The effect of diet therapy on borderline type was discussed here. First the effect of calorie restriction on preventing the development of diabetes, using OLETF rat model was shown. Second, the influence of sucrose and ethanol on insulin secretion and insulin sensitivity was studied. Third, the relation between fat and diabetes was discussed. Especially the importance of difference in the quality of fat was emphasized. Fourth, the significance of the idea of glycemic index for preventing the absorption of carbohydrate was explained. Fifth, the application of dietary fiber for preventing diabetes was shown, using WBN/Kob rat model.