In some species of growing mammals glutamine is an essential amino acid that, if inadequate in the diet, is needed for normal growth and development. It is thus sometimes considered to be a conditionally essential amino acid in some species. A review of studies that have measured L-glutamine concentrations ([glutamine]) in horses demonstrates that plasma [glutamine] has routinely been reported to be much lower (~330 µmol/L) than in other mammals (> 600 µmol/L). Plasma [glutamine] represents the balance between intestinal transport into the blood after hepatic first pass, tissue synthesis and cellular extraction. The hypothesis is proposed that sustained low plasma [glutamine] represents a chronic state of sub-optimal glutamine intake and glutamine synthesis that does not meet the requirements for optimum health. While this may be without serious consequence in feral and sedentary horses, there is evidence that provision of supplemental dietary glutamine ameliorates a number of health consequences, particularly in horses with elevated metabolic demands. The present review provides evidence that glutamine is very important (and perhaps essential) for intestinal epithelial cells in mammals including horses, that horses with low plasma [glutamine] represents a sub-optimal state of well-being, and that horses supplemented with glutamine exhibit physiological and health benefits.