We determined whether the steady-state levels of intestinal mucins are more sensitive than total proteins to dietary threonine intake. For 14 d, male Sprague-Dawley rats (158 +/- 1 g, n = 32) were fed isonitrogenous diets (12.5% protein) containing 30% (group 30), 60% (group 60), 100% (control group), or 150% (group 150) of the theoretical threonine requirement for growth. All groups were pair-fed to the mean intake of group 30. The mucin and mucosal protein fractional synthesis rates (FSR) did not differ from controls in group 60. By contrast, the mucin FSR was significantly lower in the duodenum, ileum, and colon of group 30 compared with group 100, whereas the corresponding mucosal protein FSR did not differ. Because mucin mRNA levels did not differ between these 2 groups, mucin production in group 30 likely was impaired at the translational level. Our results clearly indicate that restriction of dietary threonine significantly and specifically impairs intestinal mucin synthesis. In clinical situations associated with increased threonine utilization, threonine availability may limit intestinal mucin synthesis and consequently reduce gut barrier function.