Letter to Editor

During our hospital's latest internal medicine rounds, we reviewed the paper: " Relationship between lean body mass and serum renal biomarkers in healthy dogs " by Hall et al 1 The study examines the effects of lean body mass on renal biomarkers in dogs, and aims to provide further validation for the use of symmetric dimethylarginine (SDMA) as a marker of glomerular filtration rate (GFR) in dogs. Most clinicians would agree that SDMA is an exciting new biomarker that has the potential to improve diagnosis and management of chronic kidney disease (CKD) patients. As a result, we enjoyed reviewing the paper, and it certainly stimulated discussion among colleagues. Since this work was a retrospective analysis of samples collected from a previous study, 2 we were also prompted to read the original paper concurrently. Of particular interest was the observation of a " time on diet effect " for SDMA concentrations, which decreased significantly in dogs of all treatment groups. The authors suggested that this effect might be because of improved GFR in the study dogs, but we wondered whether there might be an alternative explanation? Therefore, we wish to make some observations on both papers, which we hope will be of value to readers of Journal of Veterinary Internal Medicine, and would very much welcome the thoughts of the authors on these matters. For the sake of clarity, we will use the term " original study " for the PLOS ONE paper, 2 and " SDMA study " for the Journal of Veterinary Internal Medicine paper. 1 The original study was a 6-month feeding trial comparing three diets: a control diet and two " test diets " , comprising the control diet with added L-carnitine, added fatty acids, or both. All dogs were healthy, without evidence of CKD, and the control diet was a commercially available phosphorus-restricted " therapeutic diet " designed for feeding to dogs with CKD. However, before the start of the study, dogs were fed a " pretrial " food, which was a standard complete and balanced adult canine food. Nutrient contents of all diets are reported in the original study, and reveal marked differences between the pretrial food and the control/test foods for protein (18 versus 14% as fed [AF]), fat (eight versus 18% AF), and phosphorus (0.7 versus 0.3% AF) content. 2 Given these differences, could the " time on diet effect " be related …