It warrants further investigation, he said.
"There is a growing incidence of IBD in children worldwide but little information about rates in New Zealand children. Our study found quite a striking difference between the North and South Islands. One theory we have for the difference is that South Island children could have lower levels of vitamin D. This has implications for their immune system as vitamin D is involved in the production of a molecule that plays a key defensive role in the gut."
Professor Day said the 'vitamin D theory' was supported by European research showing colder places such as Scandinavia had higher rates of the diseases than warmer countries on the continent.
The reason for an increasing number of children being diagnosed with IBD was a subject of intense debate and scrutiny internationally, he said.
One theory was that children's diet consists of far more processed than whole foods nowadays. Another was that children did not play outdoors as much and get exposure and immunity to germs, he said.
An IBD diagnosis was often a very big deal for a child and their family to deal with.
"It's a lifelong condition with no cure. For those with more severe forms of the disease, there are often hospital stays to administer medication or for surgery. Some children need to have regular injections or infusions of medication," he said.
Steroids used to be a common treatment for the condition but doctors were wary of prescribing them now because of better alternatives and steroids potential to affect growth, among other side-effects, he said.
"At diagnosis now we are commonly recommending a special liquid-only diet for a period of time, and later daily ongoing medication to maintain control of the condition."
Professor Day said his team hoped to follow the prevalence study with one that measured vitamin D levels in IBD sufferers, as well as their location in New Zealand.
• Find out more about research on the gut at the University of Otago, Christchurch's Showcase on October 1, from 1-5pm. There will be a giant inflatable colon you can walk through and see the effect of diseases on the gut. The Showcase will include laboratory tours, a tour of the medical simulation centre, interactive displays with the chance to speak with researchers, and a debate hosted by Gary McCormick on whether "We can all live to 120 years".
