IGF-1 LR3
Also known as Long R3 IGF-1, Long Arg3 IGF-1, LR3-IGF-1, Insulin-like Growth Factor 1 Long R3
IGF-1 LR3 is a modified, long-acting version of insulin-like growth factor 1 that resists binding proteins and stays active for most of a day. People use it for muscle growth and recovery, and it carries a real risk of low blood sugar.
Overview
IGF-1 LR3 is a synthetic analog of human IGF-1 with an arginine substituted for glutamate at position 3 and a 13 amino acid extension at the N-terminus, making it 83 amino acids long. Both changes were engineered to stop it binding to the IGF binding proteins that normally sequester IGF-1 in blood. Free of those proteins, LR3 has roughly two to three times the biological potency of native IGF-1 and a half-life of around 20 to 30 hours rather than minutes. It was created as a cell culture reagent and remains one, with no clinical development program.
Native IGF-1 is the main downstream mediator of growth hormone. It drives muscle protein synthesis, satellite cell activation, and cartilage and bone growth. A recombinant form, mecasermin, is approved for children with severe IGF-1 deficiency, which gives some human context for what sustained IGF-1 signaling does. The LR3 version itself has essentially no human trial data. Animal and cell studies show it promotes proliferation and differentiation of muscle cells, and in livestock research it has been used to increase growth. Its reputation for building muscle in humans rests on mechanism plus anecdote.
The practical issue with LR3 is that it acts like a weak, long lasting insulin. IGF-1 shares enough structure with insulin to activate the insulin receptor at higher concentrations, and because LR3 is unbound and long acting, it can push blood glucose down for hours. Users report shakiness, sweating, and brain fog if they inject without eating. There is also a theoretical concern about promoting growth of existing tumors, since IGF-1 signaling is a known driver of cell proliferation, and the long duration of LR3 means that exposure is continuous.
How it works
IGF-1 LR3 binds the IGF-1 receptor on muscle, bone, cartilage, and other tissues, activating the PI3K-Akt-mTOR pathway that increases protein synthesis and blocks protein breakdown, and the MAPK pathway that drives satellite cell proliferation. Activated satellite cells fuse with existing muscle fibers to add nuclei, which is why LR3 is discussed in the context of hyperplasia as well as hypertrophy. Because the Arg3 substitution and N-terminal extension prevent capture by IGF binding proteins, nearly all of the injected dose is bioavailable immediately, and the extended half-life keeps receptor signaling on for most of a day. At higher concentrations it also cross-activates the insulin receptor, lowering blood glucose.
What the research shows
- in vitroLR3 IGF-1 shows markedly reduced affinity for IGF binding proteins and around 2-3 times the in vitro potency of native IGF-1.
- rodentLR3 IGF-1 increased protein synthesis and reduced protein breakdown in cultured muscle cells and in rodent muscle.
- rodentInfusion of LR3 IGF-1 in rats increased body weight gain and nitrogen retention compared to native IGF-1.
- clinical (approved drug)Recombinant native IGF-1 (mecasermin) increases growth in children with severe primary IGF-1 deficiency and causes hypoglycemia as its main side effect.
Evidence labels: rodent and in vitro mean no human data for that finding. Human pilot means small, often uncontrolled. Human RCT means randomized and controlled. Clinical means an approved drug with regulatory data.
Dose calculator
U-100 insulin syringeUnits are a volume on the syringe, not an amount of peptide. Concentration changes every time you change the water volume. The standalone calculator explains the math.
Dosing and protocol
start at 20 mcg; most people find little benefit above 50 mcg and side effects climb
4-6 weeks on, then 4 weeks off
Post-training with a carbohydrate containing meal to offset the glucose drop. Because of the long half-life, injection site matters little despite the local injection folklore.
| Vial | Suggested water | Concentration | Low dose draw |
|---|---|---|---|
| 1 mg | 2 mL | 500 mcg/mL | 4 units |
Lyophilized: fridge or freezer, away from light. Reconstituted: fridge 2-8C, use within 2-4 weeks. Handle gently; do not shake.
Cautions
- ▲Hypoglycemia is the main acute risk; inject with food and keep fast carbs on hand.
- ▲IGF-1 signaling promotes cell growth indiscriminately; do not use with any history of cancer.
- ▲Long acting and unbound, so effects cannot be reversed quickly once injected.
- ▲Can cause headaches, water retention, and joint discomfort, and prolonged use may enlarge organs in theory.
- ▲Grey market quality is highly variable; underdosed and degraded product is common because the peptide is fragile.
Commonly stacked with
Frequently asked
Does IGF-1 LR3 need to be injected into the muscle I trained?
No. It has a half-life of nearly a day and circulates throughout the body regardless of where you inject it. Site specific injection made sense for short acting MGF or IGF-1 DES, but with LR3 it is mostly superstition.
How do I avoid low blood sugar?
Eat a meal with carbohydrate around the time of injection, start with a low dose, and keep glucose tablets or juice nearby for the first few doses until you know how you respond. Never inject before bed on an empty stomach.
Is it stronger than HGH?
It is more direct. HGH works by raising IGF-1 through the liver, so LR3 skips that step and delivers the anabolic signal itself. It does not give you the fat loss effects of growth hormone and it does not build muscle on its own without training and food.
Why is the cycle so short?
Partly because IGF-1 receptors downregulate with constant stimulation, and partly because nobody has long term safety data on sustained unbound IGF-1. Four to six weeks is a community convention, not a studied number.
References
- Long R3 IGF-1: a potent IGF-1 analog with reduced binding protein affinity. Journal of Molecular Endocrinology, 1992
- Effects of IGF-1 and LR3 IGF-1 on protein metabolism and growth in rats. Journal of Endocrinology, 1995
- Mecasermin (Increlex) prescribing information. US Food and Drug Administration, 2005