Researchers from Erasmus University in The Netherlands found no evidence for either a long-term protective or harmful effect associated with day care attendance in their study involving nearly 4,000 children followed from birth through age 8.
Children who attended day care did have more viral illnesses with respiratory symptoms like wheezing in the first years of life and then less wheezing and steroid use than children who did not attend day care between the ages of 4 and 8.
But whatever protection day care provided early in childhood disappeared as the children grew old enough to be diagnosed with asthma, lead researcher Daan Caudri, MD, tells WebMD.
The study appears in the Sept. 15 issue of the American Journal of Respiratory and Critical Care Medicine.
“If day care attendance truly was protective, we would expect to see this protection persist, but that is not what we saw,” he says. “We found no lasting protective effect against asthma.”
Day Care: No Lasting Impact on Asthma
During the course of the study, parents completed questionnaires designed to provide detailed histories of their children’s respiratory illnesses and symptoms.
When they reached the age of 8, most of the children were also assessed for specific allergies and some also received lung function and airway hyper-responsiveness evaluations - tests routinely used to diagnose asthma.
Children were classified as early day care attendees if their first attendance was before the age of 2. These children were found to be twice as likely as children who did not attend day care to experience wheezing in the first year of life.
By age 5, however, early day care attendees were less likely to wheeze than children who had never attended day care, but the difference was not statistically significant.
By the age of 8, there was no difference in asthma-related symptoms like wheezing in children who had early day care and those who did not.
No difference in wheeze was seen with regard to day care attendance among boys and girls, but having older brothers or sisters did impact risk.
Children with older siblings who started day care early had more than a fourfold higher risk of frequent respiratory infections and a more than twofold risk of wheezing in the first year of life compared to children without older siblings who did not attend day care.
But even in these high-exposure children, no difference in asthma-related symptoms was reported by age 8.
7 Million Kids U.S. Kids Have Asthma
Supporters of the hygiene hypothesis say it could help explain the dramatic increase in allergy and asthma in industrialized countries during a time when exposure to germs has declined.
Asthma is now the most common chronic disease of childhood, affecting close to 7 million children in the U.S., according to the American Lung Association.
Many studies examining the hygiene hypothesis have focused on day care attendance because children who go to day care tend to be exposed to more germs and infections.
Findings have been mixed, with some studies suggesting a protective role for day care attendance and others failing to show a benefit.
The new research is one of the largest studies ever to address the question, with some of the longest follow up.
But asthma specialist John Mastronarde, MD, tells WebMD that an even longer follow up would be needed to conclusively show that early day care attendance has no lasting impact on asthma risk.
Mastronarde directs the asthma center at Ohio State University Medical Center.
“This study adds another interesting piece to the puzzle, but I don’t see it as definitive,” he says.
He says it is increasingly clear that rather than being a specific disease with one specific cause, asthma is a heterogeneous disorder with many different triggers.
Asthma researcher Clare Ramsey, MD, agrees the new study is not definitive.
An assistant professor of medicine at the University of Manitoba, Ramsey co-wrote a 2005 review of the research, concluding that “the hygiene hypothesis is not likely to be the sole explanation for the ongoing asthma epidemic in industrialized nations.”
“Based on all the available data, I would agree that there is no substantial evidence that early day care should be recommended as a preventive strategy to the general public to reduce asthma,” she tells WebMD.
But she adds that the new study did not address the question of whether day care attendance protects children who are genetically predisposed to develop asthma, as previous studies have suggested.