Insulin-like Growth Factor 1 (IGF-1) is naturally occurring, but its classification is often confused due to its powerful effects on growth and muscle tissue. IGF-1 is not a steroid (a class of lipid-based hormones). It belongs instead to the family of peptide hormones, which are protein-based signaling molecules. This distinction is based on the molecule’s fundamental chemical structure and its mechanism of action.
Chemical Identity: Peptide Hormone Versus Steroid Structure
Insulin-like Growth Factor 1 is a small protein, specifically a polypeptide hormone, composed of a single chain of 70 amino acids with a molecular weight of 7,649 Daltons. Its structure is highly similar to that of insulin, which accounts for the “insulin-like” part of its name. As a protein, IGF-1 is synthesized primarily in the liver, and this peptide composition dictates how it interacts with cells.
Steroids are lipid-soluble molecules derived from cholesterol, characterized by a distinct four-ring carbon structure. This structure allows steroids to easily pass through the cell membrane to bind to intracellular receptors. IGF-1, being a protein, cannot cross the cell membrane. Instead, it binds to the IGF-1 Receptor (IGF-1R) on the cell’s surface, activating a chain of events inside the cell. This mechanism is fundamentally different from how steroids operate.
IGF-1’s Role in the Growth Hormone Axis
The primary physiological function of IGF-1 is to act as the main mediator of Growth Hormone (GH) action, forming what is known as the GH-IGF-1 axis. GH is secreted by the pituitary gland and travels through the bloodstream to the liver, where it stimulates the production and release of IGF-1. Circulating IGF-1 then drives systemic growth and metabolism throughout the body.
This hormone is important during childhood and adolescence, stimulating cell division and proliferation in almost every tissue, playing a major part in skeletal and organ development. In adults, IGF-1 contributes to tissue repair, the maintenance of muscle mass, and bone density. IGF-1 also has metabolic roles, including the regulation of glucose and lipid metabolism, which is why it is named for its similarity to insulin.
The vast majority of IGF-1 in the bloodstream is bound to a family of six Insulin-like Growth Factor Binding Proteins (IGFBPs), most notably IGFBP-3. These binding proteins regulate the hormone’s availability and extend its half-life in circulation from minutes to several hours. This complex regulatory system ensures that IGF-1’s growth-promoting effects are tightly controlled in a healthy body.
Why IGF-1 is Used for Anabolic Effects
The confusion regarding IGF-1’s classification stems from the fact that it shares a functional characteristic with anabolic steroids: its ability to promote anabolism, or tissue growth. IGF-1 directly stimulates cell proliferation, leading to muscle hypertrophy (the enlargement of existing muscle cells) and is involved in the creation of new muscle cells (hyperplasia). It also decreases protein breakdown, supporting a net gain in tissue mass.
These anabolic properties are the reason IGF-1 is sometimes misused in performance enhancement, accelerating recovery from intense physical activity and aiding in muscle regeneration. This functional similarity leads the public to incorrectly group it with anabolic steroids. However, the mechanism of action is distinct: anabolic steroids primarily boost protein synthesis and influence testosterone levels, while IGF-1 works through its cell-surface receptor to stimulate growth factors.
The World Anti-Doping Agency (WADA) has included IGF-1 on its Prohibited List, classifying it under peptide hormones, growth factors, and related substances. The ban is due to its potential to enhance athletic performance and the associated health risks when administered without medical necessity. Medically, recombinant human IGF-1 treats severe primary IGF-1 deficiency, highlighting its growth-promoting function. Its association with performance enhancement, however, drives the public’s misunderstanding of its chemical identity.