Breastfed infants require supplementation with vitamin D (vD), but little is known about the necessary dose. This double blind trial evaluated four different doses of vD.

Exclusively breastfed infants (N = 213) were randomized at 1 mo to one of four doses, which they received through 9 mo while receiving no formula. The supplements provided daily 200 IU, 400 IU, 600 IU, or 800 IU of vD. The primary endpoint was plasma 25(OH)D level, and secondary outcomes were plasma parathyroid hormone and calcium, and illness incidence. The study was conducted during winter at 41° N.

Most infants had low (<50 nmol/l) 25(OH)D levels at 1 mo, but with supplementation levels rose. Overall, levels of 25(OH)D differed significantly in proportion to vD dose. There were no effects of vD on illness incidence or growth. Low levels were common, with 7.8% of levels being <50 nmol/l and 15 infants having 2 to 4 low levels.

The four doses of vD produced different plasma levels of 25(OH)D. The higher doses were somewhat more efficacious in maintaining vD sufficiency in breastfed infants. The findings support the recommended dose of 400 IU/d, and stress the need to start supplementation at birth.