People who don't get enough vitamin D may be at increased risk for asthma, a new study suggests.

In the study, researchers analyzed information collected from more than 25,000 adults ages 18 to 79, and more than 9,700 children ages 6 to 17, who took part in a yearly U.S. national health survey conducted between 2001 and 2010. The participants were asked whether they had been diagnosed with asthma or experienced wheezing (a symptom of asthma) in the past year. The participants also had a test to determine the level of vitamin D in their blood.

Overall, of the study participants, 68 percent of the children and 70 percent of the adults had levels of vitamin D that were lower than what's usually considered adequate for healthy people (30 nanograms per milliliter), which is known as vitamin D insufficiency. In addition, about 1,200 children and 1,800 adults had been diagnosed with asthma. The disease involves inflammation and a narrowing of the airways, both of which make it difficult to breathe.

Children with vitamin D insufficiency were 1.35 times more likely to have asthma compared with children with adequate levels of vitamin D, the researchers found. Adults with vitamin D insufficiency were not at increased risk for an asthma diagnosis, but they were more likely to say they experienced wheezing in the past year, compared with those who had adequate levels of vitamin D. [9 Good Sources of Disease-Fighter Vitamin D]

The exact reason behind the link is not known, but it's thought that vitamin D decreases levels of inflammation in the body, said Yueh-Ying Han, a research assistant professor in pulmonary medicine, allergy and immunology at the University of Pittsburgh School of Medicine, who presented the new findings this week at the meeting of the American Public Health Association in Denver. It's also possible that vitamin D improves people's response to drugs that treat asthma, Han told Live Science.

The researchers also found that the portion of people with vitamin D insufficiency dropped from about three-quarters of participants in 2001 to about two-thirds of participants in 2010. Around that same time, the prevalence of asthma also dropped, from 8.2 percent in 2007-2008 to 7.4 percent in 2009-2010.

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The findings agree with previous studies that found a link between vitamin D levels and asthma. For example, some studies have found that children whose mothers consume higher amounts vitamin D during pregnancy have a lower risk of asthma than children whose mothers consume lower amounts of vitamin D. Other studies have found that, among kids with asthma, those with low vitamin D levels have worse symptoms than those with higher vitamin D levels.Still, because the new study was conducted at a single point in time, it cannot prove that low vitamin D levels cause asthma. Future studies are needed to determine if providing vitamin D supplements to children with asthma can lead to improvements in their symptoms, Han said.

In fact, the researchers are currently conducting a study in which children with asthma who have vitamin D insufficiency are given either a daily vitamin D supplement or a placebo. The researchers want to see whether vitamin D supplements reduce asthma attacks or hospital visits due to asthma.

A review study published in September found that vitamin D supplements lowered the risk of asthma attacks in children and adults with the condition, but did not seem to improve daily symptoms of asthma.

It's important to note that people with asthma should not take vitamin D as a replacement for their current asthma treatment, Han said. But because vitamin D supplements are relatively safe, they are recommended for general health in people who do not get adequate levels of this vitamin, Han said.

Original article on Live Science.

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