Children in the South Island are up to three times more likely to suffer debilitating inflammatory bowel diseases than those in the North.
Researchers say it could partly be down to sunlight, and its impact on vitamin D levels.
Data collected by Otago University researchers reveals between 40 and 60 children per 100,000 in the South Island had been diagnosed with an inflammatory bowel disease.
The rate in the North Island is between 10 and 20 children per 100,000.
Paediatrician Professor Andrew Day said that one theory is that South Island children could have lower levels of vitamin D.
However he said this claim needs more investigation.
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