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"Teens that have the lowest intake of fruits, vitamin C and omega-3 fatty acids tended to have lower pulmonary function and reported more respiratory symptoms than those with higher intake," said study author Jane Burns, a research fellow at the Harvard School of Public Health in Boston.
"This is a time in their lives when they should all have good lung function, and they may not be obtaining optimal lung function. This may affect their lung function later in life," Burns added.
Results of the study are published in the July issue of Chest.
About 20 million Americans -- 9 million of them children -- have asthma, according to the U.S. National Heart, Lung, and Blood Institute. While the exact cause of the disease is still unknown, doctors do know that underlying inflammation of the airways is an important factor in the disease. Preventive treatments for asthma are aimed at reducing that inflammation.
Burns and her colleagues included more than 2,000 12th-grade students from 12 areas in the United States and Canada in the current study. Over the course of a year, they asked the teens about their diets, their general health and respiratory symptoms, and also tested the adolescents' lung function.
Most of the teens were white, about one-third were overweight, and nearly three-quarters didn't take a daily multivitamin. About one-quarter of the adolescents studied smoked.
Many of the teens -- about one-third -- had dietary shortfalls when it came to consumption of fruits, vegetables, vitamins A and E, beta-carotene and omega-3 fatty acids. A whopping 86 percent didn't consume the recommended five to nine servings of fruits and vegetables each day.
The researchers also found that consumption of less than one-quarter of a serving of fruit daily resulted in lower average lung-function scores. A low intake of vitamin E -- less than 5.2 milligrams daily -- was associated with a increased risk of reported asthma.
Finally, a low intake of omega-3 fatty acids -- less than 22 milligrams per day -- was associated with increased odds of chronic bronchitis, wheezing and asthma. In fact, those with the lowest intake of omega-3 fatty acids were almost 70 percent more likely to report have asthma symptoms.
There was a slight increase in the adjusted odds -- about 6 percent -- of developing chronic bronchitis or acute bronchitis in teen smokers who averaged less than 85 milligrams per day of vitamin C daily compared to smokers with the highest vitamin C intake.
Burns said she believes healthy diets help airways in several ways. One is by lessening inflammation, and the other stems from the helpful effects of antioxidants.
"You really are what you eat," added Dr. Jane Krasnick, chief of allergy and immunology at St. John Macomb-Oakland Hospital in Warren, Mich. "What you put in your body may make a difference. We spend a lot of time worrying about what we inhale and environmental exposures, maybe we should think more about what we're putting into our bodies [as food] as well," she said.
Both Burns and Krasnick recommended that teens up their intake of fresh fruit and fish whenever possible. Burns added that teens should also take a daily multivitamin. While a multivitamin isn't as good as healthy food, "vitamins are very useful and easy to take," she said.
SOURCES: Jane Burns, Sc.D., research fellow, Harvard School of Public Health, Boston, Mass.; Jane Krasnick, M.D., chief, allergy and immunology, St. John Macomb-Oakland Hospital, Warren, Mich.; July 2007 Chest
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