To determine if diets are associated with different rates of interdigestive and postprandial enzyme secretion and how quickly enzyme secretion is modulated by nutrients, 27 healthy humans were randomly selected to follow one of five diets. The calorie proportions of carbohydrate, fat, and protein in each diet was assigned by a mixture design. After the subjects followed a diet for 2 weeks, they were intubated with an oroduodenal tube, and enzyme outputs were measured during the interdigestive period and after eating a meal identical to meals eaten during the previous 2 weeks. For the next 24 hours subjects either followed the same diet or a diet that contained the same amount of fat, but the percent of carbohydrate and protein was changed by 30%. Then interdigestive and postprandial pancreatic enzyme outputs were remeasured. After 2 weeks, diets containing the most carbohydrate (50%-80%) were associated with the lowest interdigestive and postprandial amylase and lipase (P less than 0.05) and trypsin outputs (P less than or equal to 0.05). In contrast, diets containing the most fat (40%) were associated with the highest interdigestive and postprandial outputs of amylase (P less than 0.05) and trypsin (P less than 0.05). Maintaining or altering diets for 24 hours did not change interdigestive pancreatic enzyme outputs, but postprandial amylase output was significantly increased (P less than 0.05) by increasing protein and decreasing carbohydrate content of the diets by 30% for 24 hours. We conclude that diets containing a high proportion of calories as carbohydrate for 2 weeks are associated with lower interdigestive and postprandial pancreatic secretion than diets that have a high fat content. In response to diets, changes in postprandial pancreatic enzyme secretion occur within 24 hours whereas changes in interdigestive secretion (no nutrients in the lumen) occur after 24 hours.