The findings don't confirm a connection between premature birth and diabetes, although other studies have hinted at a possible connection and increased risk.
But they do show that babies and young children have higher insulin levels if they were born before full term, and the levels are greatest in those who were the most premature. Higher insulin levels, in turn, could be an indicator of diabetes even decades down the line, the researchers noted.
If future studies confirm the research, it may be possible to know which newborns will face the highest risk of developing type 2 diabetes in adulthood, said study co-author Dr. Xiaobin Wang.
"We could identify babies potentially at risk right from birth and alert pediatricians and parents to pay more attention to future risk of metabolic disease," said Wang, director of the Center on the Early Life Origins of Disease at Johns Hopkins University Bloomberg School of Public Health, in Baltimore.
Scientists want to understand the early risk factors for diabetes because it's possible to prevent the illness before it appears, Wang said. They're especially interested in the possible effects of premature births, she said, because increasing rates of obesity appear to be boosting the percentage of premature births.
In the United States, she said, one of every nine babies is born prematurely, and the rate is one in five among black Americans.
The new study aimed to detect how early premature babies are different from full-term babies when it comes to insulin. Insulin is a hormone produced by the pancreas that is necessary for the body to metabolize the carbohydrates in food. People with type 2 diabetes cannot process insulin properly, putting them at risk of dangerous fluctuations in their blood sugar levels.
When it comes to the causes of diabetes, "inherited genes explain only quite a small part of the risk, maybe no more than 10 percent for some forms of diabetes," said Mark Hanson, a professor of cardiovascular science at the University of Southampton, in England.
"And even though obesity is a risk factor, our diets and lifestyles as adults are only part of the story, too. It has recently become clear that aspects of our early lives, even before we're born, can have a substantial impact on risks," said Hanson, author of a commentary accompanying the new study.
Wang and colleagues tracked 1,358 babies (born between 1998 and 2010 in Boston) into early childhood. The children's insulin levels were checked at birth and later in life, through 2012.
The researchers found that insulin levels were higher in those who were born prematurely. The investigators also discovered that size at birth affected potential risk for future diabetes.
"The effect is greater the earlier the baby is born, possibly because whatever induced the preterm labor may have altered the settings of the baby's pancreas-control system," Hanson said. And weight gain in childhood appears to make things worse, risk-wise.
For now, study author Wang said, the findings emphasize the importance of special monitoring of babies born prematurely, although more research is needed.
What about today's adults who were born prematurely? It's difficult to reverse a path toward diabetes in adulthood. But Wang said it's possible that someday doctors will ask patients routine questions about their birth as part of exams.
The study appears in the Feb. 12 issue of the Journal of the American Medical Association.
For more about premature birth, try the U.S. National Library of Medicine.