Vitamin D Metabolism and Hormonal Influences
In many species, the biosynthesis of vitamin D begins with exposure to UV light, wherein 7-dehydrocholesterol is transformed to previtamin D3. Factors that affect synthesis of vitamin D3 include quantity and quality of the UV light, coat, and skin pigmentation. Dogs and cats are unique from humans and many other species in that they lack the ability to synthesize vitamin D3 in the skin, likely because of high activity of 7-dehydrocholesterol-Δ7-reductase.1,2 For this reason, dogs and cats require dietary supplementation with vitamin D to meet nutritional requirements. There are 2 dietary forms of vitamin D: cholecalciferol (vitamin D3), which typically comes from animal food sources, and ergocalciferol (vitamin D2), which typically comes from plant sources. Cats may not utilize ergocalciferol as efficiently as cholecalciferol3; however, dogs have the ability to utilize both dietary forms equally.4,a
Dietary vitamin D is supplied in commercially available dog and cat foods in the form of various ingredients (eg, organ meat or oily fish products) and supplemental cholecalciferol. Once ingested, it is transported to the liver via the portal system and intestinal lymphatics (Figure 1). This process requires digestive enzymes, chylomicrons, bile acids, and VDBP or transcalciferon.5,6 After cholecalciferol is transported to the liver, it is hydroxylated by 25-hydroxylase to form 25(OH)D (also known as calcidiol or calcifediol), which binds to VDBP in the circulation. With a half-life of approximately 2 to 3 weeks, 25(OH)D is thought to be the most reliable indicator of systemic vitamin D status in humans.7
Comprehensive overview of vitamin D metabolism, starting with dietary intake and progressing through hepatic and renal transformation. Also notice the influences of phosphate (Pi), ionized calcium (Ca2+), FGF-23, Klotho, and PTH. CYP = Cytochrome P450. (Reproduced with permission of The Ohio State University.)
Citation: Journal of the American Veterinary Medical Association 250, 11; 10.2460/javma.250.11.1259
Then, 25(OH)D is hydroxylated via 1α-hydroxylase to form 1,25(OH)2D (the most active naturally occurring vitamin D metabolite; also known as calcitriol), which affects many target cells via a vitamin D receptor–mediated mechanism. Calcitriol binds to the vitamin D receptor much more readily (approx 500 times as readily) than does vitamin D3 or 25(OH)D.8 This activation of 1,25(OH)2D occurs predominately in the kidneys; however, it also occurs in other tissues that express 1α-hydroxylase. Although the exact mechanism has not been completely elucidated, 1α-hydroxylase activity is tightly regulated by serum concentrations of calcium, PTH, 1,25(OH)2D, FGF-23, and the Klotho gene.9–12 Within cells, 1,25(OH)2D can promote or suppress gene transcription and expression.13 Both 25(OH)D and 1,25(OH)2D are inactivated via 24-hydroxylase to form 24,25(OH)2D and 1,24,25-trihydroxyvitamin D, respectively, and other metabolites (eg, 25[OH]D-23,23 lactone) that are excreted in the urine and bile.14
A novel vitamin D epimer, which was identified as a C-3 epimer of 25(OH)D, has recently been discovered in cats by use of high-performance liquid chromatography.b Serum concentrations ranged from 18 to 30 ng/mL, which represented 29% to 75% of native 25(OH)D. This epimer has not been identified in dogs.
Vitamin D Roles
Classically, vitamin D is known for its influence on calcium-phosphorus homeostasis via the bone-parathyroid-kidney axis.15,16 However, vitamin D has been found to have multiple other effects throughout the body, given the wide variety of cells that express the vitamin D receptor. Actions induced by vitamin D receptor activation in humans include differentiation of immune cells, reductions in inflammation and proteinuria, increases in insulin secretion, and improvement of hematopoiesis.17 In people, vitamin D deficiency (hypovitaminosis D) has been associated with a multitude of clinical syndromes, including kidney disease, cancer, obesity, asthma, intestinal disease, diabetes mellitus, hypertension, and infectious diseases.18–26 Vitamin D status also affects various disease conditions in dog and cats.
Measuring Vitamin D Metabolites
The VitDQAP27 was established through joint efforts between the National Institute of Standards and Technology and the National Institutes of Health. These efforts were initiated because measurement of vitamin D metabolites was routinely performed (and results reported) by use of multiple techniques, including liquid chromatographic methods, immunoassay techniques, chemiluminescence immunoassays, and radioimmunoassays. Furthermore, large variations in 25(OH)D results attributable to interassay, intra-assay, and interlaboratory variance make comparisons among results and defined cutoff points tenuous.28,29 The VitDQAP was able to assist in the development of standard reference materials and studies to examine differences among assay performance.
The VitDQAP, which is based on assay performance characteristics, is an international external quality assessment plan that can be used to evaluate vitamin D metabolite assays provided by participating laboratories. That assessment scheme is based, in part, on findings from studies conducted by the VitDQAP. Liquid chromatography methods are currently the most commonly used methods and remain the criterion-referenced standard (liquid chromatography with tandem mass spectrometric detection) for measurement.30 Importantly, those studies were performed with human samples, and the effect of a canine or feline matrix on these variables and comparability of results is unknown. Regardless, because there is no universally accepted best method for measurement of vitamin D metabolites, it is recommended to use a Vitamin D External Quality Assessment Scheme–certified lab to increase the likelihood of accurate results when measuring vitamin D metabolites. There are age-related differences to consider as well. The 1,25(OH)2D concentrations of kittens at 3 and 6 months of age are significantly higher than concentrations of older kittens and adult cats.31
How Much Vitamin D is Enough?
Defining 25(OH)D sufficiency, insufficiency, and deficiency is controversial. In humans, vitamin D deficiency is generally defined as < 20 ng/mL and sufficiency is generally > 30 ng/mL. Optimal repletion is defined by some as > 50 or > 60 ng/mL to achieve the aforementioned pleiotropic effects on the vitamin D receptor. Consensus on optimal, adequate, or deficient vitamin D status in populations of healthy dogs and cats has not been reached. Multiple variables (including signalment, disease, assay technique, and physiologic variation) affect the reference range and the therapeutic target range.32–34
An inverse relationship exists between circulating PTH and 25(OH)D concentrations in humans; therefore, 1 method used to define vitamin D sufficiency in humans has been to determine the lowest concentration of 25(OH)D associated with suppression of PTH synthesis.35 On the basis of this method, 25(OH)D concentrations of 100 to 120 ng/mL have been recommended by 1 group to represent sufficiency in healthy dogs because PTH concentrations are most suppressed at these concentrations of 25(OH)D.36 By use of this method, the recommendation for 25(OH)D sufficiency in dogs36 (100 to 120 ng/mL) differs considerably from that recommended for humans (> 20 or > 30 ng/mL). In that study36 of apparently healthy dogs, there was an extremely wide range of circulating 25(OH)D concentrations (9.5 to 249 ng/mL). The reference range for 1 national veterinary endocrine laboratoryc is 24 to 86 ng/mL.
Wide ranges of 25(OH)D concentrations have been reported for healthy dogs37–50 and cats.51–53 (Table 1). Importantly, assay choice and technique differed among many of these studies. In general, concentrations of 25(OH)D in healthy dogs and cats are substantially higher than concentrations in healthy humans. The higher 25(OH)D concentrations in dogs and cats likely reflect intake of commercial pet foods that often are supplemented with vitamin D at concentrations far above minimal needs, whereas people often eat diets deficient in vitamin D.54–56
Concentrations of vitamin D metabolites in healthy dogs and cats.
| Species | No. of animals | 25(OH)D (ng/mL) | 1,25(OH)2D (pg/mL) | 24,25(OH)2D (ng/mL) | Reference |
|---|---|---|---|---|---|
| Dog | 282 | 68.9 (9.5–249.2) | — | — | 36 |
| 6 | — | 26.0 ± 5.0 | — | 37 | |
| 33 | — | 36.0 | — | 38 | |
| 24 | 107.0 ± 38.9* | 58.8 ± 19.2* | — | 39 | |
| 24 | 122.8 (19.2–140.2)* | 60.6 (23.1–91.9)* | — | 40 | |
| 64 | 40.7 ± 15.6 | — | — | 41 | |
| 22 | — | 60.1 (22.5–99.2) | — | 42 | |
| 36 | 30.8* | 43.4* | — | 43 | |
| 54 | 48.1 ± 14.0* | — | — | 44 | |
| 47 | 40.7 ± 16.5; 37.6 (20.2–105.0) | — | — | 45 | |
| 24 | 29.9 (15.0–52.3)* | — | — | 46 | |
| 51 | 49.3 ± 17.6* | — | — | 47 | |
| 8 | Day 0: 57.0 ± 13.0 | Day 0: 157.0 ± 30.0 | Day 0: 54.0 ± 13 | 48 | |
| Day 2: 55.0 ± 11.0 | Day 2: 127.0 ± 33.0 | Day 2: 54.0 ± 13 | |||
| Day 8: 57.0 ± 13.0 | Day 8: 129.0 ± 32.0 | Day 8: 55.0 ± 12 | |||
| 320 | 69.7 (9.5–249.2) | — | — | 49 | |
| 10 | 75.1 (50.4–97.9) | 209.6 (168.9–128.0) | 38.7 (24.0–89.5) | 50 | |
| Cat | 36 | 49.0 (22.9–83.1) | — | — | 51 |
| 23 | 45.1 (30.4–61.1) | — | — | 52 | |
| 20 | 44.7 (14.9–61.0) | — | — | 53 |
Values reported are mean, mean ± SD, or median (range).
Results were originally reported as nmol/L but have been converted.
— = Not reported.
In 1997, adequate intake of vitamin D for an adult person was 200 U/d, whereas in 2010, adequate intake had increased to 600 U/d. According to the Association of American Feed Control Officials, the minimum vitamin D recommendation for canine adult maintenance is to provide 125 U/1,000 kcal. The Association of American Feed Control Officials recommends that the maximum amount allowed in commercial dog foods is 750 U/kcal.55 For example, a 20-kg (44-lb) dog eating a maintenance energy requirement (1.6 × resting energy requirement) of approximately 1,000 kcal/d could theoretically ingest a range of 125 to 750 U of cholecalciferol/d. On the basis of the 2010 adjustment to adequate intake for humans, it is possible that optimal dietary intake in canine and feline subjects might need adjustment in the future. However, one cannot necessarily predict a dog's serum 25(OH)D concentration on the basis of its cholecalciferol intake.57,d
Vitamin D Metabolite Status in Various Diseases
The status of vitamin D and vitamin D metabolites can be affected by various diseases and conditions36–40,42–48,50–53,58–62 (Table 2).
Concentrations of vitamin D metabolites in dogs and cats with various diseases or conditions.
| Species | No. of animals | Disease or condition | 25(OH)D (ng/mL) | 1,25(OH)2D (pg/mL) | 24,25(OH)2D (ng/mL) | Reference |
|---|---|---|---|---|---|---|
| Dog | 9 | Anal sac adenocarcinoma and hypercalcemia | — | 23.0 ± 5.0 | — | 58 |
| 6 | Solid tumors and normocalcemia | — | 16.0 ± 4.0 | — | 58 | |
| 18 | Lymphoma and hypercalcemia | — | 6.0 | — | 37 | |
| 6 | Lymphoma and normocalcemia | — | 11.0 | — | 37 | |
| 25 | Lymphoma and hypercalcemia | — | 43.0 | — | 38 | |
| 11 | Lymphoma and normocalcemia | — | 28.0 | — | 38 | |
| 8 | Anal sac adenocarcinoma and hypercalcemia | — | 31.0 | — | 38 | |
| 8 | Anal sac adenocarcinoma and normocalcemia | — | 28.5 | — | 38 | |
| 7 | Miscellaneous tumors and hypercalcemia | — | 44.0 | — | 38 | |
| 10 | Acute renal failure | 52.1 ± 32.9* | 28.8 ± 9.6* | — | 39 | |
| 21 | Chronic renal failure | 39.3 ± 24.8* | 43.8 ± 29.6* | 39 | ||
| 19 | Chronic renal failure in acute crisis | 27.2 ± 20.8* | 26.9 ± 17.7* | — | 39 | |
| 12 | Lymphoma and hypercalcemia | 40.7 (25.6–116.6)* | 42.3 (10.0–127.7)* | — | 40 | |
| 5 | Primary hyperparathyroidism and hypercalcemia | 36.5 (26.4–119.4)* | 95.4 (23.5–153.1)* | — | 40 | |
| 5 | Chronic renal failure and hypercalcemia | 26.8 (14.0–73.7)* | 34.0 (10.8–119.2)* | — | 40 | |
| 11 | CKD–stage 1 | — | 49.7 (37.4–77.2) | — | 42 | |
| 10 | CKD–stage 2 | — | 48.8 (34.9–84.6) | — | 42 | |
| 25 | CKD–stage 3 | — | 34.2 (2.4–89.6) | — | 42 | |
| 8 | CKD–stage 4 | — | 18.9 (5.0–38.2) | — | 42 | |
| 49 | Hospitalized non-GI tract illness | 26.9* | 29.3* | — | 43 | |
| 21 | Infammatory bowel disease | 28.6* | 54.5* | — | 43 | |
| 12 | Protein-losing enteropathy | 5.7* | 19.9* | — | 43 | |
| 33 | Cutaneous mast cell tumor | 41.7 ± 12.0* | — | — | 44 | |
| 19 | CKD | 19.2 ± 14.1; 14.5 (2.7–53.7) | — | — | 45 | |
| 26 | Neoplastic spirocercosis | 12.3 (5.9–24.9)* | — | — | 46 | |
| 25 | Nonneoplastic spirocercosis | 21.1 (7.7–52.0)* | — | — | 46 | |
| 31 | Congestive heart failure | — | — | 47 | ||
| 31 | Splenic hemangiosarcoma | 49.2 (19.4–91.8) | — | — | 36 | |
| 62 | Cancer and hemoabdomen, including splenic hemangiosarcoma | 49.4 (19.4–151.0) | — | — | 36 | |
| 14 | CVHD–stage B1 | 21.8 (13.4–71.3)* | — | — | 59 | |
| 17 | CVHD–stage B2 | 14.3 (2.0–68.9)* | — | — | 59 | |
| 12 | CVHD–stage C or D | 5.2 (2.0–28.3)* | — | — | 59 | |
| 12 | Racing sled dogs | Day 0: 67 ± 9 | Day 0: 122 ± 34 | Day | 0: 66 ± 13 | 48 |
| Day 2: 72 ± 11 | Day 2: 119 ± 26 | Day | 2: 67 ± 15 | |||
| Day 8: 87 ± 16 | Day 8: 121 ± 22 | Day | 8: 76 ± 18 | |||
| 26 | Chronic enteropathy–survivors | 24.9 (15.6–39.5)† | — | — | 60 | |
| 15 | Chronic enteropathy–nonsurvivors | 4.3 (1.6–17.0)† | — | — | 60 | |
| 37 | CKD–total | 48.2 (3.5–95.8) | 120.8 (19.0–286.0) | 18.9 | (0.3–48.5) | 50 |
| 10 | CKD–stage 1 | 48.2 (32.6–87.3) | 157.6 (94.8–202.4) | 24.8 | (11.2–48.5) | 50 |
| 9 | CKD–stage 2 | 55.7 (34.5–93.5) | 143.2 (96.4–286.0) | 30.3 | (14.8–46.8) | 50 |
| 12 | CKD–stage 3 | 42.7 (3.5–95.8) | 104.8 (29.2–228.7) | 10.3 | (0.3–42.1) | 50 |
| 6 | CKD–stage 4 | 25.0 (19.0–91.1) | 64.7 (19.0–91.1) | 7.0 (2.0–16.4) | 50 | |
| Cat | 24 | Mycobacteriosis | 22.2 (9.7–54.8) | — | — | 51 |
| 41 | Hospitalized with illness | 33.8 (10.6–53.5) | — | — | 51 | |
| 20 | Infammatory bowel disease (n = 14) and intestinal lymphoma (6) | 12.7 (2.0–83.1) | — | — | 52 | |
| 39 | Hospitalized with illness | 30.9 (7.1–82.4) | — | — | 53 | |
| 59 | FIV infection | 32.1 (5.0–62.4) | — | — | 53 | |
| 80 | Hospitalized cats–alive | 38.5 (1.7–81.6)* | — | — | 61 | |
| 19 | Hospitalized cats–dead | 22.9 (8.7–97.1)* | — | — | 61 | |
| 148 | Neutrophil count < 12.8 × 109 cells/L | 42.3* | — | — | 62 | |
| 22 | Neutrophil count > 12.8 × 109 cells/L | 31.9* | — | — | 62 |
Values reported are mean, mean ± SD, or median (range).
Value reported is median (interquartile range).
CVHD = Chronic valvular heart disease. GI = Gastrointestinal.
See Table 1 for remainder of key.
Kidney disease
Vitamin D metabolites have been measured in dogs with several forms of kidney disease, including acute renal failure,39 CKD,39,40,42,45,50 and proteinuric kidney disease.e There are several mechanisms by which vitamin D metabolism can be disrupted with kidney disease, including decreased dietary intake of vitamin D, decreased enzymatic conversion from cholecalciferol to 25(OH)D in the liver,63 decreased activation via 1α-hydroxylase from 25(OH)D to 1,25(OH)2D, and increased inactivation of 25(OH)D and 1,25(OH)2D.18 With proteinuria, there are additional potential mechanisms to consider, including urinary loss of VDBP (with 25 [OH] D and 1,25 [OH]2D bound to vitamin D binding protein) and decreased endocytosis of 25(OH)D into renal cells because of decreased megalin expression in the proximal renal tubules.64,65 Furthermore, inflammation may act to reduce 25(OH)D concentrations.66
In several studies,39,40,42,45,50 it has been reported that dogs with CKD have lower 25(OH)D and 1,25(OH)2D concentrations, compared with concentrations in control dogs. Vitamin D metabolites are correlated with stage of kidney disease (determined via International Renal Interest Society criteria), as indicated by the fact that concentrations of 25(OH)D, 1,25(OH)2D, and 24,25(OH)2D are significantly decreased in dogs with stage 3 kidney disease, compared with concentrations in control dogs.42,50 In other studies, many dogs had 25(OH)D and 1,25(OH)2D concentrations within reference limits. One possible explanation for this lack of difference could be the inclusion of dogs with earlier stages of CKD. Alternatively, significant differences in concentrations of vitamin D metabolites may not have been detected because of relatively large reference ranges or the method used to calculate reference ranges.
One of the consequences of CKD is development of secondary hyperparathyroidism and CKD-induced mineral and bone disorders.67–69 Plasma FGF-23 concentrations are increased in cats and dogs with CKD.70,71 Concentration of FGF-23 was negatively correlated with 25(OH)D, 1,25(OH)2D, and 24,25(OH)2D concentrations in dogs with CKD50 and with survival duration in cats with CKD.72 Calcitriol treatment has been recommended for several decades for dogs and cats to reduce PTH concentrations and improve quality of life.73,74,f However, prospective, controlled clinical studies are needed to determine the manner in which supplementation with various forms of vitamin D influences FGF-23 concentrations, Klotho expression, vitamin D repletion, quality of life, preservation of renal function, and survival duration.
Finally, dogs with acute renal failure had significantly lower 25(OH)D and 1,25(OH)2D concentrations, compared with concentrations in control dogs, but most (7/10) of the dogs with acute renal failure had concentrations within reference limits.39 These findings possibly could have been attributable to acute inflammation or critical illness66,75 or could have been spurious results. Proteinuric dogs have significantly lower 25(OH)D, 1,25(OH)2D, and 24,25(OH)2D concentrations than do control dogs.e This relationship has been definitively established in proteinuric people, and vitamin D receptor activators are frequently prescribed to reduce proteinuria.65,76
Neoplasia
Decreased 25(OH)D concentrations have been linked to increased risk of numerous neoplasms in humans, and 1,25(OH)2D has been found to have antineoplastic activity.77,78 Concentrations of circulating vitamin D metabolites have been measured in dogs with various neoplasms. Serum 25(OH)D concentrations are significantly lower for various neoplastic conditions, including dogs with neoplasia and hemoabdomen,36 cutaneous mast cell tumor,44 and lymphoma.40
Serum 25(OH)D concentrations in dogs and cats prior to the development of neoplasia have not been evaluated. Thus, it is not clear whether dogs develop hypovitaminosis D secondary to neoplasia or whether hypovitaminosis D is actually a risk factor for development of cancer. Dogs with neoplasia are often ill; this puts them at risk of developing hypovitaminosis D as a result of a reduced appetite, which leads to reduced cholecalciferol intake, and potentially from decreased intestinal absorption of cholecalciferol.
Serum 1,25(OH)2D concentrations have been measured in populations of dogs with lymphoma, both with and without hypercalcemia, with wide differences in findings.37,38,40 In the earliest study,37 both hypercalcemic and normocalcemic dogs had significantly lower 1,25(OH)2D concentrations than did healthy control dogs. Most of the hypercalcemic dogs had 1,25(OH)2D concentrations lower than the limit of detection, which is an appropriate response in the face of hypercalcemia.37 In a later study,38 hypercalcemic dogs with lymphoma had a higher mean 1,25(OH)2D concentration than did control dogs, but the normocalcemic dogs with lymphoma had a lower mean 1,25(OH)2D concentration than did control dogs, although these results were not compared with a statistical assessment. Mean 1,25(OH)2D concentrations were within reference limits for both hypercalcemic and normocalcemic dogs with lymphoma.38 Finally, in the most recent study,40 median 1,25(OH)2D concentration was significantly lower in dogs with lymphoma (110.0 pmol/L) than in control dogs (157.5 pmol/L); however, there was a wide range of concentrations that extended below the low end and above the high end of the reference range.40 The 1,25(OH)2D concentrations in dogs with anal sac adenocarcinoma (with both hypercalcemia and normocalcemia) were not significantly different from 1,25(OH)2D concentrations in control dogs.38,58
From an antineoplastic standpoint, calcitriol can have in vitro activity against osteosarcoma,79 squamous cell carcinoma,80 prostatic epithelial,81 anal sac adenocarcinoma,82 mammary gland cancer,83 and mast cell tumor84 canine cell lines.79–84 One in vivo study83 revealed a synergistic effect of administration of calcitriol with cisplatin against various tumors (eg, osteosarcoma and chondrosarcoma) in dogs. Investigators of another in vivo study84 found that calcitriol treatment could induce remission of mast cell tumors; however, the trial was discontinued because of the high rate of toxic events (ie, hypercalcemia and azotemia) observed.
Primary hyperparathyroidism
Although primary hyperparathyroidism is technically a neoplastic condition, it is separated in the information provided here to avoid confusion with malignant conditions because most dogs with primary hyperthyroidism have benign parathyroid gland adenomas. Compared with concentrations in control dogs, 5 dogs with primary hyperparathyroidism had significantly lower serum 25(OH)D concentrations40; however, all values for the dogs with primary hyperparathyroidism were within reference limits.40 Serum 1,25(OH)2D concentrations also were significantly higher in dogs with primary hyperparathyroidism than concentrations in control dogs, and 1,25(OH)2D concentrations in 4 of 5 dogs with primary hyperparathyroidism were above reference limits.40 Both findings could possibly be attributed to an upregulating effect of PTH on renal 1α-hydroxylase activity, which would thus increase 1,25(OH)2D synthesis.
In a studyg of 10 dogs with primary hyperparathyroidism treated by surgical excision of parathyroid gland adenomas, all had low 25(OH)D concentrations at the time of diagnosis, compared with concentrations in control dogs, whereas 1,25(OH)2D concentrations were within reference limits. At the time of the postparathyroidectomy nadir in ionized calcium concentration, 25(OH)D concentrations were not different from concentrations at the time of initial diagnosis, but mean 1,25(OH)2D concentrations were lower.g
A diagnosis of primary hyperparathyroidism traditionally has been made on the basis of an increased ionized calcium concentration at the time of an inappropriately high concentration of PTH. The concentration of circulating 25(OH)D is an important regulatory factor for the suppression of PTH synthesis in people (likely following its conversion to 1,25(OH)2D within the parathyroid gland). Concentrations of PTH are higher in humans with concomitant lower circulating 25(OH)D concentrations. It currently is recommended for humans that the diagnosis of primary hyperparathyroidism only be made when 25(OH)D concentrations are sufficient or after 25(OH)D has been repleted following supplementation with vitamin D.85 The importance of concurrent evaluation of ionized calcium, PTH, and 25(OH)D concentrations to make an accurate diagnosis of primary hyperparathyroidism has not yet been investigated in veterinary medicine.
Gastrointestinal tract disease
Absorption of fat-soluble vitamins depends on adequate absorption of dietary fat; thus, malabsorptive intestinal diseases can adversely affect vitamin D absorption and ultimately contribute to hypovitaminosis D.86,87 Serum 25(OH)D and 1,25(OH)2D concentrations have been evaluated in dogs with inflammatory bowel disease and protein-losing enteropathy. Both vitamin D metabolite concentrations were significantly lower in the protein-losing enteropathy group than in dogs with inflammatory bowel disease and healthy dogs.43,88 Additionally, 25(OH) D concentrations were significantly negatively correlated with duodenal inflammation and death.60,88
It is possible that hypoalbuminemia contributes to hypovitaminosis D through loss of VDBP via diseased intestines.89 Alternatively, hypovitaminosis D may contribute to intestinal protein loss through the effect of vitamin D on the immune response.90 Results of experiments indicated that vitamin D receptor–knockout mice are more likely to develop induced inflammatory bowel disease.91 Additionally, vitamin D–deficient diets predisposed mice to colitis via dysregulated colonic antimicrobial activity and impaired homeostasis of enteric bacteria.92
Orthopedic disease
Osteoblasts and chondrocytes express 1α-hydroxylase and vitamin D receptor; however, it is unknown whether vitamin D plays a direct or indirect role in bone growth and mineralization. Rickets is a metabolic bone disease typically caused by dietary deficiency of vitamin D or phosphorus or by genetic defects affecting vitamin D or phosphorus metabolism. The most common clinical abnormality is widening of the physeal growth plates of fast-growing bones (eg, radius and ulna). Histologically, hypertrophic chondrocytes accumulate, which leads to thickened, irregular growth plates.93 Dogs and cats fed unbalanced meat-based diets without vitamin D supplementation are more likely to develop fibrous osteodystrophy, rather than rickets, because of the development of nutritional hyperparathyroidism.93 For an animal with dietary-induced rickets, treatment entails transitioning the animal to a complete and balanced diet.
Two autosomal recessive disorders that cause VDDR in humans have been described. Type I VDDR is caused by a defect in the gene encoding 1α-hydroxylase, which subsequently leads to inadequate activation of 25(OH)D to form 1,25(OH)2D. This leads to 25(OH)D concentrations within the reference range but low 1,25(OH)2D concentrations. Alternatively, type II VDDR is caused by a defect in the vitamin D receptor gene, which leads to hypocalcemia, secondary hyperparathyroidism, and high 1,25(OH)2D concentrations.93 A few cases of both types of VDDR have been reported in dogs94,95 and cats.96–99 Treatment of type I VDDR entails providing supplemental 1,25(OH)2D and typically has a better prognosis than does treatment of type II VDDR, which requires high doses of both 1,25(OH)2D and calcium.93,100
Cardiovascular disease
Vitamin D plays a role in the pathophysiologic processes of cardiac disease. Cardiac myocytes express vitamin D receptor and a calcitriol-dependent calcium-binding protein.47 In humans, hypovitaminosis D is associated with increased rates of myocardial infarction and cardiovascular events.101 Studies101,102 have revealed an inverse relationship between vitamin D status and hypertension in people; however, a meta-analysis of 46 trials revealed that vitamin D supplementation had no effect on lowering blood pressure.102 Additionally, both FGF-23 and Klotho have been linked to cardiovascular disease (eg, atherosclerosis, vascular stiffening, and left ventricular hypertrophy) in people with CKD.103–105
The association between vitamin D and cardiac disease has been investigated in dogs. In 1 study47 that involved evaluation of 31 dogs with congestive heart failure, mean serum 25(OH)D concentrations were approximately 20% less than those of healthy control dogs. Another study59 revealed that serum 25(OH)D concentrations were significantly lower in dogs with stage B2, C, or D chronic valvular disease (American College of Veterinary Internal Medicine criteria), compared with concentrations in dogs with stage B1 chronic valvular disease (ie, no evidence of cardiac remodeling). Serum 25(OH)D concentrations were significantly correlated with left ventricular and atrial sizes.59 Similar to results for other diseases, decreased serum 25(OH)D concentrations may be linked to decreased dietary intake or increased inflammation. To the authors' knowledge, no veterinary studies have been conducted to evaluate FGF-23 or Klotho values in relation to cardiovascular disease.
Inflammatory conditions
Vitamin D has been associated with inflammation and the immune system because most leukocytes express vitamin D receptor.106 Serum 25(OH) D is a negative acute-phase protein and is typically inversely related to inflammatory markers (eg, C-reactive protein) in humans.66,107 Furthermore, 25(OH)D and 1,25(OH)2D modulate inflammation by inhibiting production of interleukin-6 and tumor necrosis factor-α.108 In a recent study,48 investigators evaluated 25(OH)D and C-reactive protein concentrations in racing sled dogs before and after strenuous activity. Despite higher C-reactive protein concentrations, the dogs had higher 25(OH)D concentrations after racing.48 Investigators of another study36 found no correlation between 25(OH)D and C-reactive protein concentrations in dogs with cancer.
Regarding leukocyte counts, serum 25(OH)D concentrations are significantly negatively correlated with neutrophil count, monocyte count, and interleukin-2 and −8 concentrations in dogs with chronic enteropathy.88 Concentrations of 25(OH)D are significantly lower in hospitalized cats (with a variety of illnesses) with neutrophilia, compared with concentrations in hospitalized cats without neutrophilia.62
Infectious diseases
Serum 25(OH)D concentrations have been investigated for some infectious diseases of dogs and cats. Cats with both cutaneous and systemic mycobacteriosis had significantly lower 25(OH)D concentrations, compared with concentrations in healthy cats.51 Cats infected with FIV had significantly lower 25(OH)D concentrations, compared with concentrations in healthy cats.53 Dogs with both neoplastic and nonneoplastic spirocercosis had significantly lower 25(OH)D concentrations than did healthy dogs.46 Dogs with neoplastic spirocercosis had significantly lower 25(OH)D concentrations than did dogs with nonneoplastic spirocercosis.46
There are several reports in which granulomatous disease induced hypercalcemia in dogs109–113 and cats.114,115 The major mechanism originally believed to be the cause of hypercalcemia was dysregulated production of calcitriol (ie, increased production of 1,25[OH]2D)116; however, there are granulomatous diseases in humans and dogs in which hypercalcemia has been attributed to PTH-related peptide and not to calcitriol.112
Other diseases and conditions
Several diseases and conditions that have been linked to hypovitaminosis D in humans have not yet been studied in dogs and cats. These include diabetes mellitus,23,117,118 obesity,119,120 joint and nerve pain,121,122 gallbladder stasis,123,124 epilepsy,125 acute respiratory distress syndrome,126 and dry eye syndrome.127
Mortality Rate and Death
Serum 25(OH)D concentrations have been linked to in-hospital,128 30-day,129 and overall130–132 mortality rates in people. Serum 25(OH)D status is predictive of the 30-day mortality rate for hospitalized ill cats, with those in the lower tertile at higher risk.61 Serum 25(OH) D concentration at the time of diagnosis is a significant predictor of mortality rate for dogs with chronic enteropathy.60 It remains to be determined whether a low 25(OH)D concentration specifically influences the mortality rate or whether it is a consequence of more inflammation and a greater severity of underlying disease.
Vitamin D Supplementation and Toxicosis
Numerous studies have identified decreased concentrations of vitamin D metabolites in dogs and cats with various diseases; however, it has not yet been determined whether these animals should receive supplemental vitamin D or vitamin D metabolites and, if so, the manner for providing them. Potential options include vitamin D2 (ergocalciferol), vitamin D3 (cholecalciferol), calcidiol, calcitriol, or other vitamin D receptor activators (eg, paricalcitol). A modified-release formulation of 25(OH)Dh was approved by the FDA in 2016 for treatment of humans with advanced stages of CKD. It was recently reported that providing supplemental 25(OH)D to dogs rapidly and efficiently increases serum 25(OH)D concentrations.i Additional studies are necessary to elucidate appropriate dosing recommendations.
The goal of supplementation with vitamin D or 25(OH)D should be to increase serum 25(OH)D concentrations and improve outcomes specific to the disease being managed (eg, reducing proteinuria or improving the survival rate or duration). The form of supplemental vitamin D administered, half-life of the product, and potential for toxic effects may differ; thus, caution must be exercised, and treated animals must be monitored closely.
Vitamin D toxicosis is most commonly diagnosed after the development of hypercalcemia and is a subsequent risk for acute kidney injury and soft tissue mineralization. Development of hypercalcemia as a result of vitamin D toxicosis is a relatively late finding. Several factors influence the potential for vitamin D toxicosis, including lipophilicity, affinity of vitamin D metabolites for VDBP, and rates of metabolite synthesis and degradation. The fact it is fat soluble is a primary reason that vitamin D has a long whole-body half-life of approximately 2 months. Half-lives for 25(OH)D and 1,25(OH)2D are approximately 2 to 3 weeks and 4 to 6 hours, respectively.133,134
Vitamin D toxicosis in humans that results in hypercalcemia is thought to occur when 25(OH)D concentrations exceed 100 to 150 ng/mL. In various animal species (rats, cows, pigs, rabbits, dogs, and horses), plasma 25(OH)D concentrations associated with hypercalcemia have exceeded 150 ng/mL.133 The most commonly encountered forms of vitamin D toxicosis in dogs and cats include ingestion of cholecalciferol rodenticides and calcitriol- or calcitriol analogue–containing skin creams (calcipotriol and calcipotriene).135 Occasionally, misformulation of commercial pet foods may contribute to vitamin D toxicosis.116,135,136 Recently, a dog was described that had hypercalcemia and azotemia secondary to chronic ingestion of maxacalcitol.137 Iatrogenic toxicosis, typically determined by measurement of 1,25(OH)2D concentrations, may occur secondary to provision of supplemental calcitriol for management of renal secondary hyperparathyroidism, primary hypoparathyroidism, or protein-losing enteropathy or presurgical or postsurgical treatment of primary hyperparathyroidism.
Hypercalciuria develops during early phases of vitamin D toxicosis, before hypercalcemia develops. Hypercalciuria can have negative impacts by increasing the risk of developing calcium-containing uroliths and renal injury. The urinary calcium-to-creatinine ratio is used to detect hypercalciuria in humans.138 This concept has received attention in the investigation of dogs139 and catsj that form calcium-containing uroliths.
Clinical Summary
Vitamin D homeostasis is characterized by complex interactions between vitamin D metabolites, ionized calcium, phosphorus, FGF-23, and Klotho, and regulatory pathways can be disrupted in a variety of ways. Although reference limits for serum vitamin D metabolites in healthy dogs and cats remain to be determined, many diseases have been associated with lower concentrations of vitamin D metabolites, whereas some have been associated with higher concentrations. The chicken-and-egg conundrum often applies to these diseases, and it is not definitively clear whether vitamin D deficiency precedes (causes) or is the result of these diseases. Additional studies are needed to determine whether vitamin D supplementation for dogs and cats with various diseases would improve patient outcomes and, if so, the form and dosing regimen that would best provide that supplemental vitamin D.
ABBREVIATIONS
| 1,25(OH)2D | 1,25-dihydroxyvitamin D |
| 24,25(OH)2D | 24,25-dihydroxyvitamin D |
| 25(OH)D | 25-hydroxyvitamin D |
| CKD | Chronic kidney disease |
| FGF | Fibroblast growth factor |
| PTH | Parathyroid hormone |
| VDBP | Vitamin D binding protein |
| VDDR | Vitamin D–dependent rickets |
| VitDQAP | Vitamin D Metabolites Quality Assurance Program |
Footnotes
Delaney SJ. Serum ionized calcium, 25-hydroxyvitamin D, and parathyroid hormone in two dogs fed a homemade diet fortified with D2 (abstr). J Anim Physiol Anim Nutr 2015;99:818.
Sprinke MC. Previously undescribed vitamin D epimer found in cats using HPLC method (abstr), in Proceedings. Waltham Int Nutr Sci Symp 2016;65.
Heartland Assays, Ames, Iowa.
Middleton R, Nestlé Purina PetCare Research, St Louis: Personal communication, 2016.
Parker VJ, Gilor C, Rudinsky AJ, et al. Association between vitamin D metabolites and proteinuria (abstr), in Proceedings. Am Coll Vet Intern Med Research Forum 2016;953.
Polzin DJ. Clinical benefit of calcitriol in canine chronic kidney disease (abstr). J Vet Intern Med 2005;19:433.
Song J. Evaluation of parathyroid hormone and preoperative vitamin D as predictive factors for post-operative hypocalcemia in dogs with primary hyperparathyroidism. MS thesis, Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, Ohio, 2016.
Rayaldee, OPKO Health Inc, Miami, Fla.
Young L, Backus RL. Serum 25-hydroxyvitamin D3 and 24R,25-dihydroxyvitamin D3 concentrations in adult dogs are more substantially increased by oral supplementation of 25-hydroxyvitmain D3 than by vitamin D3 (abstr), in Proceedings. Waltham Int Nutr Sci Symp 2016;70–71.
Pimenta MM, Reche-Junior A, Freitas MF, et al. Calcium:creatinine urinary ratio. A predictor of occurrence of nephrolithiasis in cats? (abstr) J Feline Med Surg 2013;15:825.
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