About 70 percent of U.S. children have low levels of vitamin D, which puts them at higher risk for bone and heart disease, researchers said today.

"We expected the prevalence of vitamin D deficiency would be high, but the magnitude of the problem nationwide was shocking," said Dr. Juhi Kumar of Children's Hospital at Montefiore Medical Center.

Cases of rickets, a bone disease in infants caused by low vitamin D levels, have also been increasing, other research shows.

The new finding, from a nationwide study, adds to growing evidence that children as well as many adults also lack the vitamin.

"Several small studies had found a high prevalence of vitamin D deficiency in specific populations of children, but no one had examined this issue nationwide," said study leader Dr. Michal L. Melamed of the Albert Einstein College of Medicine of Yeshiva University.

The cause? Poor diet and lack of sunshine, the researchers conclude today in the online version of the journal Pediatrics.

Millions of Children

The researchers analyzed data on more than 6,000 children, ages 1 to 21, collected by the National Health and Nutrition Examination Survey 2001-2004.

The researchers found that 9 percent, or 7.6 million children across the country, were vitamin D deficient and another 61 percent, or 50.8 million, were vitamin D insufficient.

Low levels were especially common in girls, African-Americans, Mexican-Americans, the obese, those who drank milk less than once a week, and those who spent more than four hours a day watching TV, playing videogames, or using computers. The deficiency was more common among the older children in the data set, too.

Lighter skin is more efficient at producing vitamin D. So darker-skinned people produce less when exposed to sunlight.

The decline in vitamin D levels in the United States was reported widely a year ago and has been underway for 20 years, Melamed said.

"Kids have more sedentary lifestyles today and are not spending as much time outdoors," Melamed said. "The widespread use of sunscreens, which block UV-B rays, has only compounded the problem."

The body uses UV-B sunlight to convert a form of cholesterol in the skin into vitamin D.

What to Do

Melamed recommends that children should consume more foods rich in vitamin D, such as milk and fish. "But it's very hard to get enough vitamin D from dietary sources alone," she said.

Vitamin D supplementation can help. In the study, children who took vitamin D supplements (400 IU/day) were less likely to be deficient in the vitamin. However, only 4 percent of the study population actually used supplements.

The American Academy of Pediatrics, which recently updated its vitamin D guidelines, now recommends that infants, children, and teens should take 400 IU per day in supplement form.Supplements are especially important for children living in northerly regions where the sun may be too weak to maintain healthy vitamin D levels. Supplements are also critical for infants who are breast-fed, the researchers said in a statement today. Breast milk contains relatively little vitamin D, while formula is fortified with the vitamin.

What else can parents do?"It would good for them to turn off the TV and send their kids outside," Melamed said. "Just 15 to 20 minutes a day should be enough. And unless they burn easily, 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."

Other experts caution that extended exposure to the sun - tanning and burning - increases the risk of deadly skin cancer.