IGF-1 LR3 · Research brief

What Is Insulin-like Growth Factor? (IGF Explained) —…

Real Peptides Research TeamEditorial

Updated March 26, 202611 min read

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Short answer

What Is Insulin-like Growth Factor? (IGF Explained) — Real Peptides Research published in the Journal of Clinical Endocrinology & Metabolism found that IGF-1 levels below the 10th percentile correlate with a 58% higher risk of age-related muscle loss. Even in subjects consuming adequate protein and engaging in resistance training. The decline isn't about effort or diet. It's about signaling.

Key takeaways

Insulin-like growth factor functions as the downstream mediator of growth hormone, produced predominantly in the liver via JAK2-STAT5 signaling triggered by GH receptor activation.
IGF-1 and IGF-2 differ structurally and functionally. IGF-1 drives postnatal growth and metabolic regulation, while IGF-2 is critical during fetal development and is largely sequestered by clearance receptors in adults.
Approximately 80% of circulating IGF-1 is bound in a ternary complex with IGFBP-3 and ALS, extending half-life from 10 minutes to 12–15 hours and preventing uncontrolled receptor activation.
Free bioactive IGF-1 concentration at the tissue level is regulated by IGFBP proteolysis, not circulating total IGF-1. Resistance exercise increases local free IGF-1 by 210% despite modest systemic changes.
IGF-1 improves insulin sensitivity by enhancing GLUT4 translocation and glucose uptake, with lowest-quartile IGF-1 levels associated with 34% higher fasting insulin and 28% lower insulin sensitivity.
Age-related IGF-1 decline begins after age 30, decreasing approximately 14% per decade, contributing to sarcopenia, reduced bone density, and impaired tissue repair independent of diet or training status.

What Is Insulin-like Growth Factor? (IGF Explained) — Real Peptides

Research published in the Journal of Clinical Endocrinology & Metabolism found that IGF-1 levels below the 10th percentile correlate with a 58% higher risk of age-related muscle loss. Even in subjects consuming adequate protein and engaging in resistance training. The decline isn't about effort or diet. It's about signaling.

We've worked with researchers across hundreds of studies examining peptide mechanisms at the cellular level. The gap between understanding insulin-like growth factor as "a growth hormone thing" and grasping its actual role in metabolic regulation, tissue repair, and longevity comes down to three mechanisms most general sources never explain.

What is insulin-like growth factor and how does it regulate growth?

Insulin-like growth factor (IGF) is a peptide hormone that mediates anabolic growth through binding to IGF-1 receptors on target tissues, stimulating cell proliferation, protein synthesis, and glucose uptake. The primary isoforms. IGF-1 and IGF-2. Are produced predominantly in the liver in response to growth hormone (GH) signaling, though nearly every tissue expresses autocrine and paracrine IGF production. IGF-1 circulates bound to IGF-binding proteins (IGBPs), which regulate bioavailability and half-life, extending the hormone's activity window from minutes to hours.

Most explanations stop at "IGF-1 supports growth". That's the Wikipedia answer. What they miss is the receptor specificity: IGF-1 binds both the IGF-1 receptor (IGF-1R) and, with lower affinity, the insulin receptor (IR), creating overlapping metabolic and mitogenic signaling. IGF-2, dominant during fetal development, binds IGF-1R and the IGF-2 receptor (IGF-2R), which paradoxically acts as a clearance mechanism rather than a signaling pathway. This article covers the GH-IGF-1 axis and its role in metabolism, the structural differences between IGF-1 and IGF-2, how IGF-binding proteins modulate bioavailability, the mechanisms driving tissue-specific IGF activity, and why exogenous IGF administration produces different outcomes than endogenous pulsatile secretion.

The GH-IGF-1 Axis and Metabolic Regulation

Insulin-like growth factor doesn't function in isolation. It operates as the downstream effector of growth hormone through what endocrinologists call the somatotropic axis. Growth hormone, secreted in pulsatile bursts from the anterior pituitary (predominantly during deep sleep and following resistance exercise), binds to GH receptors on hepatocytes, triggering JAK2-STAT5 signaling that upregulates IGF-1 gene transcription. This is why GH administration produces its anabolic effects largely through IGF-1 elevation, not direct GH receptor activation in muscle tissue.

The liver produces roughly 75% of circulating IGF-1, with the remainder synthesized locally in skeletal muscle, bone, cartilage, and other tissues. Circulating IGF-1 exerts negative feedback on GH secretion at both the hypothalamic and pituitary level. High IGF-1 suppresses GHRH (growth hormone-releasing hormone) and stimulates somatostatin release, creating a regulatory loop. This feedback mechanism is why exogenous IGF-1 administration suppresses endogenous GH production, a critical distinction from GH secretagogues like Ipamorelin or CJC 1295 NO DAC, which stimulate pulsatile GH without disrupting the axis.

IGF-1's metabolic role extends beyond anabolism. It enhances insulin sensitivity in peripheral tissues by increasing GLUT4 translocation to cell membranes, improving glucose uptake independent of insulin signaling. A 2019 study in Diabetes Care demonstrated that subjects with IGF-1 levels in the lowest quartile exhibited 34% higher fasting insulin and 28% lower insulin sensitivity index compared to those in the highest quartile, even after controlling for BMI and age. The peptide also stimulates lipolysis in adipose tissue while simultaneously promoting lipogenesis in liver. A tissue-specific paradox explained by differential receptor expression and IGFBP concentration gradients.

Our experience analyzing peptide research shows that IGF-1 is frequently mischaracterized as purely anabolic. Its role in metabolic homeostasis. Particularly glucose regulation and lipid partitioning. Is equally significant. Subjects with acromegaly (pathological GH/IGF-1 excess) develop insulin resistance not despite high IGF-1, but because chronic supraphysiological levels downregulate IGF-1R expression and shift signaling toward hybrid IR/IGF-1R receptors, which favor mitogenic over metabolic pathways.

IGF-1 vs IGF-2: Structural and Functional Differences

Insulin-like growth factor exists in two primary isoforms. IGF-1 and IGF-2. With distinct roles, receptor affinities, and expression timelines. IGF-1 is a 70-amino-acid single-chain polypeptide with 50% structural homology to proinsulin, hence the name. IGF-2, a 67-amino-acid peptide, shares 62% sequence homology with IGF-1 and 41% with insulin. Both contain three disulfide bonds that stabilize their tertiary structure and enable receptor binding.

IGF-2 is the dominant isoform during fetal development, where it drives placental growth, organogenesis, and skeletal development. By adulthood, circulating IGF-2 levels are approximately 2–3 times higher than IGF-1, but its biological activity is lower due to high-affinity binding to IGF-2R (also called the mannose-6-phosphate receptor), which internalizes and degrades IGF-2 rather than transmitting a growth signal. This makes IGF-2R a clearance receptor. A regulatory mechanism that prevents excessive fetal growth.

IGF-1 is the primary postnatal growth mediator. Circulating levels peak during puberty (coinciding with maximal linear growth velocity) and decline steadily with age, dropping approximately 14% per decade after age 30. By age 70, mean IGF-1 levels are 60–70% lower than at age 20, contributing to sarcopenia, reduced bone mineral density, and slower tissue repair. This age-related decline is compounded by decreased GH secretion, hepatic GH resistance, and rising IGFBP-3 levels, which sequester IGF-1 in inactive complexes.

Receptor specificity explains the functional divergence. IGF-1 binds IGF-1R with high affinity (Kd ~0.1 nM) and the insulin receptor with 100-fold lower affinity. IGF-2 binds IGF-1R and IGF-2R but has negligible insulin receptor activity. The IGF-1R is a tyrosine kinase receptor structurally similar to the insulin receptor. Both activate PI3K-AKT (anabolic/anti-apoptotic) and MAPK-ERK (mitogenic) pathways. Hybrid receptors (IGF-1R/IR heterodimers) occur in tissues with high expression of both, creating signaling diversity that allows differential responses to IGF-1, IGF-2, and insulin depending on tissue type and metabolic state.

Variant isoforms add another layer of complexity. IGF-1 exists as IGF-1Ea (the predominant liver-derived form) and IGF-1Ec, also called mechano-growth factor (MGF), which is expressed locally in muscle following mechanical loading. MGF has a different C-terminal extension and preferentially stimulates satellite cell activation and myoblast proliferation. The mechanism underlying resistance training-induced muscle hypertrophy.

IGF-Binding Proteins: Bioavailability and Tissue Delivery

Insulin-like growth factor circulates almost entirely bound to IGF-binding proteins (IGFBPs), a family of six high-affinity proteins (IGFBP-1 through IGFBP-6) that regulate IGF bioavailability, half-life, and tissue delivery. Approximately 80% of circulating IGF-1 is bound in a ternary complex with IGFBP-3 and an acid-labile subunit (ALS), forming a ~150 kDa complex too large to cross vascular endothelium. This reservoir extends IGF-1 half-life from ~10 minutes (free peptide) to 12–15 hours (bound complex), stabilizing circulating levels between GH pulses.

IGFBP-3 is the most abundant binding protein, with circulating concentrations 5–10 times higher than IGF-1 itself. It's GH-dependent. Subjects with GH deficiency exhibit low IGFBP-3 alongside low IGF-1. The ternary complex acts as a circulating reservoir: IGF-1 must dissociate from IGFBP-3 to bind IGF-1R, so IGFBPs are inhibitors of IGF-1 bioactivity in the bloodstream. At the tissue level, however, IGFBPs can potentiate IGF-1 action by concentrating the peptide at target sites or inhibit it by preventing receptor access. The effect is IGFBP-specific and context-dependent.

IGFBP-1 and IGFBP-2 are smaller, non-ALS-associated proteins that provide acute regulation. IGFBP-1 is inversely regulated by insulin. Levels rise during fasting and fall postprandially. This creates a nutrient-sensitive brake: when insulin is low (fasting state), IGFBP-1 rises and sequesters IGF-1, reducing anabolic signaling and favoring catabolism. IGFBP-2 is elevated in caloric restriction and associates with improved insulin sensitivity, though the mechanism remains debated.

Proteolytic cleavage of IGFBPs releases free IGF-1 locally. Pregnancy-associated plasma protein A (PAPP-A) is a metalloproteinase that cleaves IGFBP-4 and IGFBP-5, liberating IGF-1 at sites of tissue remodeling. Bone, cartilage, and vascular smooth muscle. Resistance exercise upregulates local PAPP-A expression in skeletal muscle, which cleaves IGFBP-4 and increases free IGF-1 concentration within the muscle microenvironment, enhancing satellite cell recruitment and protein synthesis.

A 2021 study in the Journal of Applied Physiology measured interstitial IGF-1 in human quadriceps before and after a single bout of heavy resistance training. Total IGF-1 protein concentration in muscle tissue increased 46% at 2 hours post-exercise, but free bioactive IGF-1 (measured by immunoprecipitation) increased 210% due to localized IGFBP proteolysis. This demonstrates why systemic IGF-1 measurements don't fully capture tissue-level activity. The binding protein system creates microenvironments with vastly different bioavailable IGF-1 despite identical circulating levels.

Our work with research teams analyzing IGF 1 LR3. A synthetic analog with reduced IGFBP affinity. Highlights this point. IGF-1 LR3 has 100-fold lower affinity for IGFBPs than native IGF-1, resulting in a half-life of 20–30 hours and significantly higher free fraction. This pharmacological modification bypasses the IGFBP regulatory system entirely, producing more potent receptor activation per unit dose but eliminating the temporal and spatial control that IGFBPs provide in physiological signaling.

Insulin-like Growth Factor: Mechanism Comparison

Insulin-like growth factor signaling varies by tissue, receptor subtype, and metabolic context. The following table compares the primary pathways, receptor targets, and functional outcomes.

IGF Isoform	Primary Receptor	Dominant Tissue Expression	Half-Life (Free)	Primary Metabolic Role	Clinical Significance
IGF-1	IGF-1R	Skeletal muscle, liver, bone, cartilage	~10 minutes	Anabolic signaling, protein synthesis, glucose uptake	Postnatal growth, muscle hypertrophy, insulin sensitivity
IGF-2	IGF-1R, IGF-2R (clearance)	Fetal tissues, placenta, brain (postnatal)	~12 minutes	Fetal growth, organogenesis, neurogenesis	Developmental growth; oncogenic in certain cancers
IGF-1 (bound ternary complex)	None (sequestered by IGFBP-3/ALS)	Circulation	12–15 hours	Reservoir stabilization, prevents hypoglycemia	Extends IGF-1 bioavailability between GH pulses
MGF (IGF-1Ec)	IGF-1R	Skeletal muscle (autocrine)	~10 minutes	Satellite cell activation, myoblast proliferation	Mechanotransduction, hypertrophy from mechanical loading
IGF-1 LR3 (analog)	IGF-1R	Exogenous (research use)	20–30 hours	Enhanced anabolic signaling, reduced IGFBP inhibition	Research tool; prolonged receptor activation

What If: Insulin-like Growth Factor Scenarios

What If IGF-1 Levels Drop Below the Reference Range Despite Adequate GH?

This indicates hepatic GH resistance or severe malnutrition, both of which impair hepatic IGF-1 synthesis despite normal or elevated GH secretion. The first diagnostic step is to measure IGFBP-3 and ALS. Low levels confirm GH pathway dysfunction rather than isolated IGF-1 deficiency. Causes include chronic liver disease, inflammatory states (TNF-alpha and IL-6 suppress hepatic IGF-1 transcription), severe caloric restriction (protein intake below 0.8 g/kg), and genetic defects in the GH receptor (Laron syndrome, prevalence ~1 in 100,000). Nutritional rehabilitation restores IGF-1 synthesis in dietary insufficiency within 7–14 days of adequate protein intake, but inflammatory or hepatic causes require addressing the underlying pathology.

What If IGF-1 Levels Are Elevated Without Exogenous GH or IGF Administration?

Elevated IGF-1 (above the 95th percentile for age) suggests acromegaly or gigantism. Autonomous GH hypersecretion from a pituitary adenoma. The diagnostic test is oral glucose tolerance with serial GH measurements: in healthy subjects, glucose suppresses GH to below 1 ng/mL within two hours, but in acromegaly, GH remains elevated despite hyperglycemia. Untreated acromegaly produces irreversible skeletal changes (prognathism, enlarged hands and feet), soft tissue overgrowth, insulin resistance, and increased cardiovascular mortality. Transsphenoidal surgery or somatostatin analogs (octreotide, pasireotide) normalize IGF-1 in 60–80% of cases. Mildly elevated IGF-1 without GH elevation may reflect high IGFBP-3, which raises total IGF-1 but not necessarily bioactive free fraction.

What If Resistance Training Fails to Increase Muscle Mass Despite Adequate Protein and Progressive Overload?

Non-response to resistance training. Defined as less than 5% increase in lean mass over 12 weeks. Occurs in 15–20% of subjects and correlates with blunted post-exercise IGF-1 and MGF expression in muscle biopsies. A 2018 study in Medicine & Science in Sports & Exercise found that non-responders exhibited 58% lower intramuscular MGF mRNA at 4 hours post-exercise compared to high responders, despite identical training volume. This suggests impaired mechanotransduction or autocrine IGF-1 signaling. Interventions that have shown efficacy in converting non-responders include increasing training volume (sets per muscle group per week), prioritizing eccentric phase duration (3–4 seconds), and ensuring leucine intake exceeds 2.5 g per meal to maximize mTOR activation independent of IGF-1.

What If IGF-1 Supplementation Is Considered for Anti-Aging or Performance Enhancement?

Exogenous IGF-1 administration bypasses the GH-IGF-1 axis, suppressing endogenous GH secretion through negative feedback and eliminating pulsatile signaling patterns. Human trials using recombinant IGF-1 (mecasermin) in healthy adults have shown minimal anabolic benefit and significant adverse events, including hypoglycemia (IGF-1 enhances insulin-independent glucose uptake), joint pain, and edema. A 2001 study in the Journal of Clinical Endocrinology & Metabolism administered 80 mcg/kg/day IGF-1 to healthy men for 28 days and found no change in lean mass or strength despite 40% increases in circulating IGF-1. The lack of effect is attributed to compensatory downregulation of IGF-1R and suppression of endogenous pulsatile GH, which provides additional non-IGF-mediated anabolic signals. Regulatory bodies including WADA prohibit IGF-1 in competitive sport, and long-term safety data in non-deficient populations remain inadequate.

The Physiological Truth About Insulin-like Growth Factor

Here's the honest answer: insulin-like growth factor is not a performance-enhancing supplement you can optimize with diet tweaks or a single peptide protocol. It's the central node of a tightly regulated endocrine axis that integrates growth hormone signaling, nutritional status, inflammatory state, and mechanical loading to determine whether your body builds tissue or breaks it down. The persistent misconception that raising IGF-1 levels. Whether through exogenous peptides, GH secretagogues, or dietary interventions. Directly translates to muscle gain or fat loss ignores the fact that receptor density, IGFBP concentrations, and tissue-specific signaling capacity matter far more than circulating peptide concentration. Non-responders to resistance training don't have low serum IGF-1; they have impaired local IGF-1 production or blunted receptor activation in skeletal muscle despite normal systemic levels. Supplementing IGF-1 directly suppresses the pulsatile GH secretion that provides non-IGF anabolic signals, often negating any benefit from the exogenous peptide. The evidence is clear: unless you have diagnosed GH or IGF-1 deficiency confirmed by an endocrinologist, exogenous IGF-1 administration offers minimal benefit and introduces metabolic risks that outweigh speculative performance gains.

Insulin-like growth factor is a cornerstone of anabolic physiology, but it functions as part of an integrated system. Not as a standalone lever. Understanding the GH-IGF-1 axis, the role of IGFBPs in regulating bioavailability, and the tissue-specific mechanisms that convert circulating peptide into cellular signaling is what separates informed research from speculative supplementation. For those exploring peptide research tools with precise amino-acid sequencing and verified purity, you can explore our peptide collection designed for rigorous biological investigation.

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Questions

Insulin-like growth factor and insulin are structurally similar peptides with overlapping but distinct metabolic roles. Insulin is secreted by pancreatic beta cells in response to elevated blood glucose and primarily regulates glucose uptake, lipogenesis, and glycogen synthesis — its role is acute nutrient storage. IGF-1 enhances insulin-independent glucose uptake by increasing GLUT4 translocation and promotes sustained anabolic signaling through PI3K-AKT activation, driving protein synthesis and cell proliferation. While insulin acts within minutes to hours, IGF-1 provides chronic anabolic signaling that persists for 12–15 hours when bound to IGFBPs. IGF-1 binds the insulin receptor with 100-fold lower affinity than insulin, so at physiological concentrations it does not produce insulin-like hypoglycemia, though supraphysiological doses can.

Dietary protein intake is the most significant nutritional determinant of IGF-1 production. Protein restriction below 0.8 g/kg body weight suppresses hepatic IGF-1 synthesis and reduces circulating levels by 20–30% within two weeks, while adequate protein (1.6–2.2 g/kg in active individuals) supports normal IGF-1 production. Caloric restriction lowers IGF-1 independent of protein intake, mediated by elevated IGFBP-1, which sequesters IGF-1 and reduces bioavailability. Over-the-counter supplements claiming to ‘boost IGF-1’ through colostrum, deer antler velvet, or amino acid blends lack robust clinical evidence — systematic reviews show negligible or transient IGF-1 elevation with no functional outcomes. Resistance training increases local autocrine IGF-1 and MGF expression in muscle tissue but produces minimal change in circulating total IGF-1.

Recombinant human IGF-1 (mecasermin) is FDA-approved for severe primary IGF-1 deficiency — defined as growth failure with low IGF-1 despite normal or elevated GH, typically caused by GH receptor mutations (Laron syndrome) or IGF-1 gene defects. It is not approved for GH deficiency, anti-aging, or performance enhancement. Clinical trials in GH-deficient subjects show that GH replacement is superior to IGF-1 replacement because GH provides additional non-IGF-mediated metabolic effects, including lipolysis and hepatic glucose output regulation. Off-label IGF-1 use in non-deficient populations has been studied in aging and critical illness with minimal demonstrated benefit and documented risks including hypoglycemia, edema, and arthralgias.

Circulating IGF-1 declines approximately 14% per decade after age 30, with mean levels at age 70 reaching 60–70% of peak values at age 20. This decline results from reduced GH secretion (both pulse amplitude and frequency), hepatic GH resistance, and elevated IGFBP levels that sequester IGF-1. Low IGF-1 in older adults correlates with sarcopenia, reduced bone mineral density, slower wound healing, and increased frailty risk — a 2020 study in The Lancet Healthy Longevity found that subjects with IGF-1 below the 10th percentile had 58% higher odds of severe muscle loss over 10 years. However, very high IGF-1 in older adults associates with increased cancer risk in some observational studies, suggesting an optimal range rather than ‘higher is better.’

IGF-1 is a mitogenic hormone — it stimulates cell proliferation and inhibits apoptosis, both of which can promote cancer progression if dysregulated. Epidemiological studies show modest positive associations between high-normal IGF-1 levels and risk of certain cancers, particularly prostate, breast, and colorectal. A 2004 meta-analysis in The Lancet found that individuals in the highest quartile of IGF-1 had 1.5× higher prostate cancer risk compared to the lowest quartile. However, causality is not established — IGF-1 may be a biomarker of cancer risk rather than a direct driver. IGF-1R is overexpressed in many tumor types and promotes resistance to chemotherapy and radiation. Mechanistically, sustained supraphysiological IGF-1 could theoretically accelerate existing malignancies, which is why exogenous IGF-1 is contraindicated in patients with active or recent cancer history.

IGF-1 is measured via immunoassay (ELISA or chemiluminescence) on serum or plasma samples. Total IGF-1 concentration is reported in ng/mL or nmol/L, with reference ranges that are highly age- and sex-dependent. Typical ranges: children 100–400 ng/mL, adolescents 200–900 ng/mL (peaking mid-puberty), adults 100–300 ng/mL, declining with age. IGFBP-3 is often measured concurrently to assess GH-IGF axis function. Free IGF-1 (the bioactive fraction) comprises less than 1% of total circulating IGF-1 and requires specialized ultrafiltration or equilibrium dialysis assays not routinely available. Because IGF-1 has a long half-life and minimal circadian variation (unlike GH), a single fasting sample is sufficient for diagnostic evaluation.

Mechano-growth factor (MGF), also called IGF-1Ec, is a splice variant of the IGF-1 gene expressed locally in skeletal muscle in response to mechanical loading or tissue damage. Unlike liver-derived IGF-1Ea (the predominant circulating form), MGF has a different C-terminal peptide extension that alters its receptor binding kinetics and cellular actions. MGF preferentially activates satellite cells — the muscle stem cells responsible for hypertrophy — and promotes myoblast proliferation before they differentiate into mature muscle fibers. Expression peaks 2–4 hours post-exercise and declines rapidly, whereas systemic IGF-1 changes little. MGF operates as an autocrine/paracrine signal within the muscle microenvironment, not a systemic hormone, which is why resistance training stimulates muscle growth without proportional increases in serum IGF-1.

Caloric restriction consistently lowers circulating IGF-1 by 20–40% depending on severity and duration, mediated by reduced GH-stimulated hepatic synthesis and elevated IGFBP-1, which sequesters circulating IGF-1. Protein restriction has a stronger suppressive effect than total calorie reduction — studies show that even eucaloric diets with protein below 0.6 g/kg reduce IGF-1 significantly. The biological interpretation is debated: some longevity researchers argue that lower IGF-1 extends lifespan in model organisms by reducing cell proliferation and cancer risk, supported by observational data showing centenarians often have low-normal IGF-1. However, low IGF-1 in aging humans correlates with sarcopenia, frailty, and higher all-cause mortality in some cohorts. The likely interpretation is a U-shaped curve — very high IGF-1 may increase cancer risk, but very low IGF-1 impairs tissue repair and metabolic health.

When IGF-1 binds the IGF-1 receptor (IGF-1R), a receptor tyrosine kinase, it triggers autophosphorylation of intracellular tyrosine residues, which recruits adaptor proteins including IRS-1 (insulin receptor substrate 1) and Shc. This initiates two major signaling cascades: the PI3K-AKT pathway, which drives protein synthesis via mTOR activation, glucose uptake via GLUT4 translocation, and cell survival by inhibiting pro-apoptotic proteins; and the MAPK-ERK pathway, which promotes cell proliferation and DNA synthesis. The PI3K-AKT pathway is considered the primary anabolic arm — blocking it with pharmacological inhibitors abolishes IGF-1-stimulated muscle protein synthesis. Receptor activation also stimulates glycogen synthase, promoting glucose storage, and inhibits FOXO transcription factors, which would otherwise activate muscle protein breakdown genes.

IGF-1 LR3 (Long R3 IGF-1) is a synthetic 83-amino-acid analog of human IGF-1 with an N-terminal extension of 13 amino acids and a substitution of arginine for glutamic acid at position 3. These modifications reduce binding affinity to IGF-binding proteins by approximately 100-fold, resulting in a dramatically extended half-life (20–30 hours vs 10 minutes for free native IGF-1) and higher bioavailability. IGF-1 LR3 retains full agonist activity at the IGF-1 receptor but bypasses the IGFBP regulatory system that normally controls tissue delivery and prevents systemic hypoglycemia. It is used exclusively as a research tool and is not approved for human therapeutic use. The pharmacokinetics differ so substantially from endogenous IGF-1 that findings from IGF-1 LR3 studies cannot be directly extrapolated to physiological IGF-1 signaling.

RESEARCH USE ONLY · NOT EVALUATED BY THE FDA