More children are developing allergies, and scientists now have evidence strongly backing one of the leading theories why.
In the US, there has been a 50 percent increase in children with food allergies since the 1990s, and research undertaken at the University of Auckland has shown a rise in cases of anaphylaxis in New Zealand over the past decade.
There are a number of theories why wealthy, developed countries are experiencing an allergenic epidemic that isn't affecting poorer countries. One of the most promising has been that advances in sanitation and hygiene have left youngsters' immune systems without a training ground to learn how to fight pathogens properly.
Just-released research from Finland has now backed that up.
For three years, scientists at Aalto University collected stool samples from more than 200 infants in wealthy Finland and Estonia as well as from neighbouring Karelia, a poor area of Russia.
They found a marked difference between the bacteria found in Finland and Estonia, and that found in Karelia. In the former, gut microbiomes are dominated by Bacteroides, which can stifle the body's immune response.
"In the Finnish and Estonian infants, where Bacteroides dominates, the gut microbiome is immunologically very silent," says researcher Aleksandar Kostic.
"We believe that later on, this makes them more prone to strong inflammatory stimuli."
In the Karelien infants, Bifidobacterium dominated, and throughout their first three years of life they had a much larger range of bacteria present in their stool -- evidence of exposure to a greater range of pathogens.
The researchers suspect the Karelian microbiomes more closely match what humans evolved with, and the dominance of Bacteriodes is a "more recent phenomenon related in some way to improved sanitation and standard of living".
The New Zealand Allergy Clinic says previous studies have shown greater rates of allergy in children growing up in smaller families. Kids with three or more older siblings are less likely to have allergies than only-children, as are only children who attend daycare.
A few studies have also shown a link between probiotics and reduced severity of allergies.