More than 25 and 79 million individuals in the U.S. have diabetes and pre-diabetes, respectively. Many individuals are required to monitor blood glucose levels. Glycemic index (GI), a measurement of how a food affects the blood glucose level, is a method for selecting foods by diabetic populations. The effect of soy protein isolate (SPI) on the glycemic response of bread was determined, including the quality characteristics of bread made with various SPI percentages. Lower loaf volumes, firmer texture and open grain structure were observed in breads as the level of SPI increased. Estimated GI (eGI) significantly decreased (p<0.05) with increased SPI levels and also a significant correlation (p<0.05) was found between these indices and SPI concentrations. This indicates that fortification of SPI in the bread can reduce eGI and possibly provide a health benefit to the diabetic population. This experiment provided useful insights into SPI effect on eGI and bread quality characteristics.