Relationship of skeletal muscle inflammation with obesity and obesity-associated hyperinsulinemia in horses

Local (skeletal muscle and adipose) and systemic inflammation are implicated in the development of obesity-associated insulin resistance in humans. In horses, obesity is neither strongly nor consistently associated with systemic inflammation. The role of skeletal muscle inflammation in the development of insulin dysregulation (insulin resistance or hyperinsulinemia) remains to be determined. We hypothesized that skeletal muscle inflammation is related to obesity-associated hyperinsulinemia in horses. Thirty-five light-breed horses with body condition scores (BCSs) of 3/9 to 9/9 were studied, including 7 obese, normoinsulinemic (BCS >= 7, resting serum insulin, 30 mu IU/mL) and 6 obese, hyperinsulinemic (resting serum insulin >= 30 mu IU/mL) horses. Inflammatory biomarkers were evaluated in skeletal muscle biopsies and plasma. Relationships between markers of inflammation and BCS were evaluated. To assess the role of inflammation in obesity-associated hyperinsulinemia, markers of inflammation were compared among lean or ideal, normoinsulinemic (L-NI); obese, normoinsulinemic (O-NI); and obese, hyperinsulinemic (O-HI) horses. Skeletal muscle and plasma tumor necrosis factor alpha (TNF alpha) concentrations were negatively correlated with BCS. When comparing inflammatory markers among groups, skeletal muscle TNF alpha was lower in the O-HI group than in the O-NI or L-NI groups. In horses, neither skeletal muscle nor systemic inflammation appears to be positively related to obesity or obesity-associated hyperinsulinemia.