Changes in tear glucose and insulin concentrations following an oral glucose tolerance test.

CLINICAL RELEVANCE: Tear glucose and insulin are responsible for the health of the ocular surface; thus, it is important for clinicians to detect the tear glucose and insulin using point-of-care methods. AIM: To determine if changes in blood glucose and insulin levels following an oral glucose tolerance test are reflected in the tears and to test the association between gene expression and tear insulin and glucose. METHODS: Twenty healthy young adults were enrolled. Basal tears and peripheral blood samples were collected to assess glucose and insulin using a point-of-care glucometer and ELISA assays in fasted subjects, and 1.5 and 3 h after an oral glucose challenge. Conjunctival impression cytology was collected to determine gene expression of insulin receptor (INSR) and glucose transporters (GLUT1 and GLUT4). Changes were examined using non-parametric one-way ANOVA. Spearman tests were conducted to examine associations between variables. RESULTS: Glucose and insulin levels increased 1.5 h after oral glucose in both blood (P < 0.001) and tears (P < 0.049) and returned to near baseline values after 3 h. There was a positive correlation between glucose levels in the blood and tears (rho = 0.57, P < 0.001), but not between blood and tear insulin levels (P = 0.18). Glucose and insulin levels in tears were correlated (rho = 0.32, P = 0.048). Tear glucose concentration at 1.5 h after oral glucose was associated with INSR expression (rho = 0.49, P = 0.03), and there was a trend with GLUT1 (P = 0.06) but not GLUT4. CONCLUSION: Tear glucose reflected blood glucose levels but this correspondence was not observed for insulin. Further studies are required to determine the role of glucose and insulin on the ocular surface in both health and diabetes.