Research suggests exposure to sunshine could explain the huge discrepancy between South Island and North Island children with bowel disease.
The Otago University study shows South Island children are more than twice as likely to develop a bowel disease, with 40 to 60 out of every 100,000 children diagnosed, compared to 10 to 20 in the north.
Inflammatory bowel diseases, such as Crohn's disease, generally appear in the pre-teen or teenage years, and are incurable.
One of the lead researchers Andrew Day said the reasons for the discrepancy were not yet known definitively.
"Sunlight exposure and vitamin D - we know that that's very important in terms of chronic diseases like inflammatory bowel disease.
"Things like selenium, we know that the South Island has lower rates of selenium in the soil. But we don't know exactly. We're certainly try to work out how to...understand that more clearly."