If you have a college degree, you -- and your kids -- may have an elevated risk of developing food allergies. Being an immigrant, on the other hand, may lower your family's risk.
The study, which only showed associations and could not determine what makes certain groups more susceptible to reactions, may bring scientists one small step closer to understanding why food allergies have become steadily more common over the past few decades.
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One explanation could be that people with more schooling may, in certain places, be more likely to visit doctors or they might have followed overly strict guidelines about when to introduce foods to their babies -- a strategy that can backfire.
"We don't think education by itself is a cause of food allergy, we think it is a marker of other things," said Moshe Ben-Shoshan, an allergist and immunologist at McGill University’s The Montreal Children's Hospital. "The question is: What in the environment is affecting the rise in food allergies?"
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From 1997 to 2007, according to the Centers for Disease Control and Prevention, the number of reported food allergies in kids under age 18 rose by 18 percent.
Today, according to various estimates, between 4 and 8 percent of American kids have some kind of food allergy. Peanuts, tree nuts, fish and shellfish cause the most serious reactions. Milk, eggs, soy and wheat are also common allergy triggers. And allergies are more common in kids than in adults.
To understand which groups might be most vulnerable, Ben-Shoshan and colleagues called more than 10,000 households across Canada in an attempt to get a representative cross-section of society. When people answered the phone and agreed to participate, they answered questions about whether anyone in their household had food allergies.
Then, they offered demographic information. The study accumulated answers from about 3,600 homes, though low-income and immigrant populations were proportionately underrepresented.
Compared to households with a high-school degree or less, the researchers reported in the Journal of Allergy, Canadian families that included someone with at least a college degree were about twice as likely to report allergies to tree nuts, such as cashews, pecans, walnuts and hazelnuts.
Women were more likely than men to suffer from tree-nut and shellfish allergies, the study also found, possibly because estrogen and other hormones can affect the activity of cells involved in allergic reactions. And immigrants were half as likely as Canadian-born families to report shellfish allergies. Income level did not make a difference in allergy rates.
One possible explanation for the results is that both non-immigrants and people with more schooling might have greater access to doctors and be more likely to acquire accurate diagnoses.
But it's also possible that lifestyle choices make a difference.
A theory called the hygiene hypothesis, for example, proposes that cleaner, less crowded households that regularly use antibiotics and vaccines end up with lower rates of exposure to bacteria and other pathogens. Our immune systems, in turn, may fail to get enough practice fending off legitimate infections, making them more likely to overreact to benign triggers, like food.
Doctors are not suggesting that people avoid vaccines or antibiotics when they are needed, Ben-Shoshan said. But one choice that might matter is the timing of when parents expose babies to potentially risky foods.
Until a few years ago, parents were advised to wait until their babies were a year or older before introducing peanuts, eggs, dairy and other common food allergy triggers.
Now, based on research suggesting that this strategy might actually make allergies more likely to develop, the American Academy of Pediatrics no longer recommends delaying the introduction of foods, unless babies are considered high risk because of a family history or other reasons. Women who are pregnant or breastfeeding should also continue to eat their usual diet, Ben-Shoshan said.
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Despite the new findings, many mysteries continue to surround food allergies and what causes them, said Andy Liu, a pediatric allergist at National Jewish Health in Denver. And risk factors may vary from place to place.
In a study that he and colleagues published last year, for example, allergy testing showed no effect of education or income on rates of food allergies in American patients. Instead, age, race and ethnicity mattered most.
The discrepancy might be explained by differences in how the studies were conducted. Or, there might be differences between Canada and the United States in access to health care or other environmental factors.
The finding that immigrants had lower rates of food allergies, on the other hand, is consistent with an accumulating body of research, Liu said, and might offer some important clues about what makes people vulnerable to allergies.
"That's been seen with immigrants again and again and seems tied more to people in rural and developing areas," Liu said. "Whether or how that's being reported in food allergies is just coming into view."