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Children living closer to the equator are less likely to have eczema and certain food allergies than their peers in less sunny regions, and the researchers say the finding provides some ecological evidence that low vitamin D levels might increase allergy risk.
Dr Nicholas Osborne of the University of Exeter in Truro, UK, and his colleagues found a latitude gradient for peanut and egg allergies, as well as eczema, in their study of more than 7,000 children.
"This study provides further stimulus for research into possible causal roles of ambient UV radiation and vitamin D levels in this disease group," they wrote in a letter published online by the Journal of Allergy and Clinical Immunology.
Using indirect measures of allergies-like rates of Epi-Pen prescriptions-other groups have found evidence that food allergy is more common among children living further away from the equator, Dr. Osborne and his colleagues note.
Eczema likely in central regions
In the current study, Dr Osborne and his colleagues used data from the third wave of the Longitudinal Study of Australian children, which included 3,312 four- and five-year-olds and 4,331 eight- and nine-year-olds and was conducted in 2008.
They say Australia's north-south length of 4,500 kilometres, from 10 degrees south to 43 degrees south, as well as its high rates of allergy and asthma, make it "strongly placed to examine these associations."
Eczema had the strongest latitude gradient in both cohorts, the researchers found. In the four- to five-year-olds, 9% of children living in the north of the country (10 degrees to 29 degrees S) had eczema, compared to 15% of children in the centre of the country(29 to 35 S) and 19%% in the south (35 to 43 S).
In this group, on multivariate analysis, with the northernmost region (i.e., closest to the equator) as the reference, kids were 75% likelier to have eczema in the central region and more than two and a half times likelier if they lived in the south.
Allergies don’t change by latitude
Among the eight- and nine-year olds, eczema rates from north to south were 8%, 12%, and 13%, and adjusted odds ratios in the central and southern regions were 1.6 and 1.9.
The researchers identified similar latitude gradients for peanut allergies in both age groups, and for egg allergies in the younger children. (Egg allergy couldn't be investigated in the older children, because there were no cases of egg allergy in the north.)
The prevalence of allergies to cow's milk, wheat, or "other" foods did not change by latitude. Soy and sesame allergy also showed no latitudinal gradient, but because these allergies were rare, the researchers note, the study may have been underpowered to detect any difference.
"We were not surprised at the lack of association of other food allergies with latitude because these would include other mechanisms of food allergy, including non-IgE and food intolerances, as opposed to IgE type alone," Dr Osborne and his team wrote.
They also didn't see any latitudinal gradient in asthma prevalence, which could suggest, they said, "that developing asthma is potentially due to a different pathologic pathway to peanut and egg allergy."
"Investigating these phenomena at a finer level by using meteorological data, such as temperature and solar (irradiance), as well as serologic measures of vitamin D, might provide more clues to the nature of these associations," the authors concluded.
(Reuters Health, March 2012)
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