Comments on: ‘Exercise‐specific plasma proteomic signatures in racehorses: Candidates for training adaptation and peak load monitoring’

We read with interest the study by Grzedzicka et al.: 'Exercisespecific plasma proteomic signatures in racehorses: Candidates for training adaptation and peak load monitoring' published recently in Equine Veterinary Journal. 1 Molecular techniques give an opportunity for deeper insight into physiological and pathological processes; sometimes we are even lucky enough to find a new biomarker or a new mechanism.Grzedzicka and colleagues stated that on the basis of equine plasma proteomic profiles during training and racing, they aimed to identify novel plasma biomarkers of training adaptation and peak load.Their additional aim was to discover candidate biomarkers linked to training status, oxidative stress, inflammation and metabolic remodelling.While we respect Grzedzicka's lab's extensive work, we would like to address the fundamental concern regarding the composition of the study population.This combined horses of markedly different breeds, ages and sex into a single analytical group.In our opinion based on published literature data and our experiences with performance horses, this composition poses the severe limitation that can reduce interpretability, increase confounding and weaken confidence in attributing observed proteomic changes specifically to the training stage.Thus, it severely limits the ability to draw meaningful and clinically relevant conclusions regarding training and isolate them from other biologically relevant sources of variation.