Fructooligosaccharides from Cynoglossum tubiflorus: Effect of the molecular size on their antidiabetic activity in high-fat diet and alloxan induced diabetic rats

[Display omitted] •Eight pure fructooligosaccarides (FOS) were isolated and completely characterized from Cynoglossum tubiflorus.•FOS and ethanol extract treatment for diabetic rats led to a reduction in blood glucose level, a decrease in α-amylase activity, TC and LDL-C levels, and an increase in HDL-C level.•This is the first report demonstrating the straight relationship between the size of an FOS and its biological effect. The use of acetylation followed by silica gel column purification allowed the isolation of eight fructooligosaccharides (FOS) from the ethanol extract of Cynoglossum tubiflorus roots. Each FOS was identified by analyzing its FT-IR, HRMS/MS and NMR data, including 1H, 13C and 2D NMR HH COSY, HMBC and NOESY. In diabetic rats treated with a series of FOS from Glc-(Fru)3 to Glc-(Fru)7, a significant inhibition of intestinal α-amylase was observed. This activity increases proportionally with the FOS molecular size. It was found that they delay the absorption of total cholesterol (TC), ldl-cholesterol (LDL-C) and increase HDL-cholesterol (HDL-C) in a molecular size-dependent manner. This inhibitory effect on the activity of the digestive enzyme causes a significant (p