Rapid determination of 25-hydroxy vitamin D3 in swine tissue using an isotope dilution HPLC-MS assay

A rapid method for quantification of 25-hydroxy vitamin D3 in different swine tissues based on isotope dilution HPLC-MS has been developed and validated. Six times deuterated analyte is used as internal standard. The method is fast and can be performed with only 1 g sample. Sample preparation for kidney, liver, muscle and spleen requires only homogenisation and extraction with methanol. An additional enzymatic digest is required for skin, and clean-up of the extract by solid-phase extraction (SPE) is used for adipose tissue and skin. The lower limit of detection varies from 1 ng/g (muscle) to 5 ng/g (adipose and skin). The method has been successfully applied to various tissue samples of pigs fed for 119 days either 2000 IU of vitamin D3 or 50 μg of 25-hydroxy vitamin D3 per kg feed. For animals ingesting 25-OH-D3 supplements the highest tissue contents were observed in the skin (24.8 ± 3.5 ng/g), followed by kidney (14.2 ± 1.5 ng/g), liver and muscle (5.7 ± 0.6 ng/g). The 25-OH-D3 content in the skin was significantly higher in animals ingesting 2000 IU/kg of vitamin D3 (39.5 ± 13.4 ng/g). Levels in selected tissues of some animals were below the lower limit of quantification. No measurable amounts of 25-OH-D3 were found in spleen, abdominal fat and subcutaneous fat of the animals of both groups as well as in the liver, kidney and muscle of the animals ingesting 2000 IU/kg of vitamin D3.