Serum vitamin D, calcium, and phosphorus concentrations in ponies, horses and foals from the United States and Thailand Megan E. Pozza a , Thattawan Kaewsakhorn b , Chumnan Trinarong b , Nongnuch Inpanbutr a , Ramiro E. Toribio a,⇑ a Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, 601 Vernon Tharp Street, Columbus, OH 43210, USA b Faculty of Veterinary Medicine, Chiang Mai University, Chiang Mai, Thailand article info Article history: Accepted 3 January 2014 Keywords: Calcium Foal Horse Phosphorus Vitamin D abstract Vitamin D is essential in calcium and phosphorus regulation, bone physiology, cell proliferation and epi- thelial integrity. Literature on vitamin D in growing horses is sparse, and the effect of age on vitamin D has not been evaluated in equids in the United States or in tropical countries. The goal of this study was to determine if there was an effect of age on serum 25(OH)D 3 concentrations in equids in the US (Ohio/ Kentucky) and Thailand (Chiang Rai and Kanchanaburi) during the same time of the year. Blood samples were collected from healthy ponies (n = 21) and Thoroughbred foals (n = 13), yearlings (n = 10), and horses (n = 20) in Thailand and from Thoroughbred foals (n = 10) and horses (n = 17) in the US. Serum concentrations of 25(OH)D 3 , calcium and phosphorus were measured. In both countries, serum 25(OH)D 3 concentrations were lower in foals than in yearlings and adult horses. Serum 25(OH)D 3 concentrations were higher in horses than in ponies in Thailand, but were not different between horses from either country. Calcium concentrations were not different between groups or location. In both countries, phosphorus concentrations were higher in foals than in older groups; how- ever, were not different between ponies and horses. This study shows that independent of geography there are age-related differences in 25(OH)D 3 concentrations in horses and further confirms that 25(OH)D 3 concentrations are lower in horses compared to other species. The information will serve as the basis for future clinical studies and to help understand better the pathophysiology of equine disorders associated with calcium and phosphorus dysregulation. Ó 2014 Elsevier Ltd. All rights reserved. Introduction Vitamin D has essential functions in calcium and phosphorus reg- ulation and skeletal physiology (Dittmer and Thompson, 2011; Toribio, 2011). Vitamin D enhances gastrointestinal absorption and renal reabsorption of calcium and phosphorus by promoting the expression of genes involved in the transepithelial transport of calcium and phosphorus (Rourke et al., 2010; Toribio, 2010). Vitamin D also modulates osteoblast activity, a critical function that if absent can lead to skeletal abnormalities such as rickets (Dittmer and Thompson, 2011). Other actions of vitamin D that have been re- cently recognized include immunomodulation, anti-carcinogenic effects, anti-inflammatory actions and inhibition of cell proliferation (Toribio, 2011; Haroon and Fitzgerald, 2012; Radlovic et al., 2012). Vitamin D 3 is produced in the skin by ultraviolet light (290–315 nm) photolytic induced cleavage of 7-dehydrocholes- terol, producing previtamin D 3 , that after thermal isomerization forms vitamin D 3 (cholecalciferol). From the skin, vitamin D 3 is transported by vitamin D-binding protein (DBP) to the liver, where it is hydroxylated by a microsomal mixed function P 450 oxidase (25a-hydroxylase) to 25-hydroxyvitamin D 3 [25(OH)D 3 ]. Activa- tion of 25(OH)D 3 to 1,25(OH) 2 D 3 occurs in the kidney, via 1a- hydroxylase, and is driven by hypocalcemia, hypophosphatemia, and parathyroid hormone (PTH) (Toribio, 2011). Blood concentra- tions of 25(OH)D 3 are considered the best indicator of the vitamin D status (Holick et al., 2011; Toribio, 2011). In growing mammals, hypovitaminosis D leads to rickets while in adults it is associated with osteomalacia and osteoporosis (Toribio, 2011; Morris et al., 2012). Rickets in children still remains a serious health problem in developing countries (Pettifor, 2004; Arabi et al., 2010; Dobnig, 2011). Hypovitaminosis D in humans is associated with skin color, geographical location, social and reli- gious customs, genetics, and a multitude of diseases (Kato et al., 2002; Pettifor, 2004; Arabi et al., 2010; Springbett et al., 2010; Dobnig, 2011). Seasonal variations in serum vitamin D concentra- http://dx.doi.org/10.1016/j.tvjl.2014.01.002 1090-0233/Ó 2014 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +1 614 2926661. E-mail address: toribio.1@osu.edu (R.E. Toribio). The Veterinary Journal 199 (2014) 451–456 Contents lists available at ScienceDirect The Veterinary Journal journal homepage: www.elsevier.com/locate/tvjl