To understand diabetes, first you must understand how glucose is normally processed in the body.

How insulin works

Insulin is a hormone that comes from a gland situated behind and below the stomach (pancreas).

  • The pancreas secretes insulin into the bloodstream.
  • The insulin circulates, enabling sugar to enter your cells.
  • Insulin lowers the amount of sugar in your bloodstream.
  • As your blood sugar level drops, so does the secretion of insulin from your pancreas.

The role of glucose

Glucose — a sugar — is a source of energy for the cells that make up muscles and other tissues.

  • Glucose comes from two major sources: food and your liver.
  • Sugar is absorbed into the bloodstream, where it enters cells with the help of insulin.
  • Your liver stores and makes glucose.
  • When your glucose levels are low, such as when you haven’t eaten in a while, the liver breaks down stored glycogen into glucose to keep your glucose level within a normal range.

Causes of type 1 diabetes

The exact cause of type 1 diabetes is unknown. What is known is that your immune system — which normally fights harmful bacteria or viruses — attacks and destroys your insulin-producing cells in the pancreas. This leaves you with little or no insulin. Instead of being transported into your cells, sugar builds up in your bloodstream.

Type 1 is thought to be caused by a combination of genetic susceptibility and environmental factors, though exactly what many of those factors are is still unclear.

Causes of prediabetes and type 2 diabetes

In prediabetes — which can lead to type 2 diabetes — and in type 2 diabetes, your cells become resistant to the action of insulin, and your pancreas is unable to make enough insulin to overcome this resistance. Instead of moving into your cells where it’s needed for energy, sugar builds up in your bloodstream.

Exactly why this happens is uncertain, although it’s believed that genetic and environmental factors play a role in the development of type 2 diabetes. Being overweight is strongly linked to the development of type 2 diabetes, but not everyone with type 2 is overweight.

Causes of gestational diabetes

During pregnancy, the placenta produces hormones to sustain your pregnancy. These hormones make your cells more resistant to insulin.

Normally, your pancreas responds by producing enough extra insulin to overcome this resistance. But sometimes your pancreas can’t keep up. When this happens, too little glucose gets into your cells and too much stays in your blood, resulting in gestational diabetes.

Risk factors for diabetes depend on the type of diabetes.

Risk factors for type 1 diabetes

Although the exact cause of type 1 diabetes is unknown, factors that may signal an increased risk include:

  • Family history.Your risk increases if a parent or sibling has type 1 diabetes.
  • Environmental factors. Circumstances such as exposure to a viral illness likely play some role in type 1 diabetes.
  • The presence of damaging immune system cells (autoantibodies). Sometimes family members of people with type 1 diabetes are tested for the presence of diabetes autoantibodies. If you have these autoantibodies, you have an increased risk of developing type 1 diabetes. But not everyone who has these autoantibodies develops diabetes.
  • Dietary factors. These include low vitamin D consumption, early exposure to cow’s milk or cow’s milk formula, and exposure to cereals before 4 months of age. None of these factors has been shown to directly cause type 1 diabetes.
  • Geography. Certain countries, such as Finland and Sweden, have higher rates of type 1 diabetes.

Risk factors for prediabetes and type 2 diabetes

Researchers don’t fully understand why some people develop prediabetes and type 2 diabetes and others don’t. It’s clear that certain factors increase the risk, however, including:

  • Weight. The fattier tissue you have, the more resistant your cells become to insulin.
  • Inactivity. The less active you are, the greater your risk. Physical activity helps you control your weight, uses up glucose as energy and makes your cells more sensitive to insulin.
  • Family history. Your risk increases if a parent or sibling has type 2 diabetes.
  • Race. Although it’s unclear why, people of certain races — including blacks, Hispanics, American Indians and Asian-Americans — are at higher risk.
  • Age. Your risk increases as you get older. This may be because you tend to exercise less, lose muscle mass and gain weight as you age. But type 2 diabetes is also increasing dramatically among children, adolescents and younger adults.
  • Gestational diabetes. If you developed gestational diabetes when you were pregnant, your risk of developing prediabetes and type 2 diabetes later increases. If you gave birth to a baby weighing more than 9 pounds (4 kilograms), you’re also at risk of type 2 diabetes.
  • Polycystic ovary syndrome. For women, having polycystic ovary syndrome — a common condition characterized by irregular menstrual periods, excess hair growth and obesity — increases the risk of diabetes.
  • High blood pressure. Having blood pressure over 140/90 millimeters of mercury (mm Hg) is linked to an increased risk of type 2 diabetes.
  • Abnormal cholesterol and triglyceride levels. If you have low levels of high-density lipoprotein (HDL), or “good,” cholesterol, your risk of type 2 diabetes is higher. Triglycerides are another type of fat carried in the blood. People with high levels of triglycerides have an increased risk of type 2 diabetes. Your doctor can let you know what your cholesterol and triglyceride levels are.

Risk factors for gestational diabetes

Any pregnant woman can develop gestational diabetes, but some women are at greater risk than are others. Risk factors for gestational diabetes include:

  • Age. Women older than age 25 are at increased risk.
  • Family or personal history. Your risk increases if you have prediabetes — a precursor to type 2 diabetes — or if a close family member, such as a parent or sibling, has type 2 diabetes. You’re also at greater risk if you had gestational diabetes during a previous pregnancy, if you delivered a very large baby or if you had an unexplained stillbirth.
  • Weight. Being overweight before pregnancy increases your risk.
  • Race. For reasons that aren’t clear, women who are black, Hispanic, American Indian or Asian are more likely to develop gestational diabetes.
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