Dogs produce negligible VitD through cutaneous exposure to UV light, which makes VitD an essential dietary nutrient for dogs.1 Dietary VitD exists as ergocalciferol (VitD2), which is found in plants, and cholecalciferol (VitD3), which is derived from animals.2 Supplemental VitD can also be added to diets in the form of a vitamin premix, in addition to any VitD present in raw ingredients.2 Vitamin D has a crucial role in the maintenance of calcium and homeostasis,2 and there is growing evidence of the importance of VitD in extraskeletal roles such as immune function3 and muscle health.4 There is also an increasing volume of research on the association between an abnormally decreased blood VitD concentration and various diseases of dogs including endotoxemia,5 inflammatory bowel disease,6 congestive heart failure,7 chronic kidney disease,8 and cancer.9,10,a Insufficient dietary intake of VitD can result in hypovitaminosis D and cause rickets, osteomalacia, or osteoporosis. Excessive intake of VitD can lead to high serum 25-hydroxyvitamin D concentrations and hypercalcemia with concurrent calcification of soft tissues and possibly death.2 Adequate concentrations of VitD in commercial foods are critical because most dogs are fed commercial dog foods as their primary or sole source of nutrition.11–13
Vitamin D requirements are typically expressed as international units (U) of VitD3; 1 U is the biological equivalent of 0.025 μg of cholecalciferol, which is the predominant form of VitD used in the manufacturing of pet foods. In North America, commercial dog foods are formulated to meet AAFCO recommendations, which require that the VitD3 concentration range between 143 and 1,429 U/1,000 kcal ME for adult maintenance dog foods as well as for foods formulated for gestating and lactating bitches and growing puppies.14 That range was adapted from recommendations provided by the NRC,15 which are based on results of research on the relationship between dietary VitD intake and subsequent bone health. Clinical signs of VitD3 toxicosis can result after ingestion of rodenticides that contain cholecalciferol.16 Toxicosis can also develop following ingestion of dog foods that contain excessive VitD3, and some dog foods have been recalled because of improper VitD3 content. One such recall occurred in 2010 after 36 dogs fed specific commercial dry foods developed signs of hypervitaminosis D associated with excessive dietary intake of VitD3.17
The purpose of the study reported here was to quantify the VitD3 concentrations in commercially available dog foods and compare the measured (analyzed) concentrations with AAFCO and NRC recommendations and the VitD3 concentrations reported by the manufacturers (manufacturer-reported VitD3 concentrations). Additionally, the effects of variables such as diet type (wet vs dry), AAFCO nutritional adequacy statements, purchase location (veterinary exclusive vs commercial), and size of the pet food company (ie, manufacturer) on analyzed VitD3 concentrations were also assessed. We hypothesized that analyzed VitD3 concentrations would be lower than the corresponding manufacturer-reported concentrations owing to VitD3 degradation during processing or storage and that analyzed VitD3 concentrations of AAFCO-compliant foods would fall within AAFCO and NRC recommendations and not be significantly affected by any of the other variables assessed.
Materials and Methods
Dog food samples
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, Guelph, ON, Canada, as part of another study.a Each owner provided approximately 1 cup (70 to 220 g depending on the diet) of dog food in a lidded, opaque, plastic container. Foods with a moisture content > 70% were categorized as wet, and those with a moisture content ≤ 10% were categorized as dry. For each food, AAFCO nutritional adequacy statements were recorded on the basis of information obtained from packaging, company websites, or contact with the manufacturer. Then, each food was categorized on the basis of whether it was formulated to meet AAFCO nutrient profiles for adult maintenance or all life stages or had undergone AAFCO feeding trials for adult maintenance or all life stages. For foods that did not have an available AAFCO adequacy statement, the product website was checked and the manufacturer was contacted to determine whether the product was formulated in accordance with NRC recommendations. Manufacturers' websites were also consulted to determine whether each food was sold to veterinarians only (veterinary exclusive) or was available for purchase over the counter in commercial outlets, such as pet stores, supermarkets, and big-box retailers. An online revenue databaseb was consulted to determine the 2013 revenue for each manufacturer of the foods evaluated. Manufacturers with revenue > $1 billion (US dollars) were considered large companies, and manufacturers with a revenue ≤ $1 billion were considered small companies.
Additional food information
The manufacturer of each food evaluated was contacted to obtain the food's VitD3 concentration, energy density, and moisture content. Each manufacturer was asked to describe whether the reported VitD3 concentration represented the analyzed concentration of the final product or was based on preprocessing VitD3 supplementation. Manufacturers that stated the reported VitD3 concentration was based on analysis of the final product were asked to provide the method of VitD3 analysis.
Storage and VitD3 analysis
After food samples were obtained by the investigators, they were stored at −80°C until analysis. Prior to analysis, samples were homogenized then freeze-dried and sent to the Royal Canin Americas Satellite Laboratory in Guelph, ON, Canada, for determination of VitD3 concentration by means of high-performance liquid chromatography–tandem mass spectrometry as described.18 The method had an LOQ of 100 U of VitD3/kg of dry matter and an uncertainty of ± 30%. For statistical analyses, the VitD3 concentration for each food was expressed as U/1,000 kcal ME on the basis of the caloric content provided by the manufacturer.
Statistical analysis
Data were assessed for normality by use of Shapiro-Wilk tests, the results of which indicated that the data were not normally distributed. Therefore, a logarithmic transformation was applied to the data to normalize the distribution prior to analysis. Following analysis, data were back-transformed, and results were reported as the geometric mean and 95% CI. Separate independent-sample t tests were performed to compare analyzed VitD3 concentrations between wet and dry foods, between foods on the basis of purchase location (veterinary exclusive and over-the-counter retail), and between food produced by large and small manufacturers. Manufacturers for which 2013 revenue information could not be obtained were excluded from the latter comparison. A 2-way ANOVA was used to compare analyzed VitD3 concentrations between foods with various AAFCO nutritional adequacy statements (ie, life stage [adult maintenance vs all life stages] and formulation type [foods formulated to meet AAFCO nutrient guidelines vs those that underwent AAFCO feeding trials]) and the interaction between formulation type and life stage; foods without AAFCO adequacy statements were excluded from that comparison. A paired t test was used to compare analyzed and manufacturer-reported VitD3 concentrations, and the strength of the relationship between the analyzed and manufacturer-reported VitD3 concentration was assessed with the Pearson product-moment correlation coefficient (r). Foods for which a manufacturer-reported VitD3 concentration could not be obtained were excluded from those analyses. All analyses were performed with commercial software,c and values of P ≤ 0.05 were considered significant.
Results
Samples of 82 commercial dog foods were analyzed. One dry-food sample was formulated in accordance with NRC rather than AAFCO recommendations, and that sample was excluded from all statistical analyses because its analyzed VitD3 concentration was below the assay LOQ. The VitD3 concentrations for the 81 AAFCO-compliant foods evaluated were summarized (Table 1).
Analyzed VitD3 concentration (U/1,000 kcal ME) for 81 samples of commercial dog foods summarized on the basis of various categorization schemes.
| Variable | Category | No. of samples | Geometric mean (95% CI) |
|---|---|---|---|
| Moisture* | Dry | 72 | 421 (373–475) |
| Wet | 9 | 404 (270–606) | |
| AAFCO nutritional adequacy statement† | Nutrient profile–adult maintenance | 26 | 369 (307–445) |
| Nutrient profile–all life stages | 30 | 432 (362–515) | |
| Feeding trial–adult maintenance | 18 | 417 (318–546) | |
| Feeding trial–all life stages | 7 | 596 (377–942) | |
| Company size‡ | Large | 40 | 425 (358–504) |
| Small | 16 | 405 (316–519) | |
| Purchase location | Veterinary exclusive | 25 | 411 (330–512) |
| Over the counter | 56 | 423 (369–484) |
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, Guelph, ON, Canada, as part of another study.a Each owner provided approximately 1 cup (70 to 240 g depending on the diet) of dog food in a lidded, opaque, plastic container, and the VitD3 concentration in each food was determined by high-performance liquid chromatography–tandem mass spectrometry.
Foods with a moisture content > 70% were categorized as wet, and those with a moisture content ≤ 10% were categorized as dry.
Each food was categorized on the basis of whether it was formulated to meet AAFCO nutrient profiles for adult maintenance or all life stages or had undergone AAFCO feeding trials for adult maintenance or all life stages.
Determined on the basis of 2013 revenue; manufacturers with revenue > $1 billion (US dollars) were considered large companies, and manufacturers with revenue ≤ $1 billion were considered small companies. Information to determine company size was unavailable for the manufacturers of 25 food samples.
The geometric mean analyzed VitD3 concentration for all 81 samples was 428 U/1,000 kcal ME (95% CI, 379 to 482 U/1,000 kcal ME). One dry food with an AAFCO nutritional adequacy statement for all life stages as determined on the basis of feeding trial results had a VitD3 concentration of 131 U/1,000 kcal ME, which was below both the AAFCO minimum allowance (143 U/1,000 kcal ME) and NRC recommended allowance (136 U/1,000 kcal ME). The VitD3 concentrations for 8 samples were greater than the NRC SUL but were within AAFCO recommendations. Three food samples were labeled as adult maintenance diets (1 was formulated to meet AAFCO nutrient profiles and 2 underwent AAFCO feeding trials), and 5 food samples were labeled for all life stages. The VitD3 concentration was not significantly (P = 0.46) associated with the interaction between formulation type and life stage. The geometric mean VitD3 concentration did not differ significantly between foods formulated to meet AAFCO nutrient profiles and those that underwent AAFCO feeding trials (P = 0.14), foods formulated for adult maintenance and those formulated for all life stages (P = 0.08), dry and wet foods (P = 0.84), foods produced by small and large manufacturers (P = 0.76), or foods purchased exclusively from veterinarians and those purchased over the counter (P = 0.83).
The manufacturer-reported VitD3 concentration was unavailable for 9 samples. The geometric mean manufacturer-reported VitD3 concentration for the remaining 72 food samples (395 U/1,000 kcal ME; 95% CI, 352 to 444 U/1,000 kcal ME) did not differ significantly (P = 0.20) from the geometric mean analyzed VitD3 concentration for those samples (428 U/1,000 kcal ME; 95% CI, 374 to 470 U/1,000 kcal ME). In fact, there was a significant (P < 0.01) positive correlation (r = 0.47) between the manufacturer-reported and analyzed VitD3 concentrations. The manufacturers of 42 of the 72 food samples reported that the VitD3 concentration was analyzed in the final product. Twenty-seven manufacturers were able to provide at least some information regarding the method used to determine the VitD3 concentration in the final product, whereas 15 manufacturers were unable or unwilling to provide that information. The VitD3 concentration in the finished product was predicted on the basis of vitamin supplementation before processing for 17 foods. The manufacturers of 13 of the evaluated foods were unable to provide sufficient information on how the VitD3 concentration was determined. For 1 food formulated to meet AAFCO nutrient recommendations for all life stages, the manufacturer-reported VitD3 concentration was 138 U/1,000 kcal ME, which was less than the AAFCO minimum allowance but within NRC recommendations. Five food samples had manufacturer-reported VitD3 concentrations that were greater than the NRC SUL but within AAFCO recommendations. Of those 5 food samples, 3 were labeled for all life stages (1 formulated to meet AAFCO nutrient profiles and 2 that underwent AAFCO feeding trials), whereas the remaining 2 had undergone AAFCO feeding trials for adult maintenance.
Discussion
Vitamin D toxicosis and deficiency are diagnosed primarily on the basis of bone health measures. Skeletal abnormalities characteristic of VitD toxicosis have been associated with accidental oversupplementation of VitD319 and led to the recall of pet foods.17,20 To our knowledge, there have been no published reports of VitD deficiency in dogs fed commercial dog foods as their primary or sole source of nutrition. The 2006 NRC Nutrient Requirements of Dogs and Cats21 states that adult dogs are fairly resistant to dietary VitD deficiency, but it fails to cite any scientific evidence to support that statement. For dogs, the NRC recommends a daily intake of at least 136 U of VitD3/1,000 kcal ME during all life stages and suggests an SUL of 800 U of VitD3/1,000 kcal ME/d.22 However, the SUL for VitD3 recommended by the NRC for adult maintenance, as well as gestation, lactation, and growth, is extrapolated from a single study21 that is not referenced in the NRC chapter on vitamin requirements for dogs. Current recommendations for VitD3 requirements are based purely on mineral metabolism and bone health, and it is unknown whether the recommendations are sufficient to support the extraskeletal functions of VitD3. As more information becomes available regarding the relationship between VitD metabolism and dietary requirements in dogs, the NRC and AAFCO recommendations may need to be revisited.
Results of the present study indicated that 81 of the 82 dry and canned (wet) dog food samples evaluated contained adequate concentrations of VitD3 as outlined in AAFCO recommendations for nutritional adequacy in dogs, regardless of whether the foods were formulated to meet AAFCO nutrient profiles or had undergone AAFCO feeding trials. None of the food samples analyzed in the present study contained VitD3 concentrations that exceeded AAFCO recommendations. The VitD3 concentration for 1 sample was less than the AAFCO recommended minimum allowance, but that food passed an AAFCO feeding trial. Foods that pass AAFCO feeding trials do not have to meet AAFCO nutrient profiles. The VitD3 concentration for a food sample that was formulated in accordance with NRC rather than AAFCO recommendations was less than the LOQ for the VitD3 assay (100 U VitD3/kg of dry matter). Collectively, these results suggested that, given current dietary recommendations for VitD3, the risk for the development of VitD3 toxicosis or deficiency in dogs fed complete and balanced commercial dog foods is likely low, albeit not zero. Indeed, low concentrations of dietary VitD3 may not have a significant effect on growth and bone development in puppies. In 1 study,23 large-breed puppies fed diets without supplemental VitD3 from weaning until 2 years old did not develop skeletal signs of VitD deficiency. Thus, current recommendations for dietary VitD3 intake may be greater than required for nutritional adequacy as determined by bone health. However, to our knowledge, potential extraskeletal signs associated with VitD deficiency have not been assessed.
Although processing methods differed between wet and dry foods, type of food was not significantly associated with analyzed VitD3 concentration in the present study. It is common practice for pet food manufacturers to account for predicted losses of nutrient concentrations during the manufacturing process. However, it could not be determined whether the wet and dry food samples evaluated in the present study had similar rates of VitD3 degradation or the manufacturers added different amounts of supplemental VitD3 to those foods prior to processing to compensate for differing rates of degradation. Results of a 1996 study24 of commercial cat foods indicate that canned (wet) foods have significantly greater VitD3 concentrations than dry foods, and the VitD3 concentrations in canned foods often exceed AAFCO recommendations. The investigator of that study24 attributed those findings to the fact that canned foods tended to contain more VitD-rich raw ingredients than dry foods. In the present study, we did not evaluate the specific ingredients contained in the evaluated foods or how those ingredients may have affected the VitD3 concentrations of those foods. Given that the VitD3 concentration was significantly greater in canned foods than dry foods evaluated in the 1996 cat food study24 but did not differ significantly between the wet and dry dog foods evaluated in the present study, it remains unclear whether high VitD3 concentrations in canned diets are unique to cat foods or related to food ingredients, or whether, since 1996, pet food manufacturers have decreased VitD3 contents in commercial pet foods to minimize the risk of VitD toxicosis.
The AAFCO has 2 sets of nutrient profiles for dogs, one for maintenance of adult dogs and the other for growing, gestating, and lactating dogs. For the dog foods evaluated in the present study, the analyzed VitD3 concentration was not significantly associated with life stage (adult maintenance vs all life stages), which was likely a reflection of the fact that the AAFCO recommendations for VitD3 intake are the same for both nutrient profiles.14 Additionally, foods that undergo AAFCO feeding trials for assessment of nutritional adequacy do not need to meet AAFCO nutrient profile recommendations. In fact, in the present study, the mean analyzed VitD3 concentration did not differ between foods formulated to meet AAFCO nutrient profiles and those that underwent AAFCO feeding trials.
Results of the present study indicated that the VitD3 concentration of dog food was not significantly associated with the size of the company that manufactured the food. However, 2013 revenue information (data used to categorize companies as large or small) was unavailable for the manufacturers of 25 of the 81 (31%) dog food samples evaluated. In contrast, in a study25 of commercial canned cat foods, the thiamine concentration in foods manufactured by small companies was significantly lower than that in foods manufactured by large companies. The investigators of that study25 postulated that that finding was a reflection of differences in the quality control practices between small and large companies. Results of the present study suggested that quality control practices for VitD3 content may be comparable between large and small dog food manufacturers.
Additionally, results of the present study indicated that the VitD3 concentration did not differ significantly between veterinary-exclusive foods and foods purchased over the counter. That finding suggested that pet owners can have confidence in the VitD3 concentrations of their pets' diets regardless of whether the food is a diet that can be purchased only from a veterinarian or a food purchased at a commercial retail location.
Given that all dog food samples evaluated in the present study were obtained from pet owners and that VitD3 degrades in the presence of air and light, it was expected that the analyzed VitD3 concentration would be significantly lower than the corresponding manufacturer-reported VitD3 concentration. However, the analyzed VitD3 concentration did not differ significantly from the manufacturer-reported VitD3 concentration for the foods evaluated in this study. Vitamin D3 can degrade by up to 16% each month after extrusion in rations formulated for ruminants.26 Unfortunately, we were unable to find comparable data regarding VitD3 degradation for pet foods in the scientific literature. The manufacturer-reported VitD3 concentration was based on analysis of the final product for some foods evaluated in the present study and on the amount of VitD3 supplementation before processing for others, whereas the manufacturers of some foods did not provide that information. Pet food is a complex food matrix, and accurate measurement of VitD3 is difficult.27 The difficulty and cost associated with measurement of VitD3 may contribute to why the manufacturer-reported VitD3 concentration generally represents a theoretical estimate that takes into account VitD3 processing losses and degradation over the course of the product's shelf life, rather than the concentration immediately after processing. The assumptions used for that calculation could result in either overestimation or underestimation of the VitD3 concentration and may have contributed to the weak, albeit significant, positive correlation between the analyzed and manufacturer-reported VitD3 concentrations.
The sampling methods used for the present study were somewhat limited by its cross-sectional nature. Because only 1 sample was assessed for each food evaluated, variation between and within batches could have caused the analyzed VitD3 concentration to deviate from the manufacturer-reported VitD3 concentration. The study results may have also been affected by the small number of samples available for some of the comparisons. Additionally, there was potential for VitD3 degradation in the samples evaluated because they were obtained from dog owners instead of newly manufactured products. Owners could not always provide the expiration date for the food sampled, and some samples may have been older than others. The VitD3 in the older samples may have degraded to a greater extent than that in more recently manufactured samples. It is likely the environmental conditions in which the foods were stored prior to sampling varied among the samples evaluated, which could have affected the analyzed VitD3 concentration. All dog food samples evaluated in the present study were purchased in a specific region of Canada. Although many of the sampled foods are currently available throughout both Canada and the United States, the availability of specific foods within the study region might have affected our results.
The VitD3 concentrations in commercial veterinary-exclusive and over-the-counter dog foods adhere to AAFCO and NRC recommendations for either maintenance of adult dogs or dogs of all life stages. On the basis of currently available knowledge of the dietary VitD3 requirement for dogs, it is likely that the VitD3 intake is adequate for dogs fed AAFCO-compliant dry and wet foods as their primary food source. Nevertheless, current AAFCO and NRC recommendations for dietary VitD3 intake for dogs are based on measures of bone health and do not account for the extraskeletal functions of VitD. None of the foods evaluated in this study had an analyzed VitD3 concentration that exceeded current AAFCO recommendations; thus, the risk of VitD toxicosis for dogs fed commercial dog foods is likely low. Results of the present study suggested that dog owners can be confident in the VitD3 content of commercial dog foods that comply with AAFCO recommendations regardless of where they are sold (ie, veterinary exclusive or over the counter).
Acknowledgments
Supported, in part, by the Ontario Veterinary College Pet Trust Fund, an American Academy of Veterinary Nutrition and WALTHAM (AAVN/WALTHAM) research grant, and Royal Canin SAS.
Drs. Bayle and van Hoek are paid employees of Royal Canin Inc. Dr. Verbrugghe is the Royal Canin Veterinary Diets Endowed Chair in Canine and Feline Clinical Nutrition at the Ontario Veterinary College, University of Guelph, Guelph, ON, Canada.
Presented as an oral presentation at the 18th Annual European Society of Veterinary and Comparative Nutrition Congress, Utrecht, the Netherlands, September 2014, and as a poster at the 15th Annual American Academy of Veterinary Nutrition Clinical Nutrition and Research Symposium, Indianapolis, June 2015.
The authors thank Chantel Andre and Thomas Lamek for assistance conducting the VitD3 analyses of all food samples, and Gabrielle Monteith for assistance with statistical analyses.
ABBREVIATIONS
| AAFCO | Association of American Feed Control Officials |
| CI | Confidence interval |
| LOQ | Limit of quantification |
| ME | Metabolizable energy |
| NRC | National Research Council |
| SUL | Safe upper limit |
| VitD | Vitamin D |
Footnotes
Weidner NL, Woods, JP, Conlon P, et al. Dietary vitamin D intake and vitamin D status in canine cancer patients (abstr). J Anim Physiol Anim Nutr 2015;99:815–816.
Top Pet Food Companies [database online]. Rockford, Ill: Watt Global Media, 2015. Available at: www.petfoodindustry.com/TopPetfoodCompanies.html. Accessed Sep 1, 2015.
R, version 3.2.3, R Foundation for Statistical Computing, Vienna, Austria.
References
1. How KL, Hazewinkel HA, Mol JA. Dietary vitamin D dependence of cat and dog due to inadequate cutaneous synthesis of vitamin D. Gen Comp Endocrinol 1994;96:12–18.
2. Wedekind KJ, Yu S, Kats L, et al. Micronutrients: minerals and vitamins. In: Hand MS, Thatcher CD, Remillard RL, et al, eds. Small animal clinical nutrition. 5th ed. Topeka, Kan: Mark Morris Institute, 2010;128–129.
3. Yamshchikov AV, Desai NS, Blumberg HM, et al. Vitamin D for treatment and prevention of infectious diseases: a systematic review of randomized controlled trials. Endocr Pract 2009;15:438–449.
4. Bischoff-Ferrari HA. Relevance of vitamin D in muscle health. Rev Endocr Metab Disord 2012;13:71–77.
5. Holowaychuk MK, Birkenheuer AJ, Li J, et al. Hypocalcemia and hypovitaminosis D in dogs with induced endotoxemia. J Vet Intern Med 2012;26:244–251.
6. Gow AG, Else R, Evans H, et al. Hypovitaminosis D in dogs with inflammatory bowel disease and hypoalbuminaemia. J Small Anim Pract 2011;52:411–418.
7. Kraus MS, Rassnick KM, Wakshlag JJ, et al. Relation of vitamin D status to congestive heart failure and cardiovascular events in dogs. J Vet Intern Med 2014;28:109–115.
8. Galler A, Tran JL, Krammer-Lukas S, et al. Blood vitamin levels in dogs with chronic kidney disease. Vet J 2012;192:226–231.
9. Wakshlag JJ, Rassnick KM, Malone EK, et al. Cross-sectional study to investigate the association between vitamin D status and cutaneous mast cell tumours in Labrador Retrievers. Br J Nutr 2011;106(suppl 1):S60–S63.
10. Selting KA, Sharp CR, Ringold R, et al. Serum 25-hydroxyvitamin D concentrations in dogs—correlation with health and cancer risk. Vet Comp Oncol 2016;14:295–305.
11. Laflamme DP, Abood SK, Fascetti AJ, et al. Pet feeding practices of dog and cat owners in the United States and Australia. J Am Vet Med Assoc 2008;232:687–694.
12. Sallander M, Hedhammar Å, Rundgren M, et al. Feeding patterns and dietary intake in a random sample of a Swedish population of insured-dogs. Prev Vet Med 2010;95:281–287.
13. Thomson RM, Hammond J, Ternent HE, et al. Feeding practices and the use of supplements for dogs kept by owners in different socioeconomic groups. Vet Rec 2008;163:621–624.
14. Higgins L, ed. Model regulations for pet and specialty pet food under the model bill. In: Association of American Feed Control Officials (AAFCO) 2014 official publication. Oxford, Ind: Association of American Feed Control Officials Inc, 2014;136–187.
15. Butterwick RF, Erdman JW Jr, Hill RC, et al. Challenges in developing nutrient guidelines for companion animals. Br J Nutr 2011;106(suppl 1):S24–S31.
16. Brown A, Waddell L. Rodenticides. In: Silverstein D, Hopper K, eds. Small animal critical care medicine. St Louis: Elsevier Inc, 2008;346–350.
17. US FDA. Recalls, market withdrawals, and safety alerts. Available at: www.fda.gov/Safety/Recalls/. Accessed Sep 7, 2015.
18. Association Française de Normalisation (AFNOR). NF EN 12821, Foodstuffs—determination of vitamin D by high-performance liquid chromatography: measurement of cholecalciferol (D3) and ergocalciferol (D2). Saint Denis, France: AFNOR, May 2009.
19. Mellanby RJ, Mee AP, Berry JL, et al. Hypercalcaemia in two dogs caused by excessive dietary supplementation of vitamin D. J Small Anim Pract 2005;46:334–338.
20. Rumbeiha W, Morrison J. A review of class I and class II pet food recalls involving chemical contaminants from 1996 to 2008. J Med Toxicol 2011;7:60–66.
21. NRC. Vitamins. In: Nutrient requirements of dogs and cats. Washington, DC: National Academic Press, 2006;193–245.
22. NRC. Nutrient requirements and dietary nutrient concentrations. In: Nutrient requirements of dogs and cats. Washington, DC: National Academic Press, 2006;354–370.
23. Kealy RD, Lawler DF, Monti KL. Some observations on the dietary vitamin D requirement of weanling pups. J Nutr 1991;121(suppl 11):S66–S69.
24. Morris JG. Vitamin D synthesis by kittens. Vet Clin Nutr 1996;3:88–92.
25. Markovich JE, Freeman LM, Heinze CR. Analysis of thiamine concentrations in commercial canned foods formulated for cats. J Am Vet Med Assoc 2014;244:175–179.
26. Coelho M. Vitamin stability in premixes and feeds: a practical approach in ruminant diets, in Proceedings. 13th Annu Fla Rumin Nutr Symp 2002;127–145.
27. Huang M, LaLuzerne P, Winters D, et al. Measurement of vitamin D in foods and nutritional supplements by liquid chromatography/tandem mass spectrometry. J AOAC Int 2009;92:1327–1335.
