Insulin resistance (IR) is a key metabolic disturbance in horses that develop obesity-associated laminitis. In addition to obesity, age and diet affect tissue sensitivity to insulin but these factors have received limited investigation in horses. This study tested the hypothesis that glucose and insulin responses to a sweet feed (SF, high CHO) meal would be greater in aged horses, as compared to adult horses, as well as in horses adapted to a forage-only diet. Three diets, grass hay (G), grass hay plus sweet feed (starch and sugar-rich, SS), and grass hay plus a fat and fiber feed (FF), were fed to 17 healthy mares, 8 adult (5-12 yr) and 9 aged (>19 yr), for a 6-week adaptation period in a randomized design. Minimal model parameters during a frequently sampled intravenous glucose tolerance test (FSIGTT) and glucose and insulin responses to a standardized SF meal were determined after 30 and 42 days on each diet, respectively. Data were analyzed by repeated measures ANOVA. The acute insulin response to glucose (AIRg) was greater and tissue insulin sensitivity was lower in aged horses, regardless of diet. No differences in glucose responses to the SF meal were detected between age groups for any of the diets; however, both peak glucose concentration and AUC-G were lower after adaptation to the SS diet. In contrast, peak insulin concentration and AUC-I were greater in aged horses than adult horses on all diets but no differences were found between diets within age groups. As hypothesized, the insulin response, but not the glycemic response, to a sweet feed meal was greater in aged horses, regardless of background diet. Further, the glycemic response was greatest after adaptation to a forage-only diet in aged horses only.
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