U.S. scientists working with stem cells say they're closer to a treatment for type 1 diabetes after creating the first cells that can make insulin in response to the body's changing glucose levels.

Researchers at Harvard University report using stem cells to produce human insulin-producing beta cells in the massive quantities necessary for cell transplantation.

That could lead to effective treatments for the disease that affects around 3 million Americans, they say.

Trials using the cells for human transplantation could be conducted within a few years, says research leader Douglas Melton, who dedicated his professional life to finding a cure after his son was diagnosed with type 1 diabetes as an infant 23 years ago.

"We are now just one pre-clinical step away from the finish line," says Melton, whose daughter Emma also has been diagnosed with the disease.

In their study published in the journal Cell, the researchers describe their efforts that yielded the first batch of pancreatic beta cells capable of sensing changing blood sugar levels and creating insulin in response.

Sufferers of type 1 diabetes possess pancreatic cells that don't produce enough insulin so must depend on outside sources such as injections or an insulin pump to deliver the hormone the body requires to break down sugars in the diet.

"We wanted to replace insulin injections" with "nature's own solution," says Melton.

The Harvard researchers say they've managed to turn stem cells into the hundreds of millions of insulin-producing cells a diabetes patient would require to effect a cure and free them from their dependence on externally-provided insulin.

In experiments where the new cells where injected into mice that were then injected with glucose, almost three-quarters of the mice were able to produce sufficient insulin to break down the sugar, they say.

Stem cells have been used to create beta cells before, but while they could produce insulin, they could not respond to the body's changing glucose levels and kept pumping out insulin whether it was needed or not.

The cells created at Harvard, Melton says, "read the amount of sugar in the blood, and then secrete just the right amount insulin in a way that is so exquisitely accurate that I don't believe it will ever be reproduced by people injecting insulin or by a pump injecting that insulin."

In addition to the experiments with mice, the stem cell-derived beta cells are currently undergoing trials involving non-human primates, the researchers reported.