Antioxidative Lignin Materials Attenuate Type 2 Diabetes Mellitus (T2DM) Progression by Preserving Glutathione via Insulin Receptor Substrate 1/Phosphoinositide 3-Kinase/Protein Kinase B (IRS1/PI3K/AKT) Axis

Lignin, a natural and renewable aromatic biopolymer, has gained attention in various biomedical applications due to its diverse structure, excellent biocompatibility, and antioxidant activity. However, the effects of lignin with tailored molecular weight on treating type 2 diabetes mellitus (T2DM) remain largely unexplored. In this study, a series of heterogeneous natural phenolic kraft lignin (KL) with tailored molecular weights, derived through an anti-sugar strategy, were prepared by continuous fractionation. The lignin fractions were categorized as F1, F2, and F3, corresponding to high, medium, and low molecular weights, respectively. Their therapeutic effects on T2DM were evaluated using a fractionated lignin culture cell model and intravenous injection into the tail vein of diabetic rats. The results demonstrated that lignin's protective effects in attenuating T2DM progression were molecular weight-dependent. Specifically, F3 reduced fasting blood glucose, reversed insulin resistance, and improved insulin sensitivity by mitigating oxidative stress and inflammatory responses. Mechanistic investigations revealed that F3 positively regulated glucose and lipid metabolism, inhibited hepatic gluconeogenesis, and enhanced hepatic glycogen synthesis by activating the insulin receptor substrate 1/phosphoinositide 3-kinase/protein kinase B (IRS1/PI3K/AKT) signaling pathway. Results revealed that lignin exerts its therapeutic effects on T2DM in a molecular weight-dependent manner, with IRS1/PI3K/AKT signaling as a potential underlying mechanism. This highlights lignin with a defined molecular weight as a promising candidate for T2DM treatment.