Diabetes mellitus is a disorder that affects the body’s ability to make or use insulin. Insulin is a hormone produced in the pancreas that helps transport glucose (blood sugar) from the bloodstream into the cells so they can break it down and use it for fuel. People cannot live without insulin.

Diabetes results in abnormal levels of glucose in the bloodstream. This can cause severe short-term and long-term consequences ranging from brain damage to amputations and heart disease.

The root causes of diabetes are complex. Most cases begin with one of two processes:

• Metabolic. Unhealthy lifestyle factors such as overeating, physical inactivity and obesity can impair the body’s ability to use insulin. This is called insulin resistance. Uncontrollable risk factors including genetics, family history and age can also be involved. Metabolic forms of diabetes include:
  
  
  • Type 2 diabetes. This accounts for 90 to 95 percent of diabetic cases, according to the U.S. National Institutes of Health (NIH). Some of these patients have had prediabetes that went uncontrolled. Once considered a disease of middle and old age, type 2 is also becoming more common in youths as the incidence of childhood obesity grows.
    
    
  • Gestational diabetes. Hormonal changes contribute to this condition, which can develop in any previously nondiabetic woman during pregnancy, especially those who are overweight.
    
    
• Autoimmune. The body’s immune system can mistakenly destroy the insulin-producing beta cells of the pancreas. The causes of autoimmune diabetes are poorly understood, but genetics and family history play a role, and viruses or other environmental factors are believed to figure in. Autoimmune forms of diabetes include:
  
  
  • Type 1 diabetes. Formerly known as juvenile diabetes, this form generally develops in children and young adults.
    
    
  • Latent autoimmune diabetes of adulthood. This variation of type 1 can occur later in life.
    
    Individuals with autoimmune diabetes who overeat, are sedentary, gain weight or have certain genes can, like people with metabolic forms of diabetes, develop insulin resistance. This state is known as double diabetes.

Diabetes can also result from another disease, such as pancreatitis, or even from a medical treatment, including pancreatectomy (surgical removal of the pancreas) or certain medications. This is known as secondary diabetes. In addition, there are uncommon inherited disorders that cause diabetes, such as maturity-onset diabetes of the young and Wolfram syndrome.

Most cases of diabetes last the rest of a person’s life. However, gestational diabetes generally ends when the pregnancy does, and some cases of secondary diabetes are also temporary.

Diabetes involves chronic levels of abnormally high glucose (hyperglycemia). Many patients, especially those with type 2 diabetes, also have elevated blood pressure (hypertension), chronic high levels of insulin (hyperinsulinemia) and unhealthy levels of cholesterol and other blood fats (hyperlipidemia). All of these factors contribute to the long-term complications of diabetes, which include:

• Vascular disease (diabetic angiopathy), atherosclerosis, heart conditions and stroke. These cardiovascular disorders are the leading cause of death in people with diabetes.
  
  
• Kidney disease (diabetic nephropathy). Diabetes is the chief cause of end-stage renal disease, which requires treatment with dialysis or a kidney transplant.
  
  
• Eye diseases. These include diabetic retinopathy, glaucoma and cataracts. Diabetes is a leading cause of visual impairment and blindness.
  
  
• Nerve damage (diabetic neuropathy). This includes peripheral neuropathy, which often causes pain or numbness in the limbs, and autonomic neuropathy, which can impede digestion (gastroparesis) and contribute to sexual dysfunction and incontinence. Neuropathy may also impair hearing and other senses.
  
  
• Impaired thinking. Many studies have linked diabetes to increased risk of memory loss, dementia, Alzheimer’s disease and other cognitive deficits. Recently some researchers have suggested that Alzheimer’s disease might be “type 3 diabetes,” involving insulin resistance in the brain.
  
  
• Infections and wounds. Foot conditions and skin disorders, such as ulcers, make diabetes the leading cause of nontraumatic foot and leg amputations. People with diabetes are also prone to infections including periodontal disease, thrush, urinary tract infections and yeast infections.
  
  
• Cancer. Diabetes increases the risk of malignant tumors in the colon, pancreas, liver and several other organs.
  
  
• Musculoskeletal disorders. Conditions ranging from gout to osteoporosis to restless legs syndrome to myofascial pain syndrome are more common in diabetic patients than nondiabetics.
  
  
• Pregnancy complications. Diabetes increases the risk of preeclampsia, miscarriage, stillbirth and birth defects.
  
  
• Emotional difficulties. Many but not all of the studies exploring connections between diabetes and mental illness have found increased rates of depression, anxiety and other psychological disorders in diabetic patients.

In addition to chronic hyperglycemia, diabetic patients can experience acute episodes of hyperglycemia as well as hypoglycemia (low glucose). Severe cases can cause seizures, brain damage and a potentially fatal diabetic coma. Acute glucose emergencies include:

• Insulin shock. This advanced stage of hypoglycemia is typically due to excessive amounts of insulin medication or certain antidiabetic agents.
  
  
• Diabetic ketoacidosis. A lack of insulin can force the body to burn fats instead of glucose for energy. The result is a toxic byproduct called ketones, along with severe hyperglycemia.
  
  
• Hyperosmolar hyperglycemic nonketotic state. This involves severe hyperglycemia and dehydration.

These dangerous glucose complications are most common in patients with unstable diabetes, but they can develop even in individuals who do not realize they have diabetes. About one-third of the estimated 20.8 million Americans with diabetes have not yet been diagnosed, according to the U.S. Centers for Disease Control and Prevention (CDC).

The incidence of diabetes has soared worldwide in recent years and is expected to keep growing, with the greatest increase seen in metabolic forms of diabetes, notably type 2. This is blamed largely on the rise of obesity and the global spread of Western-style habits: physical inactivity along with a diet that is high in calories, processed carbohydrates and saturated fats and insufficient in fiber-rich whole foods. The aging of the population is also a factor. However, other factors, such as environment may also be contributing, because cases of autoimmune diabetes (type 1) are also becoming more common.

The estimated number of people with diabetes has jumped from 30 million in 1985 to 150 million in 2000 and then to 246 million in 2007, according to the International Diabetes Federation. It expects this number to hit 380 million by 2025. Seven percent of Americans have diabetes, according to the CDC, which predicts that one in three Americans born in 2000 will eventually become diabetic. Health agencies are warning that diabetes is becoming an unprecedented epidemic even as other major diseases including cancer and nondiabetic heart disease are being controlled.

Diabetes is ranked as the sixth-leading cause of death in the United States, but the actual ranking may be higher because it is underreported as a cause of death, according to the NIH. Diabetes kills more than 1 million people a year, the World Health Organization reports. It predicts life expectancy to decline worldwide for the first time in two centuries because of diabetes. Experts are urging people to help stem this epidemic by getting regular exercise and controlling their diet and weight.

Humans are not the only species that can develop diabetes. This disease also occurs in dogs, cats and other animals, as increasing numbers of pet owners are discovering.

The word “diabetes” stems from a Greek term for passing through, a reference to increased urination (polyuria), a common symptom of the disease. “Mellitus” is the Latin word for honeyed, a reference to glucose noted in the urine of diabetic patients.

Diabetes mellitus is sometimes referred to as sugar diabetes but usually is simply called diabetes. There is also a rare disease called diabetes insipidus (water diabetes) in which the kidneys release too much water. Like diabetes mellitus, it has excessive urination as a symptom, but these two endocrine disorders are otherwise unrelated.