Despite the important role of vitamin A in lung development, researchers have found that giving the nutrient to pregnant women or preschoolers in Nepal doesn't protect kids against asthma.
But the findings don't mean vitamin A isn't important, especially in regions where vitamin deficiencies are common, according to the scientists.
Women taking vitamin supplements had a lower chance of dying during pregnancy, for instance. And those who took vitamin A while pregnant had kids with larger lungs, which have been linked to better survival.
"We're kind of narrowing down what the effect of vitamin A is," said Dr. William Checkley, from Johns Hopkins University in Baltimore, who worked on the study.
The lungs need vitamin A as they are developing and the nutrient is also involved in keeping lung tissue healthy over time, the researchers explain in the European Respiratory Journal.
In addition, previous studies hinted that people with lower levels of vitamin A in their blood are more likely to have asthma. But those kinds of studies, called observational studies, can't tease out cause and effect. Checkley and his colleagues wanted to see if by adding vitamin A to kids' or pregnant women's diets, they might lower the children's risk of asthma.
So the team followed up on two different trials that gave vitamin A or vitamin-free placebo pills to Nepalese women or kids.
The studies involved more than 5,000 kids and young adults, age nine to 23, who had gotten vitamin A or a placebo as preschoolers, or whose mothers had done so before and during pregnancy. All of them were living in an area of rural Nepal where vitamin deficiency is common.
Researchers asked all the kids if they had problems with wheezing or coughing or had ever had asthma. They also tested how well the kids' lungs were working using a device called a spirometer.
Between zero and two percent of the kids said they had had asthma at some point, and less than one percent currently did so — with no differences between the placebo and vitamin groups.
There were no differences in how many kids reported wheezing or coughing in the two groups either, or in how well their lungs worked.
Still, Checkley said the findings might have looked different in another location.
"The effect of vitamin A may vary as to the setting," he told Reuters Health. "The prevalence (of asthma) was low in Nepal."
In the U.S., for example, nearly 10 percent of kids are diagnosed with the disease.
It's possible that in an urban area where asthma is more common to begin with, giving pregnant moms or kids vitamin A may better protect kids against asthma, Checkley said.
It's also not clear how the findings would apply to a population where vitamin A deficiency wasn't such a problem.
More than 300 million people worldwide have asthma, Checkley said, and increases in asthma rates have put researchers on a search for possible culprits. Pollution and allergies have been linked to asthma, and food and nutrition are other targets of investigation.
"Obviously diet is still one of those questions — is it important or not?" Checkley said.
Researchers are still wondering, "Can we prevent or reduce the risk of asthma by giving (vitamin A) supplements?"
So far, his work suggests the answer is no — at least in this group of kids, in one part of the world.