Improved method for quantifying vitamin D in proprietary infants' formulas and in breast milk.

Abstract

Abstract In this new method for quantifying vitamin D in infants' formulas and breast milk, after repeated lipid extraction, samples are further purified by passage through Sep-Pak cartridges, followed by liquid chromatography, then quantified by competitive protein-binding assay. Analytical recovery is estimated by use of added 3H-labeled vitamin D. For repeated assays of a reconstituted proprietary powdered milk formula in two runs, the intra-assay vitamin D values were 9.0 (SD 1.4) (n = 6) and 8.9 (SD 1.6) micrograms/L (n = 7) (t = 0.07). Assay of a proprietary liquid formula yielded values of 14.9 (SD 0.9) micrograms/L (n = 6). For each, the results agreed with the vitamin D content shown on the label. Vitamin D concentrations in breast milk from two groups of lactating mothers of different social class and nutritional status were 2.3 (SD 1.4) (n = 7) and 2.0 (SD 1.7) micrograms/L (n = 7). Overall recoveries ranged from 65 to 75%. Only 2 mL of milk is required, which facilitates sample collection, and the assay is less time-consuming than other current methods.