Vitamin D, 25-Hydroxyvitamin D, is a fat-soluble vitamin that promotes calcium absorption, fortifying bones and teeth, and maintaining the level of calcium and phosphorus in the blood.

Vitamin D deficiency can result in conditions such as rickets in children (usually <11 ng/mL), and osteomalacia and osteoporosis in adults (typically levels <25 ng/mL). Low levels of vitamin D also increase the risk of bone and heart disease, and are also associated with higher blood pressure and lower calcium and HDL cholesterol levels in later life.

Based on the information obtained from the survey, researchers found that, of more than 6,000 of children followed, nine percent (equal about 7.6 million) were vitamin D deficient, and a greater number were found to be getting an insufficient amount—61%, representing 50.8 million U.S. children. Only 4% received the recommended daily amount of vitamin D.

Children classified as “insufficient” had 15-29 ng/mL, while those who were classified as “deficient” had levels of 15 ng/mLor less. Normal range, according to the National Institute of Health, is 30-74 ng/mL.

The study is the largest of its kind to date, gaining nationwide rather than just regional data. "We expected the prevalence of vitamin D deficiency would be high,” says lead author Juhi Kumar, M.D., M.P.H., of the Children's Hospital at Montefiore Medical Center, “but the magnitude of the problem nationwide was shocking."

Vitamin D deficiency is more common for female children, those that are older, obese, drink less than one glass of milk week, and those who spend four hours or greater on the computer or watching television per day. African-American and Mexican-American children also had a greater risk.“It’s very hard to get enough vitamin D from dietary sources alone,” says Dr. Michal Melamed of the Albert Einstein College of Medicine in Bronx, New York.

Melamed, co-leader of the study, attributes part of this problem to lifestyle, recommending parents send their children outdoors. One source of vitamin D is UV-B sunlight, which converts cholesterol in the skin into vitamin D. She also recommends ditching the sunscreen for a while: “Just 15 to 20 minutes a day should be enough…don't put sunscreen on them until they've been out in the sun for 10 minutes, so they get the good stuff but not sun damage.” Individuals with darker skin and those who live in northern regions with less sun exposure should spend more time outdoors.

The American Academy of Pediatrics recommends that children and teens get at least 400 IU per day, double the previous recommendation of 200IU per day. Good sources of vitamin D include fish, milk, eggs, cod liver oil, and fortified foods. Vitamin D levels can also be increased with the use of supplements.

The study's abstract is available online, and will be published in September's issue of Pediatrics.