Spending the first few years of life someplace other than the United States seems to protect against allergy and asthma. An analysis finds that children who were born elsewhere and moved to the U.S. are less prone to these immune ailments than U.S.-born kids. But for some allergic complaints, the mysterious protection weakens after long exposure to an American lifestyle, researchers report April 29 in JAMA Pediatrics.
The beneficial effect has all the earmarks of the hygiene hypothesis, which posits that young children exposed to dirt, infections, animal dander and other aspects of a grubby life develop an immune system that neatly distinguishes disease-causing from innocuous organisms and compounds in nature. Such a child doesn’t overreact to harmless substances such as grasses or pollen later in life, the hypothesis holds.
“This is a somewhat surprising finding,” says Doug Brugge, a public health researcher at Tufts University School of Medicine in Boston. It is well-known that moving to the United States can be detrimental to one’s health in general — an effect mainly attributed to a Western diet. But in the narrower realm of allergy and asthma, the risk has been less clear, he says.
Physician Jonathan Silverberg of St. Luke’s-Roosevelt Hospital Center in New York City and his colleagues analyzed health data from a nationwide questionnaire involving more than 90,000 U.S. children up to age 17, some of whom had been born in other countries. Those kids were roughly half as likely to report any brush with asthma as were children born in the United States.
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The foreign-born children were also only 39 percent as likely as U.S.-born kids to have hay fever, 43 percent as likely to have eczema and 60 percent as likely to report a food allergy. Foreign-born children with a parent who was also born outside the United States had an even lower chance of having hay fever or asthma.
Children in the study were of many races and ethnicities. The protective effect of being foreign born remained even after adjusting for family income, race, age, sex and urban living.
The researchers found that foreign-born kids who had lived in the United States for at least 10 years didn’t seem to have the same advantage against hay fever and eczema as did foreign-born kids who had arrived in the last two years. “This suggests that the protective effects of the hygiene hypothesis may not be lifelong,” the study authors note.
Earlier research among Mexican-Americans had hinted that a child needed to live in Mexico for at least two years to garner a benefit, says Fernando Holguin, a physician and epidemiologist at the University of Pittsburgh Medical Center. Many factors may generate this protection, Holguin says. For example, the parental benefit found in the new study suggests that behaviors or diet linked to the country of origin might contribute, he says.
Vitamin D may matter, too, Brugge says. “Suppose you have immigrants coming from equatorial places,” he says, “and they move to Boston.” Without supplements, their natural vitamin D levels — dependent on sun exposure — would plummet, particularly if they are dark skinned, since more skin pigmentation impedes vitamin D manufacture from sun exposure. “I cannot dismiss the vitamin D hypothesis,” Brugge says, particularly since the vitamin is involved with regulating immune responses (SN: 7/16/2011, p. 22).
Brugge says it is time to look for causes. “We need to move beyond documenting these changes and try to figure out what might be behind them,” he says.