Back ground: Aging is generally associated with alterations in physical activity, weight status and energy metabolism, which predisposes aged individuals to metabolic syndrome. In this manuscript, age effects on energy metabolic indicators of similar physical activity and weight status but of varying ages were investigated. Materials and Methods: Energy metabolic indicators, such as plasma adiponectin, leukocytic AMP-activated protein kinase, plasma malate dehydrogenase and lactate dehydrogenase along with common plasma metabolites, were measured in healthy young (AV = 7.1 years) and aged (AV = 14.1 years) riding horses of similar physical activity, diet and weight status. Malate dehydrogenase and lactate dehydrogenase ratio was also calculated as the indicator of energy metabolism. Results: Plasma adiponectin concentration and leukocytic AMP-activated protein kinase activity in aged horses were significantly lower than those in young horses (p&lt
0.05, Mann-Whitney U test). Although not significant, energy metabolism indicators, malate dehydrogenase, lactate dehydrogenase and their ratio were lower in aged group when compared to those of young group. Conclusion: The present results indicate the decline in energy metabolism with aging in healthy horses even without any visible changes in adiposity. Such changes reflect dysfunction of energy metabolism and predispose the aged individuals to the development of metabolic syndrome.