Hyaluronic acid (HA) is arguably the most ubiquitous skincare ingredient of the past decade, appearing in everything from five-dollar sheet masks to five-hundred-dollar serums. Its simplicity, a glycosaminoglycan capable of holding 1,000 times its weight in water, belies a formulation complexity that dramatically affects clinical outcomes.
Molecular Weight Spectrum
Native hyaluronic acid exists in the body as a high-molecular-weight (HMW) polymer ranging from 1,000 to 1,400 kDa. However, commercial HA ingredients span from ultra-low molecular weight at 5 kDa to very high molecular weight exceeding 2,000 kDa, and each behaves completely differently on and within the skin.
High molecular weight HA above 1,000 kDa sits on the skin surface, forming a hydrating film that reduces TEWL and provides immediate plumping. It does not penetrate the stratum corneum, but its film-forming properties create an excellent occlusive-humectant layer.
Low molecular weight HA at 50 kDa and below penetrates beyond the stratum corneum into the epidermis and upper dermis. This is where controversy begins. While LMW-HA delivers hydration deeper in the skin, some research suggests it may trigger inflammatory cascades via CD44 and TLR2/4 receptor activation.
The Fragmentation Paradox
Endogenous HA fragmentation occurs naturally during wound healing and inflammation, where HMW-HA is cleaved into smaller fragments that signal tissue damage. Administering exogenous LMW-HA could potentially mimic this pro-inflammatory signal.
The current consensus leans toward multi-weight formulations that provide surface benefits of HMW-HA without high concentrations of pro-inflammatory LMW fragments. Sodium acetylated hyaluronate represents a promising alternative with enhanced penetration and a better safety profile.