City kids more likely to have food allergies: study

TORONTO – Kids who live in cities have a much higher prevalence of food allergies than those living in rural areas, according to a new study.

American researchers, led by Northwestern University’s Ruchi Gupta, looked at 38,465 children from 0-18 years. Parents reported whether or not their child had a food allergy based on physician diagnosis, allergy testing as well as the child’s food reaction history.

Results showed 9.8 per cent of kids in urban centres have food allergies, compared to 6.2 per cent in rural communities – a 3.6 per cent difference that Gupta calls “very significant.”

“It’s a big difference,” says Gupta. “It’s clear that as population density starts to decrease, so does food allergy.”

Peanut allergies were the most common type of food allergy for kids in urban areas (twice as common than in rural areas), and milk was the most dominant food allergy in rural communities.

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Dr. Moshe Ben-Shoshan is a professor at McGill University and an allergist at Montreal Children’s Hospital. He has led one of the only studies looking at similar trends in Canada, involving 9,667 Canadians of all ages.

While his research didn’t include milk and showed no significant difference with peanuts between urban and rural areas, he did see an increase in shellfish allergies in city dwellers.

“Shellfish allergy is almost two-folds higher in urban areas [in Canada]. We looked at fish and tree nut also, but it didn’t reach significance, maybe because we had a lower sample of children,” says Ben-Shoshan.

While there is no definitive cause for the urban-rural divide, there are a few theories on the table. One is that the pollutants in urban areas have an effect on food allergies, though Ben-Shoshan notes there is very sparse Canadian data regarding pollution and food allergy.

Another possible explanation currently gaining attention is the “hygiene hypothesis.” This suggests when kids aren’t exposed to normal germs, bacteria and dirt – things their immune system should be fighting as they develop – their immune system will instead start fighting things unnecessarily.

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“The idea is that in urban areas, we’re becoming too clean, using too many cleaning supplies and then kids aren’t able to go outside and play in the dirt,” says Gupta. “In rural areas when kids may be exposed not only to the outdoors, but potentially to farm animals, it has been shown to be protective.”

This theory also includes the way people eat in cities, in terms of processed foods and pesticides and has been linked to asthma and environmental allergies.

But Ben-Shoshan warns of taking the hygiene hypothesis too far.

“That [theory] led to undesirable speculation about the fact that it suggests vaccination is not good because you are less exposed to infection agents, and then you are more likely to have allergies,” he says. “However, this has been shown in numerous studies not to be true.”

The Canadian research points to lifestyle and environmental differences between city and rural areas as the strongest link to development of food allergies, but is far from conclusive.

“It could be that city dwellers are more likely to visit physicians more often and maybe change accordingly the time of introduction of potentially allergenic food,” says Ben-Shoshan.

“I don’t think we can really pinpoint at this time of our studies what is the factor exactly in rural life that is beneficial for the development of food allergies. It could be so many things: activity, you run more, you are exposed to less pollution, you have a different lifestyle, you eat differently… there’s no one answer.”

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He notes that if the reason city dwellers are more affected by food allergies than those in rural areas is because of an environmental factor or lifestyle (versus a genetic factor, for example), then there’s a good chance it can be changed to avoid food allergies.

Both Ben-Shoshan and Gupta look forward to future research on the topic where they can further tease out the factors that contribute to the development of better treatment strategies for food allergies.

Ben-Shoshan’s work was supported by AllerGen NCE Inc. (the Allergy, Genes and Environment Network), a member of the Government of Canada’s Networks of Centres of Excellence Canada program.

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