Breast milk is nearly perfect — but it's missing one thing

Breast milk covers almost everything a baby needs — with one well-known gap: vitamin D. Even a healthy, well-nourished parent tends to have low levels of it in their milk, so exclusively and partially breastfed babies usually don't get enough from feeding alone. Vitamin D matters because it helps the body absorb calcium and build strong bones; a serious shortage can lead to rickets, where soft bones bend under a growing child's weight.

The old fix was sunlight, since skin makes vitamin D from UV rays. But babies burn easily, sunscreen (rightly) blocks the same rays, and pediatricians advise keeping infants out of direct sun — so sun isn't a reliable source. That's why the American Academy of Pediatrics recommends giving breastfed and partially breastfed infants a daily 400 IU vitamin D supplement, usually as a few drops, beginning in the first days after birth.

Formula-fed babies are a different story: infant formula is already fortified with vitamin D. Once a baby is drinking about a liter (roughly 32 oz) of formula a day, they're generally getting enough without extra drops. If your baby mixes breast and bottle, it's worth asking your pediatrician whether drops still make sense.