Aug. 3, 2009 -- Seven out of 10 U.S. children have too-low vitamin D levels, putting them at risk of heart disease, rickets, and weak bones.

Nearly one in 10 kids -- 7.6 million American children -- are actually deficient in vitamin D. Low vitamin D is risky, but vitamin D deficiency is a serious health threat in which the body begins to reabsorb calcium from the skeleton.

The new findings come in a study by Juhi Kumar, MD, MPH, of Montefiore Medical Center; Michal Melamed, MD, of Albert Einstein College of Medicine; and colleagues.

"We expected the prevalence of vitamin D deficiency would be high, but the magnitude of the problem nationwide was shocking," Kumar says in a news release.

Rickets, a bone disease of infants caused by too little vitamin D, has been on the rise. It's not the first time: A U.S. epidemic of rickets at the end of the 1800s ended only when the U.S. fortified milk with vitamin D.

It's now apparent that more must be done. People get vitamin D from foods like milk and fish, but it's hard to get enough from diet alone. The body makes its own vitamin D, but only when a person gets at least 10 minutes of direct sunshine a day, before putting on sunscreen.

Most people need regular vitamin D supplements. Indeed, the Kumar study found that children who took vitamin D supplements were least likely to have low vitamin D levels. But only 4% of kids get these supplements.

Some kids were at particularly high risk of low vitamin D levels:

Older children

Girls

African-American children

Mexican-American children

Obese children

Kids who drank milk less than once a week

Kids who spent more than four hours a day watching TV, playing video games, or using computers

Melamed noted that the widespread use of sunscreens keeps kids from getting vitamin D from sunlight.

"It would be a good idea for parents to turn off the TV and send their kids outside," she says in the news release. 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 the sun damage."

The Kumar study is an analysis of data gathered in the 2001 to 2004 National Health and Nutrition Examination Survey (NHANES) of a nationally representative sample of 6,275 children aged 1 to 21.