Dec. 27, 2010 -- Infants at age 3 months who had newborn blood levels of 25-hydroxyvitamin D -- a measurement of vitamin D -- below 25 nanomoles per liter (nmol/L) were twice as likely to develop respiratory infections as infants who had levels at 75 nmol/L or higher, according to an international study.
That finding is based on umbilical cord blood samples taken from more than 900 infants to measure blood vitamin D levels. Earlier research has suggested that mothers who have higher levels of vitamin D in their blood during pregnancy are more likely to give birth to infants who are at a lower risk for wheezing.
Investigators led by Carlos Camargo, MD, DrPH, an associate professor of medicine and epidemiology at Harvard Medical School, examined whether vitamin D levels in the infants’ umbilical cord blood were associated with risk for respiratory infections, wheezing, or asthma.
Camargo and researchers from New Zealand analyzed data from the New Zealand Asthma and Allergy Cohort Study, which followed more than 1,000 children in the cities of Wellington and Christchurch.
Umbilical cord samples were available from 922 infants. Most infants were born to term at 40 weeks, and the average was about 3.6 kilograms or about 7 pounds and 9 ounces.
Mothers were also frequently interviewed about their children’s history of asthma, wheezing, and respiratory infection from age 3 months until the children turned 5 years old. Very few children in the study took vitamin supplements; their vitamin D status came mostly from sunlight exposure.
Researchers found that:
- About 20% of infants had 25-hydroxyvitamin D levels below 25 nmol/L, which is considered below normal vitamin D levels.
- The average level of 25-hydroxyvitamin D was 44 nmol/L, which was still considered low.
- Lower vitamin D levels were more common among children born in the winter, children of lower socioeconomic status, children who had family histories of asthma and smoking and who had been exposed to secondhand smoke at an early age.
- Low vitamin D levels were associated with wheezing and respiratory infection, but not associated with being diagnosed with asthma. The findings do not establish cause and effect.
The study is published in the January issue of Pediatrics, a journal of the American Academy of Pediatrics.
Benefits of Vitamin D
Vitamin D, found abundantly in fortified dairy products such as milk and cheese, is known for helping children build strong bones, but it can also help bolster the immune system, researchers note. Vitamin D is also produced by the body when exposed to sunlight.
Acute respiratory infections are a major health problem in children, the researchers say. Bronchiolitis, in particular, is a leading cause of hospitalization among infants in the U.S. Researchers want to evaluate whether vitamin D supplements could provide some benefit.
"Our data suggest that the association between vitamin D and wheezing, which can be a symptom of many respiratory diseases and not just asthma, is largely due to respiratory infections," Camargo says in a news release. "There's a likely difference here between what causes asthma and what causes existing asthma to get worse. Since respiratory infections are the most common cause of asthma exacerbations, vitamin D supplements may help to prevent those events, particularly during the fall and winter, when vitamin D levels decline and exacerbations are more common. That idea needs to be tested in a randomized clinical trial, which we hope to do next year."