Serum 25-Hydroxyvitamin D Status Across Vertebrate Species: A Comparative Review

Vitamin D plays a fundamental role in calcium and phosphate homeostasis, skeletal development, immune regulation, and numerous metabolic processes in both humans and animals. Serum 25-hydroxyvitamin D [25(OH)D] is the accepted biomarker of vitamin D status, but its physiological concentrations vary considerably among animal species due to differences in metabolism, diet, ultraviolet B (UVB) exposure, and environmental conditions. This review summarizes current knowledge on vitamin D status across vertebrate species and identifies the major determinants responsible for interspecies variation. A structured literature search was conducted using Google Scholar, PubMed, Scopus, and Web of Science. Studies reporting serum or plasma 25(OH)D concentrations in companion animals, livestock, wildlife, reptiles and fish were evaluated, together with publications addressing vitamin D metabolism, analytical methods, and supplementation. Available evidence demonstrates marked differences in serum 25(OH)D concentrations among vertebrate species. Vitamin D status is primarily influenced by UVB exposure, dietary intake, husbandry conditions, season, and species-specific metabolic adaptations. In companion animals such as dogs and cats, dietary intake is the predominant source of vitamin D, whereas in livestock both UVB exposure and supplementation substantially influence circulating 25(OH)D concentrations. Wild animals often show greater seasonal variation in reported 25(OH)D concentrations, although comparisons with domestic counterparts are complicated by differences in diet, supplementation, environmental exposure, and analytical methods. Across multiple species, supplementation with 25-hydroxyvitamin D₃ appears more effective than cholecalciferol in increasing circulating 25(OH)D concentrations. Reported vitamin D status differs among vertebrate species and cannot be interpreted using universal reference values, particularly when analytical methods and study conditions differ. Species-specific physiology, environmental factors, and nutritional management should all be considered when evaluating serum 25(OH)D concentrations. Future studies should prioritize standardized analytical methodologies, establish species-specific reference intervals, and expand research on wildlife to improve understanding of vitamin D physiology across vertebrates.