People who use oral steroid medications may be at greater-than-average risk of a serious vitamin D deficiency, a new study suggests.

The findings, reported in the Journal of Clinical Endocrinology & Metabolism, do not prove that the drugs themselves are the cause.

But they do suggest that people on the medications should have their doctors check their blood levels of vitamin D, researchers say. And that may be especially important for children.

Steroid medications help control inflammation and are used for a number of medical conditions -- including asthma, certain types of arthritis such as rheumatoid arthritis and autoimmune diseases like Crohn's disease, lupus and multiple sclerosis.

In the new study, researchers found that among nearly 23,000 Americans in a government health survey, those using oral steroid medications were twice as likely as non-users to have a severe vitamin D deficiency.

Overall, 11 percent of those on steroids had a vitamin D level below 10 ng/mL -- which is considered too low to keep your bones, or the rest of you, healthy. That compared with five percent of study participants not on steroids.

Vitamin D levels that low can lead to a serious softening of the bones or muscle pain. Blood levels of about 20 ng/mL or above are considered sufficient for health.

"When doctors write that prescription for steroids and they're sending the patients for lab tests, they should also get the vitamin D level measured," lead researcher Dr. Amy L. Skversky, of Albert Einstein College of Medicine in New York, said in a news release from the university.

Skversky's team based their findings on a national health survey done between 2001 and 2006. It included 22,650 U.S. adults and children who had blood samples taken and reported on their medication use.

Just under one percent of the group said they'd used oral steroids in the past month -- which would translate to 2.1 million Americans likely taking the drugs nationwide.

The steroid users in the study were twice as likely to have a vitamin D deficiency, even after researchers accounted for several other factors that affect D levels -- including obesity, milk intake and vitamin D supplement use.

The link was especially strong among children. Steroid users younger than 18 were 14 times more likely to have a vitamin D deficiency than kids not taking the medications.

It's not clear that the medications themselves were to blame, or fully to blame. It's likely, according to Skversky's team, that the conditions the drugs treat contribute to low vitamin D levels.

Some of those disorders can lead to poor nutrient absorption, for example, or limit people's physical activity -- which may mean more time indoors, away from the sunlight that triggers the body's natural ability to make vitamin D.

On the other hand, there is evidence that steroid medications may lead to vitamin D deficiency, possibly by boosting an enzyme that curbs the vitamin's activity in the body.

The bottom line, according to Skversky, is that patients and doctors should be aware of the higher risk of vitamin D deficiency linked to oral steroids.

The latest recommendations from the Institute of Medicine, an advisory body to the U.S. government, are for most children and adults to get 600 IU of vitamin D per day. Adults older than 70 are advised to get 800 IU.

There are no special recommendations for people on steroid medication.

The sun is the major natural source of vitamin D. Food sources are relatively few and include fatty fish like salmon and mackerel, as well as dairy products and cereals that have added vitamin D. Multivitamins also contain vitamin D.

Not all steroids are taken orally. The study did not look at inhaled steroids, like those often used by people with asthma, so it's not clear if the findings would apply to those medications.