Fine-Gradient Low-Molecular-Weight Hyaluronic Acid Supplementation Modulates Gut Microbial Profiles and SCFA Output in a Starch-Containing In Vitro Fecal Fermentation Model

Hyaluronic acid (HA) is a glycosaminoglycan commonly administered orally, and its molecular weight (MW) influences its physicochemical behavior and potential interactions with the gut microbiota. However, MW-dependent effects on community assembly and fermentation-derived metabolites within the low-molecular-weight (LMW) range remain insufficiently resolved. In this study, five HA samples (6.9–35 kDa) were evaluated using an in vitro human fecal fermentation model. Microbial composition was profiled by 16S rRNA gene sequencing, and SCFAs were quantified by UPLC. Compared with the control under the same basal medium, HA supplementation was associated with shifts in community structure and higher alpha diversity. The 6.9 and 9.5 kDa groups were associated with significantly higher total SCFA concentrations, particularly butyrate, than the 13, 17, and 35 kDa groups under the same basal medium. Because soluble starch was present in the fermentation medium, these differences should be interpreted as modulation effects rather than direct evidence of HA-specific fermentation. 16S-based functional prediction suggested MW-dependent differences in predicted central carbohydrate metabolism potential, which were consistent with the observed SCFA patterns but should be interpreted as inferred functional potential rather than direct evidence of pathway activity. These findings indicate that HA molecular weight is associated with differential microbial and SCFA response patterns under the present in vitro conditions. Lower-MW HA within the tested range was associated with higher SCFA output, particularly butyrate, under a shared starch-containing basal medium, highlighting molecular weight as a potential formulation parameter in HA microbiota-centered applications.