THURSDAY, Dec. 17, 2015 (HealthDay News) -- The number of U.S. kids living with type 1 diabetes has increased by almost 60 percent since 2002, and experts are not sure why.

Using a national database, researchers found that the prevalence of type 1 diabetes stood at just under 1.5 cases per 1,000 children and teenagers in 2002. By 2013, that figure had risen to 2.3 per 1,000.

The study, published online Dec. 17 in the journal Diabetes Care, adds to evidence of a global -- and puzzling -- rise in type 1 diabetes.

Unlike type 2 diabetes -- a common adulthood disease -- type 1 diabetes has nothing to do with obesity. It's an autoimmune disease in which the immune system mistakenly attacks the cells that produce insulin, a hormone that regulates blood sugar.

And it's usually diagnosed in childhood.

People with type 1 diabetes have to take daily insulin injections, or have an insulin pump implanted, to survive.

It's a difficult disease to manage, and it carries long-term complications like nerve damage, kidney failure and heart disease, said Dr. Steven Griffen, vice president of translational development for the JDRF -- a nonprofit that supports research into type 1 diabetes.

The fact that type 1 is growing more common means that it's increasingly important to understand its causes, he said.

Griffen, who was not involved in the new study, said the rising prevalence among U.S. kids is worrisome, but not surprising. "We're seeing this trend globally. Studies in other countries have had similar results," he said.

"As to the question of why," Griffen added, "we don't have an answer."

There are theories, he said, and researchers are digging into them.

One theory, according to Griffen, centers on the gut "microbiome" -- the huge collection of bacteria that normally dwells in the digestive system. Some research has hinted that a lack of diversity in those gut bacteria may trigger type 1 diabetes in children with a genetic susceptibility.

It's thought that certain trappings of modern life -- antibiotic use, processed foods and even C-section births -- may be diminishing the diversity of gut microbiomes.