“There’s more to treating type 1 diabetes than just bringing down high blood sugar,” said Justin Gregory, M.D., a pediatric endocrinologist at Vanderbilt University Medical Center.
Gregory has been studying insulin resistance and looking for ways to restore insulin sensitivity in type 1 diabetes. People with type 1 diabetes have approximately 40 percent lower insulin sensitivity than other populations. Gregory’s most recent study, published in Diabetes, shows chronic exposure of muscle tissue to insulin injections contributes substantially to resistance – even more so than hyperglycemia.
Injecting insulin into fat tissue bypasses the liver, which normally breaks down approximately half of insulin made by the pancreas, Gregory said. As a result, insulin levels are high in muscle tissue of people with type 1 diabetes.
For the new study, Gregory and colleagues compared three distinct populations: healthy people (controls), people with type 1 diabetes, and people with glucokinase-maturity-onset diabetes of the young (GCK-MODY, also known as MODY2). People with GCK-MODY produce insulin, but are hyperglycemic, due to a rare genetic mutation in their glucokinase gene. All study participants were between the ages of 13 and 51.
Each group had a different mix of insulinemia and glycemia. People with type 1 diabetes are hyperinsulinemic and hyperglycemic, while people with GCK-MODY are hyperglycemic only. The differences allowed Gregory’s team to assess the relative contribution of each factor to insulin resistance.
Comparing Insulin Sensitivities
The researchers used clamp techniques to adjust and standardize blood glucose levels among participants. Then, they compared insulin levels between groups.
People with GCK-MODY had the same level of insulin as healthy controls, indicating their muscle tissues were equally sensitive to circulating insulin. But people with type 1 diabetes had insulin levels that were two-and-a-half times higher than either of the other groups, indicating their tissues were resistant to insulin and required much higher doses to move glucose out of the bloodstream. The results suggest hyperinsulinemia as a major driver of insulin resistance.
Explained Gregory, “In type 1 diabetes, people have too much insulin at muscle and not enough at liver – all because they are putting insulin in the wrong place. Restoring that balance is important toward helping people with type 1 diabetes reduce their risk of heart disease.”
Insulin resistance is closely linked to microvascular disease and other comorbidities in type 1 diabetes. Overcoming insulin resistance is the next challenge, Gregory says, to help prevent these complications.
“The study brings to light the need to develop therapeutic strategies to keep the appropriate balance of insulin between the liver and peripheral insulin-sensitive tissues.”
Gregory is collaborating with colleagues at the Vanderbilt Diabetes Center and elsewhere to develop new therapies. One option may be to develop insulin analogs that preferentially target the liver, or to implant pumps inside the abdominal space so insulin more quickly reaches the liver. Either approach could better recapitulate how insulin naturally produced by the pancreas is processed in the body.
“We need to come up with a way of delivering insulin that replicates that normal balance of insulin,” Gregory said. “That’s the best way to mitigate insulin resistance in these patients.”