Eating a Mediterranean diet -- one high in fruits, fish, and vegetables and low in saturated fat -- is associated with a reduced likelihood of asthma in children, a large observational study reaffirmed.
Overall, choosing foods increasingly similar to a Mediterranean diet was associated with a lower prevalence of both wheeze and asthma, Dr. Gabriele Nagel, of Ulm University in Germany, and colleagues reported in the June issue of Thorax.
In contrast, children who ate burgers at least three times a week had increased odds of having asthma.
"Fast food is rich in industrially hydrogenated vegetable fats such as margarine and meat from ruminant animals which are dietary sources of trans-fatty acids," the researchers noted. "There is some evidence that dietary intake of trans-fatty acids is associated with asthma and atopy [allergic sensitivity]."
The findings support previous studies that identified links between increased consumption of fish, fruits, and vegetables with reduced asthma symptoms, although the researchers noted that causal connections could not be established.
"These findings show the need for further studies to determine if this type of diet in an interventional study will have the same effect," Dr. Wesley Burks, chief of pediatric allergy and immunology at Duke University Medical Center in Durham, N.C., commented in an e-mail.
"But," he added, "I would not deliberately change someone's diet based on this observation alone."
There is some biological plausibility regarding a link between the Mediterranean diet and risk of asthma and allergy, the study authors noted.
Certain fish are rich in omega-3 polyunsaturated fatty acids, which have anti-inflammatory properties and have been shown to counterbalance pro-allergic T helper (Th)2 activity. Fruits and vegetables are high in antioxidants, which have been shown to be inversely related to asthma in adults.
So, according to Dr. Stephen Cook, a pediatrician at the University of Rochester Medical Center in New York, "it would make some sense that higher levels of consumption of these [Mediterranean] diets would be beneficial, especially among those who would benefit the most from the protective effect."
Nagel and her colleagues looked at data from the second phase of the International Study on Allergies and Asthma in Childhood (ISAAC), a collection of cross-sectional studies performed at 29 centers in 20 countries.
Parental questionnaires were used to collect information on allergic diseases and diet from 50,004 children ages 8 to 12. Countries were divided into affluent and non-affluent.
After adjustment for sex, age, current exposure to environmental tobacco smoke, number of siblings, exercise, and maternal atopic disease, consumption of the following foods was associated with a lower prevalence of wheeze:
Consumption of meat in general was not associated with odds of having wheeze.
When the researchers looked at how often certain foods were eaten, they found that the odds of ever having asthma were lower among those who consumed fruit, raw green vegetables, and cooked vegetables at least three times a week.
The findings are consistent with previous studies in children, according to Nagel and colleagues.
Addressing the limitations of the study, Keith-Thomas Ayoob, of Albert Einstein College of Medicine in New York City, pointed out that there was no information on dietary supplements or consumption of certain food groups, such as dairy.
"It's very possible that kids whose diets lack adequate fruits and vegetables and fish are also lacking in certain nutrients, that they are overweight, that their diets lack calcium, and on and on," Ayoob said in an e-mail.
"There are missing pieces here," he continued, "so I would advise parents to make it a priority of ensuring that children get enough fruits, vegetables, and low-fat dairy foods on a daily basis, whether or not their kids have asthma."
The study was also limited by the retrospective collection of dietary information, which is subject to recall bias, and by the inability to adjust for total energy intake and body mass index, according to Nagel and her colleagues.
They and several physicians contacted about the study also noted that numerous comparisons were performed, apparently without statistical adjustment.
This "raises the likelihood of 'fluke' associations," according to Dr. David Katz, director of Yale's Prevention Research Center. "This is partly, but not entirely, mitigated by the findings being in accord with a priori hypotheses (i.e., the researchers finding what they were looking for)."