Many U.S. teenagers — including half of African Americans — would be considered vitamin D-deficient if the definition of deficiency were changed to what many experts recommend, a new study finds.

Right now, people are considered to have an overt deficiency in vitamin D when blood levels drop below 11 nanograms per milliliter (ng/mL), but there is debate over how the optimal vitamin D level should be defined.

Some experts consider a level of 30 ng/mL or higher to be desirable for overall health, and many argue that the cutoff for deficiency should be 20 ng/mL.

In the new study, published in the journal Pediatrics, researchers found that adopting the 20 ng/mL standard would push many more U.S. teenagers into the vitamin D-deficient category.

Using data from a government health survey of nearly 3,000 12- to 19- year-olds, they found that 14 percent would be deficient in vitamin D — compared with 2 percent when the current standard was applied.

What's more, 50 percent of black teenagers would be considered vitamin D-deficient, up from 11 percent under the current definition.

Also at elevated risk were overweight teens, who were twice as likely as their thinner peers to have a deficiency, according to Dr. Sandy Saintonge and colleagues at Weill Cornell Medical College in New York.

Vitamin D has been the subject of much research of late, including one recent study showing that 40 percent of U.S. babies and toddlers may have inadequate blood levels of the vitamin.

Vitamin D is necessary for healthy bone development and maintenance, and it also plays a role in nerve, muscle and immune system function. Some studies have linked low vitamin D levels to a higher risk of type 1 diabetes in children and, in adults, heart disease and certain cancers.

The latest findings, according to Saintonge's team, suggest that teenagers should take supplemental vitamin D, and possibly have their blood levels of the vitamin routinely checked — especially if they are at high risk of deficiency.

The American Academy of Pediatrics recommends that infants, children and teenagers get 400 IU of vitamin D each day. Milk, breakfast cereals and orange juice fortified with the vitamin are the main food sources, though some fatty fish naturally contain high amounts of vitamin D. Experts recommend vitamin pills for children who do not get enough of the vitamin from food.

Vitamin D is naturally synthesized in the skin when it is exposed to sunlight. This process is less efficient in people with darker skin, which is one reason African Americans are at higher risk of deficiency.

Overweight children and adults appear to be at elevated risk because vitamin D is stored in body fat. The more vitamin D that gets taken up by fat tissue, the less active vitamin there is in the blood.