The in vivo glycemic and insulin responses and in vitro starch digestibility were determined for six staple foods (corn, lima beans, white and yellow teparies, mesquite, and acorns) traditionally consumed by Pima Indians. Equivalent carbohydrate portions (25 g) of the foods were fed to eight healthy Caucasian volunteers. The calculated glycemic indices (GIs) (mean +/- SEM with glucose as the standard) were all low, ranging from 16 +/- 2 for acorns to 40 +/- 5 for corn. Insulin responses and in vitro starch digestibilities correlated with the GI. These results provide further support for the hypothesis that the slow digestion and absorption of starch in traditional foods was a factor that helped protect susceptible populations from developing diabetes.