Age-dependent changes in plasma concentrations of 25-hydroxyvitamin D may complicate vitamin D status assessment of immature cats.

Background: Vitamin D deficiency and excess in clinically presented cats conventionally is diagnosed by comparison of patient plasma 25-hydroxyvitamin D (25 (OH)D) concentration with plasma reference intervals determined in healthy adult cats. For immature cats, validity of this vitamin D status assessment method is uncertain. Objective: The overall objective was determination of whether plasma concentration of 25 (OH) D and other vitamin D metabolites in immature cats markedly change with developmental age as has been reported in other species. Methods: Four male and 4 female domestic short-hair kittens from weaning were continuously presented a single nutritionally adequate growth-diet. Concentrations of 25 (OH) D and 24,25-dihydroxyvitamin D (24,25 (OH)²D), and calcitriol were quantified in plasma of jugular venous blood collected at 12, 15, 18, and 21 weeks and 1 year of age. Plasma was liquid and solid-phase extracted and fractionation by normal-phase HPLC, and 25 (OH) D and 24,25 OH)₂D quantified by reverse-phase HPLC-UV and calcitriol by RIA. Results: Plasma 3-epi-25 (OH) D and 25 (OH) D concentrations increased (p < 0.001) with age so that by study end the concentrations rose by 1-and 2-fold, respectively. Concentrations of 3-epi-25 (OH) D relative to 25 (OH) D were 30% at 12 weeks and 20% at 1 year. Between ages 12 and 21 weeks, rises in 25 (OH) D concentration were positively correlated with body weight gains (ρ = 0.952, p < 0.001) and 24,25 (OH)₂D concentrations were consistently greater than 25 (OH) D concentrations (p < 0.001). At 1 year of age, concentrations of 24,25 (OH)₂D declined below those of 25 (OH) D and 3-epi-24,25 (OH)₂D consistency occurred in low concentrations. Vitamin D2 metabolites and sex differences in metabolite concentrations were not observed. Conclusion: Reliance on quantification of plasma 25 (OH) D concentration for vitamin D status assessment in kittens may be confounded by developmental changes in 25 (OH) D independent of vitamin D intake. High 24,25 (OH)₂D concentration and occurrence of 3-epi-25 (OH) D in plasma additionally may interfere with the quantification. [ABSTRACT FROM AUTHOR]