A Study of the Seasonal Variation of Vitamin D in Normal Cow’s Milk

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Abstract
A method is presented for the concentration of the antirachitic factors in milk fat thus making possible the biological assay of fats of low potency. The monthly assay of milk fats from several sources over a period of 2 years shows that milk may vary as much as 900 per cent in antirachitic potency. Highest values were obtained during July, August or September and lowest usually in February. Vitamin D values ranging from 4.8 to 43.8 U.S.P. units per quart of milk were observed in the case of Guernsey milk whereas the extreme values for Holstein milk were 3.1 to 27.7 U.S.P. units per quart. The close correlation between the antirachitic potency of milk and the amount of available sunshine indicates that the exposure of cows to sunlight is the major factor contributing to the vitamin D content of milk. Apparently the cow has little or no opportunity to store vitamin D during lactation under ordinary dairy management conditions.
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