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Insulin sensitivity decreases during pregnancy, presumably an adaptation ensuring sufficient glucose supply to feto-placental tissues. Feeds high in non-structural carbohydrates are also linked to diminished insulin sensitivity in horses. Because the equine fetus is highly glucose reliant, maternal glucose and insulin dynamics during pregnancy may have implications for optimal fetal development in horses. Mismanagement of maternal nutrition during gestation could predispose the offspring to metabolic disorders (e.g. insulin resistance) later in life. In horses, insulin resistance is associated with increased risk for development of laminitis. These studies measured insulin sensitivity and glucose dynamics in pregnant and non-pregnant mares fed high sugar and starch (SS) or high fat and fiber (FF) feeds, as well as neonatal foals born from pregnant mares fed SS and FF feed. Insulin modified frequently sampled intravenous glucose tolerance tests (FSIGT) were applied to pregnant Thoroughbred mares (n = 22) at 28 ± 3 wks (Period 1) and 47 wks (Period 2) gestation, as well as non-pregnant mares (n=10) measured simultaneously. Following the first FSIGT mares were fed SS or FF feed for the remainder of the study. After 11 wks adaptation to feeding, a subset of mares were evaluated with hourly blood samples for 24 h to assess glycemic and insulinemic response to three times daily feeding while on pasture. Neonatal foal FSIGTs (n=20) were conducted at 5 ± 1 d of age. The minimal model of glucose and insulin dynamics was used to determine insulin sensitivity (SI), glucose effectiveness (Sg), acute insulin response to glucose (AIRg) and disposition index (DI). Pregnant mares during Period 1 exhibited lowered SI, Sg and elevated AIRg relative to non-pregnant mares. Pregnant mares demonstrated greater glycemic and insulinemic responses to feeding of both SS and FF meals than non-pregnant mares consuming the same feeds. Also, SS feed elicited greater glycemic and insulinemic areas under the curve following feeding than FF feed in pregnant mares. These data support that pregnancy in mares in associated with lowered SI by 28 wks gestation and that altered SI, Sg and AIRg are associated with different responses to consuming SS and FF feeds. Foals exhibited high basal glucose, basal insulin, SI and Sg relative to mature horses, indicating a large capacity for glucose uptake with or without insulin. Basal glucose concentrations were higher and basal insulin concentrations tended to be higher in SS than FF foals (P = 0.016 and P = 0.071, respectively). Glucose and insulin dynamics in late gestation mares and neonatal foals exemplify the adaptive nature of energy metabolism in horses. Furthermore, dietary energy composition affects glucose and insulin responses to feeding in late gestation mares, which in turn was associated with different basal blood concentrations of these variables in the resulting neonatal foals. / Master of Science