Effect of increased adiposity on insulin sensitivity and adipokine concentrations in different equine breeds adapted to cereal-rich or fat-rich meals
Introduction
Laminitis associated with insulin dysregulation is an important cause of morbidity in domestic equine populations (Harris et al, 2006, Katz, Bailey, 2012). Insulin dysregulation is an umbrella term that refers to insulin resistance, fasting hyperinsulinaemia and/or exaggerated insulin responses to oral carbohydrates (Frank and Tadros, 2014). Together with obesity (generalised or regional adiposity), insulin dysregulation is considered to be a central component of equine metabolic syndrome (EMS), the clinical phenotype of many equids predisposed to pasture-associated laminitis (Frank et al., 2010). Pasture-associated laminitis also occurs in non-obese horses and ponies (Bailey et al, 2007, Geor, 2010); therefore, the link between obesity and insulin dysregulation requires further investigation. Other aspects of EMS that warrant additional study include alterations to adipokines (adipose-derived hormones such as leptin and adiponectin) and proinflammatory cytokines (Burns et al, 2010, Caltabilota et al, 2010, Wooldridge et al, 2012, Wray et al, 2013).
An apparent association between the induction of obesity and the development of hyperinsulinaemia and insulin resistance was demonstrated in a controlled study of Arabian geldings (Carter et al., 2009a). These changes occurred when horses were provided with multiple ‘sweet feed’ (cereal-rich) meals per day. The role of diet in the development of insulin dysregulation is an important consideration, because the adaptation of horses to ‘sweet feed’ meals can induce insulin resistance independent of obesity (Hoffman et al, 2003, Treiber et al, 2005). There is also evidence that weight gain can occur without reduced insulin sensitivity when horses and ponies are provided with relatively low-glycaemic rations (Quinn et al, 2008, Bamford et al, 2015a). Additionally, a once-daily oral glycaemic load appeared to improve insulin sensitivity in a group of horses and ponies (Bamford et al., 2016). Therefore, multiple daily episodes of hyperinsulinaemia may be necessary for insulin resistance to develop because of chronic over-stimulation of insulin receptors (Kronfeld et al, 2005, Suagee et al, 2011). The breed of animals studied is another consideration, as differences in the innate insulin sensitivity of different breeds can influence the insulinaemic response of an individual to oral non-structural carbohydrates (Bamford et al., 2014).
We aimed to examine the relative influence of a prolonged twice-daily dietary glycaemic load, compared with an isocaloric intake of vegetable fat, on insulin sensitivity and adipokine concentrations after the induction of obesity in horses and ponies. In addition, the metabolic responses of different equine breeds were compared by enrolling three groups with previously-documented differences in innate insulin sensitivity: Standardbred horses, mixed-breed ponies and Andalusian horses (Bamford et al., 2014). We hypothesised that animals gaining weight on a cereal-rich diet would demonstrate lower insulin sensitivity than animals that gained weight on a fat-rich diet.
Section snippets
Animals
Eleven Standardbred horses (9.5 ± 1.8 years, 457 ± 8 kg, body condition score [BCS] 5.0 ± 0.2), 11 mixed-breed ponies (9.0 ± 1.2 years, 305 ± 17 kg, BCS 5.3 ± 0.3) and 11 Andalusian-cross horses (8.3 ± 1.2 years, 475 ± 17 kg, BCS 5.5 ± 0.2) were studied. No animals demonstrated evidence of pituitary pars intermedia dysfunction when screened with a low-dose dexamethasone suppression test (McFarlane, 2011), nor did they have clinical or radiographic evidence of prior laminitis. They were kept in
Animals and diets
All animals remained clinically healthy throughout the study period and no episodes of laminitis were observed. The study diets were well tolerated; meal refusals were negligible and there were no signs of gastrointestinal disturbance. Hay consumption (%BW on a dry matter basis) was 2.21 ± 0.06%, 2.04 ± 0.11% and 2.39 ± 0.08% for the CHO, FAT and CON groups, respectively. Hay consumption was lower for the FAT group compared with the CON group (P = 0.027), but was not different between other
Discussion
In the present study, the induction of obesity was associated with reduced insulin sensitivity in horses and ponies that consumed a cereal-rich ration. In contrast, animals that consumed an isocaloric fat-rich (low-glycaemic) ration did not exhibit a change in insulin sensitivity despite reaching levels of adiposity that did not differ significantly from the CHO group. There was a significant effect of breed across all diet groups, with ponies and Andalusians demonstrating lower insulin
Conclusions
The glycaemic load of the diets used in this 20-week study appeared to be a more important influence on insulin sensitivity than the induction of obesity per se. Differences in the glucose and insulin dynamics of horse and pony breeds persisted regardless of the diet consumed, with ponies and Andalusian horses less insulin sensitive than Standardbred horses. Adiponectin may play a role in equine insulin dysregulation and warrants further investigation. These data suggest that the development of
Conflict of interest statement
P.A. Harris is both a collaborating author and an employee of WALTHAM who part-funded this work. None of the authors has any other financial or personal relationship with other people or organisations that could inappropriately influence or bias the content of the paper.
Acknowledgements
This study was supported by a linkage grant from the Australian Research Council (grant number: LP100200224) and the WALTHAM Centre for Pet Nutrition. The authors thank Mr Garry Anderson and Dr Graham Hepworth for statistical advice.
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