In someone with diabetes, the body’s ability to secrete insulin -- and the counter-regulatory hormone glucagon -- is impaired. Johns Hopkins professor Christopher D. Saudek, M.D. explains the path of glucose in diabetes.
The pancreas is an elongated organ that extends across the abdomen, below the stomach. In addition to secreting certain enzymes that aid in food digestion, the pancreas also manufactures hormones responsible for regulating blood glucose levels.
Scattered throughout the pancreas are more than a million tiny nests of cells known as the islets of Langerhans. Each islet contains several different types of cells. The majority are beta cells, which produce and store the hormone insulin until it is needed. Also located in the islets are alpha cells, which make and store glucagon, a hormone that counteracts the effects of insulin.
After a meal, carbohydrates are broken down into smaller molecules as food travels through your digestive tract. Complex carbohydrates (found in starchy foods, such as pasta and potatoes) are long strings of glucose that require more digestion than simple sugars (such as sucrose in candy or table sugar). Digestion begins in the mouth. A salivary enzyme called amylase breaks down carbohydrates into smaller molecules that pass through the esophagus and stomach and into the small intestine. Another type of amylase from the pancreas and enzymes in the intestine then split the partially digested carbohydrates into simple sugar molecules small enough to be absorbed across the intestinal wall.
The absorbed glucose and other simple sugars then travel to the liver via the portal vein. Once there, the simple sugars are converted into glucose in the liver, which in turn releases glucose into the bloodstream according to how much your body needs for energy. Some of the unused glucose is stored in the liver and muscle tissue as glycogen for future energy needs and the rest is stored as triglycerides in adipose (fatty) tissue.
After you eat a meal that contains carbohydrates and glucose enters the bloodstream, it triggers a response by the pancreas and causes the cells in the islets of Langerhans within the pancreas to produce and release insulin. The insulin, in turn, allows glucose to move from the bloodstream into the cells in your body, where it is used for energy.
If you have type 1 diabetes, the pancreas produces little or no insulin, and glucose remains in the bloodstream instead of entering cells. If you have type 2 diabetes, the pancreas produces and releases insulin, but the cells are not sensitive enough to it and insufficient glucose enters the cells. The glucose remaining in the bloodstream signals the pancreas to produce even more insulin. Eventually, however, the pancreas is not able to produce enough insulin to overcome the reduced responsiveness of cells to insulin.
To recap: In someone without diabetes, beta cells sense the rising blood glucose levels and secrete insulin into the blood. Once in the bloodstream, insulin helps glucose enter the body’s cells, where it is “burned” for energy or converted to glycogen by the liver and muscles and stored there for future energy needs. As a result, blood glucose levels return to normal, and insulin secretion decreases.
On the other hand, a drop in blood glucose levels -- for example, when one hasn’t eaten for several hours -- stimulates the alpha cells to secrete glucagon into the blood. Glucagon raises blood glucose levels by signaling the liver to convert stored glycogen back into glucose and release it into the bloodstream. Normally, the secretion of these hormones by the pancreas is perfectly balanced: Beta and alpha cells continuously monitor blood glucose levels and release insulin or glucagon as needed.
But in someone with diabetes, this delicate balance is impaired because the beta cells produce little or no insulin, the body’s cells are resistant to insulin, or a combination of both is at work. Regardless, glucose cannot enter cells effectively and remains in the bloodstream. The result is persistently high blood glucose levels (hyperglycemia). Without treatment, hyperglycemia can lead to serious long-term complications, such as heart, eye, and kidney disease.
