New data suggests that a lack of vitamin D can play a role in diabetes, autoimmune diseases, and some cancers. As a result, the American Academy of Pediatrics is doubling the amount of vitamin D it recommends for infants, children, and adolescents to 400 IU a day, starting in the first few days of life. The academy recommends vitamin D supplements for breast-fed infants and partially breast-fed infants. The supplements are also recommended for non-breast fed babies and children who drink less than1 liter per day of vitamin D-fortified formula or milk. Earlier in 2008, researchers at Children's Hospital Boston discovered "suboptimal" levels of vitamin D in 40 percent of 380 otherwise healthy infants and toddlers. In the past, the chief source of vitamin D used to be sunlight, which allows vitamin D to be manufactured in the skin, but doctors say many people are now not outdoors long enough to get the right amount of sunlight, or they cannot absorb sunlight because of sunscreen use. Doctors do not recommend that people sunbathe or visit tanning salons, however, because there is no right amount for a given individual.

(http://www.boston.com/news/health/articles/2008/10/13/doctors_double_vitamin_d_for_children/)