archive: doi confirmed, no local PDF (closed access); abstract verified via Europe PMC ↩
doi:10.1016/j.archger.2010.10.004 · Adunsky A, Chandler J, Heyden N et al. · Arch Gerontol Geriatr 2011 Sep-Oct;53(2):183–189 · rct (Phase IIb, multicenter, double-blind, placebo-controlled) · n=123 (MK-677 n=62, placebo n=61) · model: older adults recovering from hip fracture · MK-677 25 mg/day during rehabilitation; stair-climbing p=0.292 (NS); gait speed p=0.011; IGF-1 +51.4 ng/mL (p<0.001); trial terminated early: congestive heart failure safety signal; authors conclude “unfavorable safety profile in this patient population” · archive: doi confirmed, no local PDF (closed access); abstract verified via Europe PMC ↩ ↩²
doi:10.2337/db11-1300 · Barzilai N, Huffman DM, Muzumdar RH, Bartke A · Diabetes 2012 Jun;61(6):1315–1322 · review · 74 references; synthesis of metabolic signaling pathways in aging · Reviews: IIS pathway mutations extending lifespan in model organisms; caloric restriction lowering GH/IGF-1; paradox of somatopause (GH decline) vs longevity benefit of GH axis reduction. Does not originate specific quantitative lifespan-extension percentages for dwarf mice — cites primary sources (Ames/Snell work by Bartke group and others). The “reduced somatotropic signaling” section confirms the paradox framing. · archive: doi confirmed; local PDF downloaded (PMC3357299) ↩ ↩²

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