The purpose of this study is to evaluate the effect of mulberry leaf extract on blood glucose in subjects with impaired fasting glucose. After 2 weeks run-in period for wash-out, subjects will randomly assigned to two groups, i.e. control group (placebo) or mulberry leaf extract group. They will intake placebo or mulberry leaf extract for 4 weeks. Biomarkers will be analyzed in blood samples in 0 and 4 weeks.