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 the pancreas, a gland situated behind and below the stomach.
- 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 main 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 insulin levels are low, such as when you haven't eaten in a while, the liver metabolizes stored glycogen into glucose to keep your glucose level within a normal range.
Causes of type 1 diabetes
In type 1 diabetes, 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 as in type 1 diabetes, it's believed that genetic and environmental factors play a role in the development of type 2. 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. As your placenta grows larger in the second and third trimesters, it secretes more of these hormones — making it even harder for insulin to do its job.
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. This is 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, genetic factors likely play a role. Your risk of developing type 1 diabetes increases if you have a parent or sibling who has type 1 diabetes. Environmental factors, such as exposure to a viral illness, also likely play some role in type 1 diabetes. Other factors that may increase your risk include:
- The presence of damaging immune system cells that make 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 type 1.
- Dietary factors. A number of dietary factors have been linked to an increased risk of type 1 diabetes, such as low vitamin D consumption; early exposure to cow's milk or cow's milk formula; or exposure to cereals before 4 months of age. However, none of these factors has been shown to cause type 1 diabetes.
- Race. Type 1 diabetes is more common in whites than in other races.
- 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 more fatty 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. Exercising less than three times a week may increase your risk of type 2 diabetes.
- 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 Asians — 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 140/90mm Hg is linked to an increased risk of type 2 diabetes.
- Abnormal cholesterol levels. If you have low levels of high-density lipoprotein (HDL), or "good," cholesterol, your risk of type 2 diabetes is higher. Low levels of HDL are defined as below 35 mg/dL.
- High levels of triglycerides. Triglycerides are a fat carried in the blood. If your triglyceride levels are above 250 mg/dL, your risk of diabetes increases.