Low circulating 25-hydroxyvitamin D (25[OH]D) concentrations have been observed in canine diseases, sparking interest in defining new 25(OH)D reference ranges in dogs and making it imperative to understand what factors might be associated with 25(OH)D measurement. Inflammation may confound 25(OH)D measurement, and is an enabling characteristic of cancer, a disease linked to low 25(OH)D concentrations in dogs. Therefore, this thesis explored relationships between vitamin D and inflammation in dogs with cancer with three studies:

1. Plasma 25(OH)D and inflammatory marker concentrations were measured in dogs with cancer before treatment. Dogs with B-cell lymphoma were the only cancer group with decreased plasma 25(OH)D concentrations compared with healthy dogs(p=0.03), and these dogs also had increased concentrations of multiple inflammatory markers, including C-reactive protein (CRP)(p<0.001).

2. Plasma 25(OH)D and inflammatory marker concentrations were measured during cancer treatment in dogs with lymphoma. Lower pre-treatment plasma 25(OH)D concentrations, and higher plasma CRP concentrations were observed in dogs with B-cell lymphoma compared to healthy dogs (p=<0.001 and 0.001). At week 25 of chemotherapy treatment, plasma 25(OH)D concentrations were increased(p=0.006), and plasma CRP concentrations were decreased(p=0.002) in dogs with B-cell lymphoma compared to their pre-treatment concentrations, and were no longer different from the healthy group.

3. An explanation for lower pre-treatment 25(OH)D concentrations observed in dogs with B-cell lymphoma, namely that inflammation causes upregulation of the CYP24A1 enzyme (responsible for conversion of 25(OH)D to 24,25(OH)2D), was investigated. Circulating concentrations of 25(OH)D and 24,25(OH)2D, and the 25(OH)D/24,25(OH)2D ratio (representative of CYP24A1 activity) were measured. Plasma 25(OH)D and 24,25(OH)2D concentrations were lower in dogs with B-cell lymphoma than in healthy dogs (p=0.02 and 0.04), but the 25(OH)D/24,25(OH)2D ratio was not different between groups, suggesting upregulation of the CYP24A1 enzyme is not responsible for decreased plasma 25(OH)D concentrations. Findings support a relationship between circulating 25(OH)D and inflammatory marker concentrations in dogs with B-cell lymphoma and warrant investigation into concentrations of other vitamin D metabolites in affected dogs. Findings provide clinicians with evidence-based information for owners of dogs with lymphoma who are interested in learning more about vitamin D in relation to their dog’s cancer diagnosis.