Only 5 to 37 percent of American infants meet the standard for vitamin D set by the American Academy of Pediatrics.  Vitamin D strengthens bone and the immune system and also appears to prevent type 1 diabetes, heart disease and cancer.

Although breast milk is the perfect food in every other way, it's often low in vitamin D. Since humans originated in equatorial areas with year-round sunshine, babies in the distant past wouldn't have needed to get vitamin D from breast milk.

Many mothers also are vitamin D-deficient.