A chemical used to preserve meat has been implicated in New Zealand's type 1 diabetes epidemic.

Auckland University research points the finger at nitrates and nitrites as a potential cause of some child-onset diabetes. They are used to preserve meats such as salami and ham and nitrates occur naturally in vegetables. The body also converts some nitrate to nitrite.

Around 15,000 people in New Zealand have type 1 diabetes and the incidence has doubled in the past 20 years.

Type 2 diabetes, for which obesity is a risk factor, has a different cause, although the effects can be the same.

Type 1 diabetes has a genetic component, but it requires an environmental trigger. This trigger has not been found, despite researchers considering viruses and a huge range of potential dietary causes. Auckland University scientist Dr Shiva Reddy said his research on mice strongly supported the theory that nitrates caused some type 1 diabetes.

He gave tiny doses of a drug called streptozotocin to mice who were genetically prone to developing type 1 diabetes and to another group who were not prone.

The chemical is involved in a similar cellular pathway to nitrates and is known to destroy insulin- producing cells when given in large doses.

Most mice in the first group quickly developed diabetes; few in the second did, and much more slowly.

Dr Reddy hopes to test the theory overseas in the next two years on human insulin-producing cells taken from the pancreas of an organ donor.

Diabetes researcher Professor Bob Elliott, of the biotech company Living Cell Technologies, said Dr Reddy was doing pioneering work on the disease which backed the nitrates theory.

Dr Reddy was the first to do laboratory experiments underpinning overseas survey work that had suggested a link between the disease and nitrates.

Professor Elliott said a study in Iceland found a link between heavy consumption of nitrate-preserved meat by newly pregnant women in the month before Christmas and type 1 diabetes in the children.

Scientists in Melbourne had found a substance like streptozotocin on the skins of potatoes. "There are other possibilities, particularly for foods that are heavily processed."

A study by the Institute of Environmental Science & Research said more than 97 per cent of exposure to nitrates from 24 vegetables and processed foods it checked was from the vegetables, especially lettuce and potato.

But the levels in New Zealand lettuce and potato were not high compared with European results. The population's average exposure to nitrates and nitrites were well within the acceptable daily intakes, the study said.