Other experts worry about the potential harm that could result if children consume excess vitamin D. Like vitamins A and E, vitamin D is fat-soluble, so it is stored in the body's fat cells instead of eliminated in the urine. According to the National Institutes of Health, too much vitamin D can cause nausea, vomiting, constipation and weakness.
"Meeting the need is great, but exceeding the need is not always good," said Madelyn Fernstrom, professor and director of the UPMC Weight Management Center in Pittsburgh. "Especially with infants and toddlers, I think we should proceed with caution."
Keith Ayoob, associate professor of pediatrics at Albert Einstein College of Medicine in New York, agrees that routine supplementation is unnecessary at this point.
"The first line of defense is to find out why kids are not getting enough vitamin D and correct that," he said.
Ayoob and Fernstrom suggest that this deficiency is primarily a lifestyle problem, resulting from the fact that children are not consuming enough vitamin D to meet the guidelines.
"It doesn't matter how great a recommendation is if a parent is not using it," Fernstrom said.
In the study, low vitamin D levels were linked to lack of milk intake for many of the children. Therefore, Ayoob said dietary deficiencies should be addressed first.
"If you supplement them, they're still going to have a bad diet," he said. "They're going to have a very well-supplemented bad diet."
Fernstrom suggests a few simple changes that might remedy the problem: Giving toddlers vitamin-D fortified milk instead of juice and making sure at least a small part of the body, such as the hands or legs, are exposed to sunlight for about 30 minutes a day.
Experts do agree that one group should be routinely supplemented: infants who are breast-fed, as breast milk just doesn't have enough vitamin D.
The next step? Dr. James Taylor, professor in the department of pediatrics at the University of Washington, said a long-term study is needed to determine the lasting effects of vitamin D deficiency on infants and toddlers. Researchers should also look at whether supplements have positive effects on bone density.
"There's a lot of research on vitamin D right now," Taylor said. "I think we're not to the final answer yet. This study is just a piece of the puzzle."