Feeding silk protein hydrolysates to C57BL/KsJ-db/db mice improves blood glucose and lipid profiles

The hypothesis for the research is that hydrolyzed silk protein has an antidiabetic effect by reducing plasma glucose levels. To investigate this potential antidiabetic activity of hydrolyzed silk protein by protease-N (silk protein hydrolysate E5K6) in vivo, male C57BL/KsJ-db/db mice were separated into 3 groups: control group, db/db mice treated with vehicle (distilled water); SP-1 group, db/db mice treated with silk protein hydrolysate E5K6 at 0.1 g/kg body weight; and SP-2 group, db/db mice treated with silk protein hydrolysate E5K6 at 0.2 g/kg body weight. After 4 weeks of treatment, plasma glucose levels were lower in the SP-1 (177.3 ± 20.8 mg/dL) and SP-2 (151.8 ± 9.2 mg/dL) groups as compared to those in the control group (236.0 ± 31.2 mg/dL). Furthermore, blood glycated hemoglobin was significantly reduced in the SP-2 (6.6% ± 0.1%) compared to that in the control mice (7.7% ± 0.1%). The SP-2 group also had significant reductions in plasma concentrations of total cholesterol, low-density lipoprotein cholesterol, and the atherogenic index by 11%, 27%, and 26%, respectively, compared to the control group. Insulin levels on plasma concentrations were significantly increased in the silk protein hydrolysate E5K6 groups (SP-1, 4.2 ± 1.1 ng/mL; SP-2, 4.8 ± 0.4 ng/mL) compared to those in the control group (2.9 ± 0.9 ng/mL). The silk protein hydrolysate E5K6-treated db/db mice (SP-1, 62.8 ± 1.6 arbitrary units [AU]; SP-2, 63.0 ± 4.0 AU) displayed pancreatic islets with significantly enhanced (P < .05) insulin staining as compared to the intensity of staining of those from the control group (55.8 ± 2.5 AU). The results suggest that silk protein hydrolysate E5K6 has insulin-releasing activity through the induction of β-cell activity in the pancreatic islets.