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Pregnant women who get ample amounts of vitamin D may be protecting their unborn children from developing asthma later in life, a U.S. study suggests.

Researchers at Massachusetts General Hospital in Boston used questionnaires to assess vitamin D levels of women when they were pregnant.

They then matched this data with the children who were showing early signs of asthma -- recurrent wheeze -- by the age of 3.

"The higher the maternal level of vitamin D, the lower the child's risk [of asthma]" said Carlos Camargo, lead researcher of the study published in the American Journal of Clinical Nutrition.

Vitamin D is obtained from some foods, including oily fish such as salmon. It is also produced in the skin as a result of exposure to sunlight.

But, during winter months, the sun's rays just aren't strong enough to trigger vitamin D production.

"I think adults in northern latitudes should take vitamin D supplements," Dr. Camargo said in an e-mail.

"The optimal dose is not yet clear, but a recommendation of 800 to 1,000 International Units a day is safe and is known to increase blood levels of vitamin D during these winter months."

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