Researchers tested the effect of insulin pills on 560 children and adults whose relatives had type 1 diabetes. For most of them, the drug had no effect on whether or not they developed type 1 diabetes, or how quickly they developed it.

But for those at the highest risk of developing type 1 diabetes sooner rather than later, insulin pill therapy delayed the time it took to develop the full-blown disease by about two-and-a-half years, the researchers said.

"This is the largest study using oral insulin," said the study's lead author, Dr. Carla Greenbaum. Participants also had known autoantibodies that indicated a very high risk of developing type 1 diabetes in their lifetimes, said Greenbaum, chair of Diabetes TrialNet.

Study participants came from Canada, the United States, Australia, New Zealand, the United Kingdom, Italy, Sweden, Finland and Germany. They were mostly white. Sixty percent were male. Average age was about 8 years old.

The group was split into four groups based on their diabetes risk. Then they were randomly placed into an active treatment group given 7.5 milligrams of insulin in pill form daily or a placebo group. Half were followed for more than 2.7 years and half less.

In a small subset of study participants, the researchers saw that insulin pills did make a difference. In people who already showed lower early insulin secretion in response to food, insulin pill therapy delayed the onset of type 1 diabetes by 31 months compared to a similar group taking a placebo, Greenbaum said.

Greenbaum and her team suspect these folks had a response because the autoimmune attack might have been particularly active at that time. But, she added, that's just a theory.

The article, titled "Effect of Oral Insulin on Prevention of Diabetes in Relatives of Patients With Type 1 Diabetes," was published in the JAMA journal.