Breastfed infants are routinely supplemented with Vitamin D to prevent deficiency because human breast milk typically contains insufficient amounts of this vitamin. Vitamin D is essential for calcium absorption and bone mineralization, and deficiency can lead to rickets, characterized by bone softening and deformities. Since sunlight exposure may be limited or inadequate, supplementation ensures adequate serum levels of 25-hydroxyvitamin D. Routine supplementation is recommended starting soon after birth to promote healthy skeletal development.

Reference: Nelson Textbook of Pediatrics, Kliegman et al., 21st Edition.