Components of 25-hydroxyvitamin D in serum of young children in upper midwestern United States.

The relative importance of cholecalciferof (vitamin D3) and ergocalciferol (vitamin D2) in maintaining the vitamin D level in children (1/2 to 6 years old) living in the upper midwestern United States was determined by measurement of total 25-hydroxyvitamin D (25-OH-D), its components, and other indices of calcium homeostasis in serum. In 38 normal children, mean (range) serum total 25-OH-D was 32.8 (less than 5 to 53) ng/ml; in 25 of the 28 sera partitioned, the major component was 25-OH-D3. Significant seasonal variation in serum 25-OH-D3 (mean, range: 35.2, 17 to 51 ng/ml in summer and 15.9, less than 5 to 32 ng/ml in winter) was not accompanied by changes in mean serum 25-OH-D2, calcium, phosphorus, or alkaline phosphatase values. However, individual serum total 25-OH-D values correlated with serum phosphorus values (r = 0.37; P less than 0.05). The proportion of the total represented by 25-OH-D3 varied widely, with a a mean of 83% in summer and 67% in winter. Sources of D3, which include both dermal synthesis and intestinal absorption of D3 added to milk, appear to be more important than sources of D2 in maintaining vitamin D nutrition of young children throughout the year. However, sources of D2 offset the decrease in total 25-OH-D in winter months.