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.

“I’m sure that most people would agree that pregnant women and children, with or without asthma, should eat a ‘healthy’ diet,” said Dr. Nancy Lange of Brigham and Women’s Hospital, who was not involved in the study.

But she added that the findings from this study “are not sufficient to suggest that any specific changes in diet will affect allergy or asthma risk, either increasing or decreasing it.”

What the field needs now, said Lange, is interventional studies, in which investigators control a person’s intake of specific nutrients, and note the effects.

“Diet and specific dietary elements - nutrients, foods, etc. - can be difficult to analyze because there are so many confounding factors, therefore it is difficult to say anything conclusively without results from interventional studies,” Lange said in an e-mail.

Ultimately, these studies may show diet has some impact on asthma risk, perhaps by affecting development of the lungs or immune system, reducing inflammation, or curbing the generation of free radicals, Lange added.

SOURCE: Journal of Allergy and Clinical Immunology, 2010.