Washington ? Millions of U.S. children have disturbingly low vitamin D levels, possibly increasing their risk for bone problems, heart disease, diabetes and other ailments, according to two new studies that provide the first national assessment of the crucial nutrient in young Americans.

About 9 percent of those ages 1 through 21 — about 7.6 million children, adolescents and young adults — have vitamin D levels so low they could be considered deficient, while another 61 percent — 50.8 million — have higher levels, but still low enough to be insufficient, according to the analysis of federal data being released today.

“It’s astounding,” said Michal Melamed of the Albert Einstein College of Medicine in New York, who helped conduct one of the studies published online by the journal Pediatrics. “At first, we couldn’t believe the numbers. I think it’s very worrisome.”

Low vitamin D levels are especially common among girls, adolescents and people with darker skin, including African Americans, according to the analysis of a nationally representative sample of more than 6,000 children. For example, 59 percent of black teenage girls were vitamin D deficient, Melamed’s study found.

The researchers and others blamed the low levels on a combination of factors, including children spending more time watching television and playing video games instead of going outside, covering up and using sunscreen when they do go outdoors, and drinking more soda and other beverages instead of consuming milk and other foods fortified with vitamin D.

The analysis and an accompanying federal study also found an association between low vitamin D levels and increased risk for high blood pressure, high blood sugar and a condition that increases the risk for heart disease and diabetes, known as the metabolic syndrome.