Maternal serum 25-OH-vitamin D levels in the 6,400 IU group increased from 34 to 58.8 mcg/L over the 7-month study period.

Milk level changes paralleled maternal vitamin D status changes. Over 7 consecutive monthly measurements, average milk vitamin D levels increased from 59.6 IU/L at baseline to 76.3 IU/L at 7 months in the 400 IU daily group, and steadily increased in the 6,400 IU daily group from 82.4 IU/L at baseline, to 374.4 IU/L after 4 months, 555.2 IU/L after 5 months, 624.5 IU/L after 6 months, and 873.5 IU/L after 7 months. Average maternal baseline vitamin D serum levels were >30 mcg/L. Dietary intake and skin pigmentation changes did not differ between the two groups. Nineteen fully breastfeeding women were given either 400 or 6,400 IU daily of vitamin D 3 beginning at 1 month postpartum. Based on this reported average milk level, an exclusively breastfed infant would receive 20 IU/kg daily of vitamin D. In a study of eighteen breastfeeding mothers with nondeficient vitamin D status given 4000 IU daily of vitamin D, average milk levels were approximately 130 IU/L after 3 months of supplementation. Based on these reported milk levels, an exclusively breastfed infant will ingest only 8 to 12 IU/kg daily of vitamin D. Human vitamin D deficiency is defined as a 25-OH-vitamin D serum level 20 mcg/L or >50 nmol/L) taking 400 to 2000 IU daily have an average or median reported total vitamin D antirachitic activity in their milk of 50 to 80 IU/L. The clinical relevance of this effect on the total vitamin D antirachitic activity in milk has not yet been determined.

Detection of this isomer by standard vitamin D quantification techniques can overestimate the true 25-OH-vitamin D 3 level. The low potency C-3 stereoisomer of 25-OH-vitamin D 3 is also present in milk and blood. Dihydroxylated metabolites are also present in milk, but in such low quantities that their contribution to total milk vitamin D activity is not routinely accounted for. The major forms of vitamin D present in breastmilk are cholecalciferol (vitamin D 3), ergocalciferol (vitamin D 2), and their respective 25-hydroxylates (25-OH) also known as calcidiols. Vitamin D partitions extensively into the fat fraction of whole milk after 1 to 2 days when stored under refrigeration.

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