The effect of changes in body condition on insulin sensitivity, leptin, and adiponectin in horses fed forage-only diets.

Abstract

An association between insensitivity to insulin and obesity has been reported in horses. Adipocytokines leptin and adiponectin have been identified as regulators of energy intake and an insulin-sensitizing hormone, respectively, where a positive correlation exists between adiposity and leptin, while the opposite is true for adiponectin. The nature of these relationships to obesity in horses is not fully understood, nor is there a recommended ideal level of adiposity. Therefore, this study was designed to determine how differences in body composition, achieved through differences in forage-only dietary energy intake, affect insulin sensitivity (IS), leptin and adiponectin in the horse. Seventeen mature, light-breed gelding horses, 8.0 ± 4.6 yr, were used in this two-phase study. Prior to day 0 horses were started on grass-alfalfa mix hay cubes and fed to achieve a moderate body condition score (BCS) of 5. Horses were randomly assigned to one of three treatment groups: gain to lose (GL), control (C), and lose to gain (LG). Three types of cubes were fed for weight gain or loss (High Energy and Low Energy cubes, respectively), or maintenance of condition (Bale-in-a-Bag cubes) during Phase 1 (P1). Diets were reversed following day 130 when approximate target changes in BCS were observed in GL (BCS = 7) and LG (BCS = 3) treatment groups, and horses were fed to return to a BCS of 5 in Phase 2 (P2). Body weight was assessed weekly. Rump fat depth (RFD) and abdominal fat depth (AFD), BCS, IS assessed via the euglycemic-hyperinsulinemic clamp (EHC), and serum for leptin and adiponectin were collected and assessed at days 0, 65, 130, 195, and 260, and analyzed using a switch-back, repeated measures ANOVA. Parameters for each treatment group (12 horses, GL n = 3, C n = 5, LG n = 4) were analyzed using PROC MIXED of SAS. Pearson correlations were also assessed at day 130. Mean BCS for treatment groups at day 0 was 4.8 ± 0.1 and 5.1 ± 0.1 at day 260. Significant changes in BCS were observed at day 130 compared to days 0 and 260 in the GL and LG groups, where BCS at day 130 was 6.5 ± 0.3 and 3.6 ± 0.4, respectively. At day 130 significantly smaller RFD was observed in the LG group versus day 0 (1.2 ± 0.8 cm), while mean RFD in the GL group tended to be larger (8.3 ± 0.8 cm), however no differences were seen in either group when day 130 and day 260 were compared. No significant changes in IS or leptin were observed as a result of body condition gain or loss, nor were there any correlations with measures of adiposity. However, leptin was positively correlated with IS (r = 0.83, P = 0.01). A significant time effect (P < 0.01) on adiponectin was observed in control horses, such that adiponectin was higher in summer and lower in winter/early spring. Our results indicate the differences in adiposity achieved were not enough to elicit the alterations in IS seen in other studies. However, it does support the notion that moderate changes in adiposity in horses fed forage-only diets should not increase their risk of developing problems associated with metabolic disturbance. Additionally, adiponectin dynamics are different from previous findings, indicating potential seasonal influences. Further research is needed to identify mechanisms behind the regulation of insulin sensitivity, leptin, and adiponectin and their application to individual animal populations.