There is emerging interest in linking vitamin D status to physiological health and disease states in the dog, as evidenced by the recent increase in publications in this area. This research has most likely been spurred by the studies exploring vitamin D and disease in humans. However, there are important differences in vitamin D intake and metabolism between humans and dogs that should be accounted for. The understanding of basic vitamin D metabolism and the relationship between vitamin D intake and vitamin D status in dogs remains even more limited than current knowledge in humans. This review will summarize current knowledge of vitamin D in the dog, including metabolism and dietary recommendations. Emphasis is placed on the limitations to current knowledge. Studies investigating links between vitamin D and disease will be discussed in light of this knowledge. Suggestions for future research, including the development of reference ranges to define blood vitamin D sufficiency, are provided.

BackgroundLow blood 25‐hydroxyvitamin D (25(OH)D) concentrations have been associated with cancer in dogs. Little research has examined what other factors may affect 25(OH)D concentrations.Objectives(1) To determine whether the presence of cancer (lymphoma, osteosarcoma, or mast cell tumor [MCT]) in dogs is associated with plasma 25(OH)D concentrations and (2) identify other factors related to plasma 25(OH)D concentrations in dogs.AnimalsDogs newly diagnosed with osteosarcoma (n = 21), lymphoma (n = 27), and MCT (n = 21) presented to a tertiary referral oncology center, and healthy, client‐owned dogs (n = 23).MethodsAn observational study design was used. Dietary vitamin D intake, sex, age, body condition score (BCS), muscle condition score (MCS), and plasma concentrations of 25(OH)D, 24,25‐dihydroxyvitamin D (24,25(OH)2D) (a marker of CYP24A1 activity), as well as ionized calcium (ICa), parathyroid hormone, and parathyroid hormone‐related protein concentrations were measured. An analysis of covariance was used to model plasma 25(OH)D concentrations.ResultsCancer type (P = 0.004), plasma 24,25(OH)2D concentrations (P < 0.001), and plasma ICa concentrations (P = 0.047) had significant effects on plasma 25(OH)D concentrations. Effects of age, sex, body weight, BCS, MCS, and plasma PTH concentrations were not identified. A significant interaction between ICa and cancer was found (P = 0.005). Plasma 25(OH)D concentrations increased as ICa concentrations increased in dogs with cancer, whereas plasma 25(OH)D concentrations decreased as ICa concentrations increased in healthy dogs.Conclusions and Clinical ImportanceResults support a relationship between cancer and altered vitamin D metabolism in dogs, mediated by plasma ICa concentrations. The CYP24A1 activity and plasma ICa should be measured in studies examining plasma 25(OH)D concentrations in dogs.

Decreased circulating 25‐hydroxyvitamin D (25[OH]D) and increased inflammatory marker concentrations have been reported separately in canine cancer. Correlations between the two exist in humans, but little work has examined links in dogs. This study aimed to determine plasma 25(OH)D and inflammatory marker concentrations in healthy dogs and dogs with cancer and to assess correlations in each group. Newly diagnosed dogs with B‐cell lymphoma (B‐cell, n = 25), T‐cell lymphoma (T‐cell, n = 9), osteosarcoma (OSA, n = 21), and mast cell tumour (MCT, n = 26) presenting to a tertiary oncology centre, and healthy dogs (n = 25), were enrolled. Plasma samples were analysed for 25(OH)D, C‐reactive protein (CRP), haptoglobin (HP), serum amyloid A (SAA), alpha‐1‐acid glycoprotein (AAG), and 13 chemokines and cytokines. Dogs with B‐cell had decreased plasma 25(OH)D (P = .03), and increased plasma CRP, AAG, HP, KC‐like and MCP‐1 concentrations (P < =.001, .011, <.001, .013 and .009, respectively) compared with healthy dogs. Plasma CRP, HP and SAA concentrations were increased in dogs with OSA compared with healthy dogs (P = .001, .010 and .027, respectively). No differences were noted in dogs with T‐cell and MCT. Negative correlations were observed between plasma 25(OH)D concentrations and: AAG concentrations in dogs with T‐cell (Rs = −0.817, P = .007); GM‐CSF concentrations (Rs = −0.569, P = .007) in dogs with OSA; and IL‐7 concentrations (Rs = −0.548, P = .010) in dogs with OSA. Decreased 25(OH)D concentrations and increased concentrations of multiple inflammatory markers were observed in B‐cell patients, supporting an association between 25(OH)D and inflammation. The cross‐sectional study design meant the timing of changes could not be determined. Prospective cohort studies are warranted.

OBJECTIVE To quantify vitamin D (VitD) concentrations in commercial dog foods and compare those concentrations with Association of American Feed Control Officials (AAFCO) recommendations and manufacturer-reported concentrations. DESIGN Cross-sectional study. SAMPLE 82 commercial dog foods. PROCEDURES Samples of commercially available dog foods were obtained from owners of healthy dogs in the Guelph, ON, Canada, area and owners of dogs that were patients at the Ontario Veterinary College Health Sciences Centre's Mona Campbell Centre for Animal Cancer. For each food, the VitD concentration was determined by high-performance liquid chromatography-tandem mass spectrometry, and adherence to AAFCO and National Research Council recommendations was assessed. Analyzed VitD concentrations were compared with manufacturer-reported VitD concentrations and between wet and dry foods, among AAFCO nutritional adequacy statements (nutrient profiles vs feeding trials and adult maintenance vs all life stages), between foods sold only by veterinarians and those sold over the counter, and between small and large manufacturers. RESULTS The analyzed VitD concentration was below both AAFCO and National Research Council recommendations for one sample and below the assay detection limit for another. Analyzed VitD concentrations did not differ significantly from manufacturer-reported VitD concentrations or between wet and dry foods, among foods with different AAFCO nutritional adequacy statements, between foods sold only by veterinarians and those sold over the counter, or between foods produced by small and large manufacturers. CONCLUSIONS AND CLINICAL RELEVANCE Results suggested that manufacturer-reported VitD concentrations were accurate and that dog owners can be confident that VitD intake is adequate for AAFCO-compliant commercial dog foods.