New research shows New Zealand's inflammatory bowel disease rates are expected to double in the next 20 years

A landmark international study - contributed to by Otago University - has for the first time identified four distinct stages of the disease's development, and charts its growth globally.

Study co-author Otago University Professor Richard Gearry says we'll reach 40-thousand cases by 2045, but it's still unclear why rates are rising.

But he says they do know increases are happening in communities which have been westernised, or industrialised.