A large glycosaminoglycan (sugar-based molecule) that is a major component of the extracellular matrix in skin, synovial fluid (joints), and vitreous humor (eyes). Hyaluronic acid’s defining property is its extraordinary water-binding capacity—a single gram can hold up to six liters of water—which provides tissue hydration, volume, and mechanical cushioning.

Hyaluronic acid production declines with age, contributing to skin thinning, joint stiffness, and reduced tissue hydration. It is widely used in dermal fillers (injected) and skincare products (topical). In the peptide context, hyaluronic acid appears alongside cosmetic peptides like GHK-Cu and Matrixyl in skin formulations, and its production is one of the downstream effects measured in peptide skin studies. It is not itself a peptide—it is a polysaccharide—but understanding its role is essential for evaluating the cosmetic peptide evidence base.