What hyaluronic acid actually is
Hyaluronic acid (also called hyaluronan, or its sodium salt, sodium hyaluronate) is a glycosaminoglycan — a long chain of repeating sugar molecules. It is one of the most abundant components of the human dermis, where it fills the space between collagen and elastin fibres, retaining water and providing the visco-elastic "bounce" of healthy young skin. Roughly half of the body's HA is in the skin.
Your own HA production declines noticeably from the late 20s onwards, contributing to the loss of skin volume and elasticity that becomes visible in the 30s and 40s. Topical HA does not replace what the dermis produces — the molecules are too large to penetrate that deeply through intact skin — but it works at the stratum corneum and upper epidermis to deliver immediate hydration and a measurable plumping effect.
The naming convention in skincare is confusing. "Hyaluronic acid" and "sodium hyaluronate" are functionally equivalent for topical use (sodium hyaluronate is the more stable form usually preferred in formulation). "Hydrolyzed sodium hyaluronate" means the molecule has been chemically broken into smaller fragments. None of this matters as much as the molecular weight.
