Short communication: Limitations of glucose tolerance tests in the assessment of peripheral tissue insulin sensitivity during pregnancy and lactation in dairy heifers

The aim of the present study was to point at the limitations of glucose tolerance tests (GTT) to assess peripheral tissue insulin sensitivity in dairy heifers in different physiological states (pregnancy and lactation). Intravenous GTT were performed in 5 nonpregnant, nonlactating heifers, 5 heifers at the end of pregnancy (12-7 d before calving), and 5 lactating primiparous cows (11-14 d after calving). Glucose and insulin concentrations were determined and area under the curve (AUC) and clearance rate of glucose and insulin were calculated. Additionally, data were analyzed using the minimal model to derive the insulin sensitivity parameter (Si). Basal glucose and insulin concentrations were greater in the nonpregnant, nonlactating heifers. Clearance rate of glucose and Si were lowest, whereas the AUC for glucose was greatest in the nonpregnant, nonlactating heifers. Insulin concentrations during the GTT were greater for the nonpregnant, nonlactating heifers. Results from the GTT in pregnant heifers and lactating primiparous cows are biased by the fact that a large part of the glucose disappearance during an intravenous GTT occurs independently of insulin by the pregnant uterus or the lactating mammary gland. As such, greater AUC of glucose, lower clearance rate of glucose, or lower Si derived from GTT performed in nonpregnant, nonlactating dairy heifers in the present study might indicate decreased peripheral tissue insulin sensitivity of the glucose metabolism or decreased insulin-independent glucose disappearance. Based on the results from a GTT, it is impossible to discriminate between both metabolic pathways. It can be concluded that parameters derived from GTT are not suited to compare peripheral tissue insulin sensitivity of the glucose metabolism between dairy heifers in different physiological states due to the large variation in insulin secretion and the substantial difference in insulin-independent glucose disposal associated with these physiological states.