You can get vitamin D from standing in the sun—your skin makes the vitamin in response to UV light—or from your diet, but recently, researchers have found evidence of vitamin D deficiency in many adults. Not as much is known about vitamin D levels in children, so researchers from Harvard Medical School measured vitamin D in kids at Children's Hospital Boston.

What the researchers wanted to know: Is vitamin D deficiency common among healthy adolescents?

What they did: Three hundred and seven patients ages 11 to 18 were recruited when they had routine physical exams at Children's Hospital Boston between 2001 and 2003. Patients who were visiting because they were sick weren't eligible for the study. Participants filled out questionnaires on their eating and exercise habits; researchers used those to figure out how much vitamin D the children were getting and how much time they spent outside. Each patient also had 15 milliliters of blood taken to be analyzed for vitamin D, calcium, phosphorus, magnesium, and a hormone.

What they found: Nearly a quarter of the adolescents didn't have enough vitamin D in their blood. Vitamin D deficiency was most common among black adolescents, and more common in winter and spring than summer and fall. Darker skin may block UV light from reaching the part of the skin where vitamin D is synthesized. Kids who drank a lot of soft drinks and juice also had lower vitamin D levels; this may be related to the trend for children to drink less milk. Overweight adolescents were also more likely to be low on vitamin D.

What it means to you: Vitamin D is important for bone growth, and kids with dark skin or who live in a northern place like Boston may not be getting enough vitamin D.So perhaps parents ought to consider giving their kids vitamin D-fortified milk or vitamins, or insisting their kids spend more time in the sun. (But not so much that they're in danger of skin cancer.)

Caveats: The kids in the study weren't a random sample; researchers took any children who walked in the hospital, fit the criteria for the study, and agreed to take part. The study group happened to have more who were at higher risk for vitamin D deficiency, including black, Hispanic, and overweight kids. The study only found a correlation between vitamin D and race; it can't show that one causes the other. And the study depended on kids to be accurate on questionnaires about nutrition and exercise.

Find out more: Learn about Vitamin D from the National Library of Medicine and the National Institute of Health: http://www.nlm.nih.gov/medlineplus/ency/article/002405.htm.

Read the article: Gordon, C.M., et al. "Prevalence of Vitamin D Deficiency Among Healthy Adolescents." Archives of Pediatric and Adolescent Medicine. June 2004, vol. 158, pp. 531-537.

Free abstract: http://archpedi.ama-assn.org/cgi/content/abstract/158/6/531