A new study shows that diet may be a key culprit in asthma, a chronic swelling of the airways that affects some 20 million Americans, six million of them children.

Researchers report in the journal Chest that adolescents are more likely to experience respiratory problems if their diets are deficient in certain nutrients.

"Teens with the lowest intake of fruit, vitamins C and E, and omega-3 fatty acids had lower lung function and higher reports of respiratory symptoms such as cough and wheeze," says study author Jane Burns, an epidemiologist at Harvard School of Public Health in Boston.

Burns and her colleagues surveyed 2,115 U.S. and Canadian high school students from 16 to 19 years of age over the course of a school year about their eating habits—and, also, tested their lung function.

They discovered that teens who consumed less than 25 percent of one serving of fruit each day were more likely to have less efficient lung function than their compeers.

Burns and her colleagues speculate that fruit plays a role because it is rich in vitamin C—also associated with healthy lungs — as well as in flavonoids, antioxidants that hamper the production of free radicals. Free radicals are compounds composed of unstable atoms generated both by the body and environmental sources such as smoking and pollution that damage cells, possibly leading to increased disease susceptibility and aging. "The associations of fruit with pulmonary function and respiratory symptoms may be a result of a joint protective effect of flavonoids and vitamin C on airway and alveolar epithelium," the researchers wrote. Burns says that low levels of vitamin E and omega-3, meantime, were associated with a higher incidence of reported asthma. It did not seem to matter if the vitamins came from natural sources or supplements.

Lewis Smith, a professor of medicine at Northwestern University in Evanston, Ill., says this research confirms many previous studies showing the benefits of a healthy diet on respiratory function. The difficulty is determining which nutrients among the thousands in various foods are actually responsible for the health benefits, says Smith, who has conducted studies that link soy isoflavones (also powerful antioxidants) with a decrease in the severity of asthma symptoms.

"This is a very complicated disease [and] an awful lot of work needs to be done [to understand] the causes," Burns says. She is currently involved in two new studies—one probing the effects of air pollution and the other examining the synergistic effects of smog and diet on respiratory well-being. The bottom line: "We really have a pervasive problem with teens not meeting the recommended allowances [of foods and micronutrients]" Burns says. "A poor diet is definitely a factor affecting asthma symptoms."