Effects of age and diet on glucose and insulin dynamics in the horse.

BACKGROUND: Age and diet may affect insulin sensitivity (SI) but these factors have received limited investigation in horses.

OBJECTIVES: To measure minimal model parameters during an insulin-modified frequently sampled intravenous glucose tolerance test (FSIGTT) after adaptation to a forage only diet (HAY) or forage supplemented with either starch/sugar-rich (SS) or oil/fibre-rich (FF) concentrate feeds; and to assess glucose and insulin responses to a standardised meal challenge (SMC, 4 g/kg BW of SS) after diet adaptation in adult and aged mares.

STUDY DESIGN: Latin square design with eight adult (5-12 years) and nine aged (>19 years) healthy mares.

METHODS: Diets were fed for 6 weeks, and the FSIGTT and SMC were performed after 31-32 and 41 days on each diet respectively. Data were analysed by a mixed ANOVA for repeated measures.

RESULTS: Acute insulin response to glucose (AIRg) was greater and SI was lower in aged horses, compared with adults, regardless of diet. Both AIRg and SI were greater in aged mares after adaptation to SS, as compared with HAY. Similar trends, although not statistically significant, were observed after adaptation to SS in adult mares. Peak insulin concentration and area under the insulin vs. time curve during the SMC were greater in aged than adult mares with all diets. Furthermore, area under the glucose vs. time curve was lower after adaptation to SS, when compared with other diets, in both groups.

MAIN LIMITATIONS: Transient weight loss occurred at the beginning of the study and only one sex was included. Incomplete ingestion of the SMC by four mares was another limitation.

CONCLUSIONS: Insulin responses to i.v. and enteral nonstructural carbohydrate challenge increase with age in healthy horses, regardless of diet fed.