IGF-1 LR3 Doesn't Build Muscle (Here's Why)
A short solo talk where William Day argues IGF-1 LR3 does not build muscle, explains the local versus systemic IGF-1 distinction, and ends with a coaching pitch.
Key moments
Tap a moment to play the video from there.
- 0:00Claim and disclaimerDay says IGF-1 LR3 is widely promoted for muscle growth and that this is wrong. He states the video is not medical advice and that he is not a doctor.
- 1:01How muscle growth worksHe explains satellite cell activation and says mechanical tension makes the muscle produce a local IGF-1 splice variant, mechano growth factor. He calls this the actual mechanism.
- 2:01Systemic IGF-1 and LR3Day separates liver-derived systemic IGF-1 from the local signal and describes LR3's modifications and its stated half-life of roughly 20 to 30 hours.
- 3:36Why the belief spreadsHe gives two reasons: promoters with financial incentives such as affiliate links, brands and coaching, and sincere users who credit the compound for results.
- 5:09Local versus injected IGF-1Day contrasts autocrine, paracrine and endocrine signalling. He says injected IGF-1 circulates everywhere and cannot target the muscles just trained.
- 6:09Evidence and possible usesHe says hypertrophy research in healthy, trained people is negligible. He grants possible roles in recovery, connective tissue integrity and collagen synthesis.
- 6:40What he recommends insteadDay ranks mechanical tension first, nutrition second and sleep and recovery third. He says IGF-1 LR3 is not part of that conversation.
- 7:11Confounding variablesHe suggests people who credit IGF-1 LR3 usually also train hard, eat and sleep well, and may be using testosterone.
- 8:12Coaching offer and sign-offDay describes his one-to-one coaching covering training, nutrition and protocols, links to apply in the description, and asks viewers to share their own IGF-1 LR3 experiences.
What the video says
William Day, who says he is not a doctor and just likes experimenting with peptides, argues that IGF-1 LR3 does not build muscle. He calls this settled rather than debatable. The video is a single-speaker talk of about nine minutes, and he frames it as entertainment based on his own research and experience.
His core argument is about where IGF-1 acts. Day says muscle growth depends on satellite cell activation, driven by a local splice variant of IGF-1 (mechano growth factor) that the loaded muscle makes in response to mechanical tension. He describes that signal as autocrine and paracrine, meaning the muscle signals to itself or to neighbouring cells. Systemic IGF-1, made in the liver in response to growth hormone and carried in the blood, is described as having little to no effect on hypertrophy.
Day describes IGF-1 LR3 as a synthetic analog with an arginine substitution at position three and a 13-amino acid extension. He says these changes sharply reduce binding to IGF-1 binding proteins and lengthen the half-life from roughly 20 minutes to around 20 to 30 hours. In his view, injecting it is an endocrine route that cannot target the muscles you just trained, so it spreads to bone, organs and connective tissue. He says research on it for hypertrophy in healthy, trained people is negligible at best.
He offers two reasons the belief persists: sellers with affiliate links, brands or coaching programmes have a financial incentive, and sincere users credit the compound for gains that came from hard training. He allows that it may support general anabolism, connective tissue integrity, recovery and collagen synthesis, but says it is not worth the cost for muscle. His alternative hierarchy is mechanical tension first, then nutrition, then sleep and recovery. The video closes with an invitation to apply for his one-to-one coaching.
On each compound
IGF-1 LR32:01
Day describes it as a synthetic IGF-1 analog with an arginine substitution at position three and a 13-amino acid extension. He says it binds IGF-1 binding proteins less and stays active about 20 to 30 hours, but argues it does not build muscle.
See how your DNA reads IGF-1 LR3IGF-11:01
Day says the local, muscle-produced form drives satellite cell proliferation and hypertrophy, while liver-derived systemic IGF-1 has little to no effect on muscle size. He says systemic IGF-1 supports collagen synthesis and connective tissue repair.
See how your DNA reads IGF-1“Systemic IGF-1 has little to no effect on muscle hypertrophy”
What this video doesn’t settle
- Day cites no studies by name, so his claims about local versus systemic IGF-1 and the lack of hypertrophy research are not backed with sources in the video.
- He says the evidence is clear, but the video does not present data on IGF-1 LR3 use in people, so effects on muscle remain his interpretation.
- Day sells one-to-one coaching and promotes it at the end, which is a commercial interest of his own alongside his criticism of others' incentives.
Questions this video answers
Does IGF-1 LR3 build muscle according to William Day?
Day says no. He argues muscle growth is driven by local IGF-1 produced in the muscle under mechanical tension, not by injected systemic IGF-1. He says research on IGF-1 LR3 for hypertrophy in healthy, trained people is negligible at best.
What is the difference between local and systemic IGF-1?
Day says local IGF-1, including mechano growth factor, is made in the loaded muscle and acts on the cell itself or its neighbours. Systemic IGF-1 is made in the liver in response to growth hormone and travels through the blood to many tissues.
What does IGF-1 LR3 do in the body, according to this video?
Day says its modifications reduce binding to IGF-1 binding proteins and extend its half-life from roughly 20 minutes to around 20 to 30 hours. He says it may support general anabolism, connective tissue integrity, recovery and tissue repair, but not muscle growth in trained muscles.
What does Day suggest doing instead of using IGF-1 LR3?
He recommends progressive overload with enough mechanical tension, correct protein and calories, and good sleep and recovery. He ranks them in that order and says peptides are not the bottleneck if those basics are off.
Video by William Day on YouTube, embedded as published. The summary is written by PeptidesDNA from the video; every claim in it is the speaker’s. Not affiliated with or endorsed by the creator. Not medical advice.
