Can diet protect against asthma?

Researchers found zinc, vitamins A, D, and E, as well as fruits and vegetables, seemed to have a protective role, but they stress the data are too preliminary to draw any firm conclusions.

"The short answer is that it is too early to tell," said Dr. Graham Devereux of the University of Aberdeen in the UK, who worked on the study.

As a result, pregnant women and parents should not change their diets solely for the purpose of protecting their kids from allergies, Devereux added in an e-mail to Reuters Health.

"Currently there is no evidence that changing diet makes any difference," he said.

More than seven percent of adult Americans, and even more kids, have asthma, and the lung disease has been on the rise in recent decades for unknown reasons.

According to the U.S. Centers for Disease Control and Prevention, it now causes more than 13 million annual visits to emergency rooms and doctors' offices.

Devereux and his colleagues, whose findings appear in the Journal of Allergy and Clinical Immunology, reviewed 62 recent studies that looked at diet and risk of allergy and asthma.

All studies were based on records of women's diets and other observations, a much weaker design than clinical trials in which people are given supplements or foods to eat.

Of the 22 studies that looked at fruit and vegetable intake, 17 linked healthier diets to lower risks of asthma and allergies. And two reports suggested that children with higher levels of vitamin A in their bodies had a 75 percent lower risk of developing asthma.

Pregnant women who ate a lot of vitamins D -- found in fatty fish -- and E -- found in nuts and seeds -- were between 30 and 40 percent less likely to have a child who wheezed, often a sign of asthma.

Also, sticking to a Mediterranean diet -- rich in vegetables, fish and monounsaturated fats from olive oil and nuts, but low in saturated fat from meat and dairy -- during pregnancy was also tied to a drop of nearly 80 percent in babies' risk of wheezing.

But the studies didn't find any apparent benefits from vitamin C or selenium.