Breast milk may not be enough.

A new study finds a high incidence of vitamin D deficiency in breast-fed babies, mostly during winter. Such a deficiency limits the body's use of calcium, which is essential for healthy bones and teeth.

As part of a trial of iron supplementation, Ekhard E. Ziegler of the University of Iowa in Iowa City and his colleagues regularly took blood samples over 2 years from 84 newborns who were initially breastfed exclusively. The researchers noticed that few infants were getting supplemental vitamin D.

The scientists evaluated vitamin D in the infants' blood. They report in the August Pediatrics that 78 percent of breastfeeding youngsters not receiving vitamin D in supplements were deficient in that nutrient during winter, but only 4 percent showed the deficiency in summer. None of the 49 infants getting vitamin supplements showed the deficiency at any time.

Iowa's northerly location keeps its residents from getting enough sun exposure in winter to produce much of the vitamin in their skin, the researchers note (SN: 10/16/04, p. 248). Although breast milk delivers vitamin D, mothers in the study were probably deficient in the vitamin during winter.

The current recommended dietary intake of 200 international units per day for nursing women isn't enough, says study coauthor Bruce W. Hollis of the Medical University of South Carolina. His earlier research (www.sciencenews.org/articles/20050430/food.asp) suggested that "lactating women need about 6,000 international units a day to ... supply adequate amounts to a nursing infant," he says.