The effect on energy metabolism of delayed absorption of starch by inhibition of alpha-amylase was examined by considering levels of plasma glucose and 3-hydroxybutyric acid (3-OHBA) in rats. Addition of alpha-amylase inhibitor (alpha AI) to a high starch diet delayed the plasma glucose response after feeding: peak plasma glucose levels in the control group occurred 15 min after feeding, whereas in the alpha AI group this peak did not occur until 30 min after. The total plasma glucose response was not different between the two groups. Plasma 3-OHBA levels 1 day after food restriction increased approximately five-fold in both groups. After 3 days of food restriction, the alpha AI group maintained the same level of plasma 3-OHBA as after 1 day of food restriction, while the control group showed significantly decreased levels of 3-OHBA. After 3 days of food restriction, plasma insulin levels were significantly decreased in the alpha AI group compared with the corresponding levels of the control group and with levels before the restriction. There was no significant difference in body weight between the two groups. These findings suggest that delayed hyperglycemia due to delayed absorption of starch following alpha AI loading may attenuate insulin secretion, leading to altered metabolism of 3-OHBA during the delayed response to energy deficit.