Posted May 18, 2007, this is the second of a two-part series.
Part I: "Childhood Vitamin D—A Dark Side?" is available here.
Asthma incidence has been rising among people of all ages, but especially children. According to the Centers for Disease Control and Prevention, its prevalence among youngsters rose from 3.6 percent in 1980 to 6.2 percent in 1996. It's the third leading cause of hospitalization in U.S. children, and treatment costs for the potentially lethal disease now run some $3.2 billion a year. There are currently no preventive measures or cure. But two new studies suggest ensuring that children get plenty of the sunshine vitamin might limit its toll.
The idea that vitamin D might be helpful came to Carlos A. Camargo Jr., of the Harvard Medical School during a colleague's lecture. While looking at a U.S. map that displayed the geographical distributions of vitamin D deficiency and a cancer it's been linked to, he recalls, "I realized it also looked like a map of asthma."
His group had already been at work on a study probing for possible dietary links to asthma—such as low consumption of fish or antioxidants—among 1,194 Boston-area youngsters. But during the lecture, he says, the proverbial light bulb switched on.
He thought: Which segment of the population makes the least vitamin D? Answer: people in northern states such as Massachusetts, where, for much of the year, sunlight isn't strong enough to trigger the vitamin's production by skin. Who consumes the least vitamin D? Blacks, the same ethnic group that makes the least vitamin D, because their heavily pigmented skin screens out much of the ultraviolet light needed to trigger vitamin D production. Blacks also experience the highest rates of asthma.
Immediately following the lecture, Camargo decided to investigate vitamin D intake among the children his group had been studying. "And boom! There it was," he told Science News Online. Children with frequent bouts of wheeze—an early indicator of asthma—were far more likely to have had mothers whose vitamin D intake during pregnancy fell well below average.
Some members of Camargo's team worked with researchers in Scotland. After seeing the link in their Boston data, these scientists asked their colleagues at the University of Aberdeen to investigate whether a similar association existed in their data.
To their great surprise, Camargo says, the Scottish researchers turned up the same trend as seen in the Boston children. Both groups wrote up their findings and submitted them simultaneously to the same journal. Those papers both appeared in the March American Journal of Clinical Nutrition.
Why look at wheeze?
Asthma is a chronic lung disease characterized by intermittent episodes of airway obstruction that lead to wheeze, cough, and shortness of breath.
Unfortunately, Camargo notes, "It's really difficult to diagnose asthma in very young children." They can't perform the diagnostic tests, such as one in which they deliberately breathe as hard and as long as they can into a machine. That's why physicians either put off a diagnosis until children become old enough to perform the exhalation tests—or offer a preliminary diagnosis on the basis of symptoms such as wheezing.
The Boston team used wheeze as a marker for potential asthma in the children it studied from birth through age 3. These kids came from middle-class homes and most of the mothers had a college education. Their average vitamin D intake during pregnancy was about 550 international units (IU) per day—above the 300 IU/day average for U.S. women. The highest intake—1,145 IU/day—was nearly 20 times as great as the lowest intake.
The researchers periodically asked moms to describe respiratory illnesses that their youngsters had recently experienced. The researchers categorized a child as potentially asthmatic if his or her parents had taken the child to a doctor at least once a year for some illness that involved wheezing.
"If you don't want to call this asthma, that's fine," Camargo says. "But we're telling people more and more that if they have recurrent wheezing, it might very well be asthma."
In the Boston study, a 100-IU increase in a mom's daily vitamin D intake during pregnancy corresponded to a 20 percent reduction in risk of recurrent wheezing events in her child. This held up whether that vitamin D had come from diet or from supplements. The researchers didn't have data on the children's sun exposure, so they don't know how much of the vitamin the kids' bodies made from sunlight. But young, middle-class children don't get much sun these days, Camargo notes, owing to their parents' propensity to slather them with sunscreen creams before letting them play outside.
In Scotland, Graham Devereux's team in Aberdeen collected data on a woman's diet during her eighth month of pregnancy. When the 1,200 children these researchers were following reached age 5, the team compared a mom's vitamin D intake with her child's history of wheeze-related respiratory illnesses. Here, too, moms whose pregnancy diets had been richest in the sunshine vitamin were least likely to have a child that experienced wheezing events.
In fact, their kids were only a third as likely to experience recurrent wheeze as were children whose moms' vitamin D intakes during pregnancy had been in the bottom 20 percent.
What's behind the benefit?
There's no way to know, from these epidemiological studies, what mechanism might make vitamin D protective against respiratory disease, Camargo says. However, he notes that a spate of recent studies have found that a deficiency in the vitamin appears to render people more susceptible to autoimmune disease (SN: 10/9/04, p. 232) and infections (SN: 11/11/06, p. 312). So, he speculates, any protection afforded by the vitamin may reflect some immune enhancement.
A study published about a year and a half ago offers further support to the idea that vitamin D is good for lung health. Its authors analyzed data collected from a representative sampling of 14,000 U.S. adults. Among these people, consumption of ample amounts of vitamin D was correlated with an individual's ability to breathe deeper and more easily. This trend held even for people suffering from chronic obstructive pulmonary disease (see Breathing Easier with Vitamin D).
As for Camargo's group, they're going to continue studying their youngsters. By the time these children reach age 5, the Boston researchers may be able to validate whether the wheeze they're tallying now was indeed a marker for asthma risk in the low-D group.
Part I: "Childhood Vitamin D—A Dark Side?" is available at Childhood Vitamin D—A Dark Side?.
If you would like to comment on this Food for Thought, please see the blog version.
Carlos A. Camargo Jr.
Center for D-Receptor Activation Research
Massachusetts General Hospital
Harvard Medical School
326 Cambridge Street, Suite 410
Boston, MA 02114
Department of Environmental and Occupational Medicine
University of Aberdeen
Aberdeen, Scotland AB25 2ZP
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