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What Is Diabetes?

Many people think of diabetes as the “sugar disease.” But that’s only part of the story. Diabetes mellitus, more commonly referred to simply as diabetes, is a chronic disease in which high levels of glucose (sugar) build up in the bloodstream. The term “diabetes” is derived from the Greek word for siphon (a tube bent in two through which liquid flows) and the Latin word “mellitus,” which means sweet as honey.

The disease is aptly named: Persistent thirst and frequent passing of urine containing glucose are characteristic symptoms of diabetes. These symptoms result from insufficient production of insulin by the pancreas or resistance of the body’s tissues to insulin action—or a combination of both. To understand what this means, you have to know where your body’s glucose comes from, what it’s used for, and how it is regulated by insulin.

What is glucose?

Glucose is a sugar that serves as the fuel that provides energy for the body’s cells. Your liver produces some glucose and your body gets the rest by digesting sugars, starches and other foods you eat.

What is insulin?

Insulin is a hormone produced by beta cells in a part of the pancreas known as the islets of Langerhans. Insulin controls how much glucose the liver produces and also helps to move glucose from the bloodstream into your cells, where it is needed as a source of energy.

The uptake of glucose into your cells occurs through a complex series of events. It begins when insulin attaches (“binds”) to receptor sites on the surface of cells in muscles and other tissues and causes carrier proteins (called glucose transporters) to move from inside the cell to the cell’s surface. Like little fuel trucks, these transport proteins deliver glucose from outside the cell to the inside. In the absence of enough insulin to bind to receptor sites and normal processing of signals within the cells, glucose enters the cells too slowly and builds up in the blood.

In a healthy person

Normally, the pancreas makes enough insulin to keep the supply and use of glucose in balance. When the blood contains enough insulin, the liver temporarily shuts down its production of glucose, and glucose is transported from the blood into your cells. Cells use some of the glucose immediately for energy. Excess carbohydrate is stored in the liver and muscles as a substance called glycogen and kept for future use. The body’s ability to store glycogen is limited, and any excess glucose that does not get stored as glycogen is converted to triglycerides and stored in adipose (fat) tissue.

Pancreatic cells in the islets of Langerhans continuously monitor blood glucose levels. After a meal, the carbohydrates you eat are digested and broken down into glucose and other sugars, which pass into the bloodstream. As your blood glucose levels rise, beta cells in the pancreas respond by secreting insulin into the blood. Glucose then passes into your cells and the liver shuts down glucose production. Between meals, insulin also prevents excessive release of glucose from the liver into the bloodstream. If blood glucose levels drop too low between meals, alpha cells in the pancreas secrete a hormone called glucagon. This hormone signals the liver to convert amino acids and glycogen into glucose that is sent into the blood.

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When someone has diabetes

In diabetes, this glucose balancing system is disrupted, either because too little insulin is produced or because the body’s cells do not respond to insulin normally, a condition called insulin resistance. The result is an unhealthy rise in blood glucose levels. (For more information, see “Why Blood Glucose Levels Rise in Diabetes.”) If diabetes is left untreated, the two principal dangers are the immediate results of high blood glucose levels (which include excessive urination, dehydration, intense thirst, and fatigue) and long-term complications that can affect your eyes, nerves, kidneys, and large blood vessels.

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