Diabetes is a disease that occurs when your blood glucose, also called blood sugar, is too high. Blood glucose is your main source of energy and comes from the food you eat. Insulin, a hormone made by the pancreas, helps glucose from food get into your cells to be used for energy. Sometimes your body doesn’t make enough—or any—insulin or doesn’t use insulin well. Glucose then stays in your blood and doesn’t reach your cells.

Causes of diabetes

• Type 1 diabetes

Doctors don’t know exactly what causes type 1 diabetes. For some reason, the immune system mistakenly attacks and destroys insulin-producing beta cells in the pancreas. Genes may play a role in some people. It’s also possible that a virus sets off the immune system attack.

• Type 2 diabetes

Type 2 diabetes stems from a combination of genetics and lifestyle factors. Being overweight or obese increases your risk too. Carrying extra weight, especially in your belly, makes your cells more resistant to the effects of insulin on your blood sugar. This condition runs in families. Family members share genes that make them more likely to get type 2 diabetes and to be overweight.

• Gestational diabetes

Gestational diabetes is the result of hormonal changes during pregnancy. The placenta produces hormones that make a pregnant woman’s cells less sensitive to the effects of insulin. This can cause high blood sugar during pregnancy.

• Insulin resistance

Type 2 diabetes mainly results from insulin resistance. Insulin resistance happens when cells in your muscles, fat and liver don’t respond as they should to insulin. Several factors and conditions contribute to varying degrees of insulin resistance, including obesity, lack of physical activity, diet, hormonal imbalances, genetics and certain medications.

• Autoimmune disease:

Type 1 diabetes and LADA happen when your immune system attacks the insulin-producing cells in your pancreas.

• Autoimmune disease

Type 1 diabetes and LADA happen when your immune system attacks the insulin-producing cells in your pancreas.

• Hormonal imbalances

During pregnancy, the placenta releases hormones that cause insulin resistance. You may develop gestational diabetes if your pancreas can’t produce enough insulin to overcome the insulin resistance. Other hormone-related conditions like acromegaly and Cushing syndrome can also cause Type 2 diabetes.

• Pancreatic damage

Physical damage to your pancreas — from a condition, surgery or injury — can impact its ability to make insulin, resulting in Type 3 diabetes

• Genetic mutations

Certain genetic mutations can cause MODY and neonatal diabetes.

Complications

• Cardiovascular disease

Diabetes dramatically increases the risk of various cardiovascular problems, including coronary artery disease with chest pain (angina), heart attack, stroke and narrowing of arteries (atherosclerosis). If you have diabetes, you're more likely to have heart disease or stroke.

• Nerve damage (neuropathy)

Excess sugar can injure the walls of the tiny blood vessels (capillaries) that nourish your nerves, especially in your legs. This can cause tingling, numbness, burning or pain that usually begins at the tips of the toes or fingers and gradually spreads upward.

• Kidney damage (nephropathy)

The kidneys contain millions of tiny blood vessel clusters (glomeruli) that filter waste from your blood. Diabetes can damage this delicate filtering system. Severe damage can lead to kidney failure or irreversible end-stage kidney disease, which may require dialysis or a kidney transplant.

• Eye damage (retinopathy)

Diabetes can damage the blood vessels of the retina (diabetic retinopathy), potentially leading to blindness. Diabetes also increases the risk of other serious vision conditions, such as cataracts and glaucoma.

• Foot damage

Nerve damage in the feet or poor blood flow to the feet increases the risk of various foot complications. Left untreated, cuts and blisters can develop serious infections, which often heal poorly. These infections may ultimately require toe, foot or leg amputation.

• Skin conditions

Diabetes may leave you more susceptible to skin problems, including bacterial and fungal infections

• Hearing impairment

Hearing problems are more common in people with diabetes.

• Alzheimer's disease

Type 2 diabetes may increase the risk of dementia, such as Alzheimer's disease. The poorer your blood sugar control, the greater the risk appears to be. Although there are theories as to how these disorders might be connected, none has yet been proved.

• Depression

Depression symptoms are common in people with type 1 and type 2 diabetes. Depression can affect diabetes management.

Sign and Symptoms

• Increased thirst

• Frequent urination

• Extreme hunger

• Unexplained weight loss

• Fatigue

• Irritability

• Blurred vision

• Slow-healing sores

• Frequent infections, such as gums or skin infections and vaginal infections

How insulin works

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

• The pancreas releases insulin into the bloodstream.

• The insulin circulates, letting sugar enter the cells.

• Insulin lowers the amount of sugar in the bloodstream.

• As the blood sugar level drops, so does the secretion of insulin from the 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 the liver.

• Sugar is absorbed into the bloodstream, where it enters cells with the help of insulin.

• The liver stores and makes glucose.

• When glucose levels are low, such as when you haven't eaten in a while, the liver breaks down stored glycogen into glucose. This keeps your glucose level within a typical range.

• The exact cause of most types of diabetes is unknown. In all cases, sugar builds up in the bloodstream. This is because the pancreas doesn't produce enough insulin. Both type 1 and type 2 diabetes may be caused by a combination of genetic or environmental factors. It is unclear what those factors may be.