Type 1 diabetes (previously called "juvenile diabetes") is an autoimmune in which the insulin producing beta cells are destroyed by the body's immune system. As a result the body is unable to produce insulin. Without insulin, the body is unable to use glucose (sugar) as energy for everyday activities. Individuals with type 1 diabetes must take insulin by injection or pump every day to survive. This type of diabetes occurs in 5 percent to 10 percent of Americans who are diagnosed with diabetes. Children and adolescents are most often diagnosed with type 1 diabetes although a significant portion of those with type 1 diabetes are diagnosed as adults.

Type 2 diabetes (previously called "adult onset diabetes") is the most common form of diabetes, accounting for about 90 percent to 95 percent of all diabetes cases. In this type of diabetes, the body does not produce enough insulin and/or the body's cells become resistant to insulin. Insulin resistance occurs when the body's muscle, fat, and liver cells do not respond to insulin. The pancreas tries to keep up with the demand for insulin by producing more. Since insulin helps to mobilize glucose from the blood stream into cells, excess glucose builds up in the blood stream. Many people with insulin resistance have high levels of blood glucose and high levels of insulin circulating in their blood at the same time indicating that the cells are not responding properly to insulin.

A related condition, called pre-diabetes, occurs when a person's blood sugar levels are higher than normal, but not high enough for a diagnosis of diabetes. People with prediabetes have impaired fasting glucose (fasting blood sugar level is 100 to 125 milligrams per deciliter [mg/dl]) or impaired glucose tolerance (blood sugar level is 140 to 199 mg/dl after a 2-hour oral glucose tolerance test). (23)

People with pre-diabetes and type 2 diabetes often do not show symptoms and they do not know that they have the conditions. The Diabetes Prevention Program — a major clinical trial in 3,234 people with impaired glucose tolerance-showed that in some individuals the loss of 5 percent to 7 percent of body weight reduced their risk of developing type 2 diabetes by 58 percent. (24) This study also suggested that people with pre-diabetes can prevent or delay the development of type 2 diabetes through lifestyle changes that include eating a low-calorie, low-fat diet to lose weight and getting 150 minutes of physical activity a week. (25) Gestational diabetes is a form of diabetes that occurs in some women who have high blood glucose levels during pregnancy but have never had diabetes before. This type of diabetes may disappear after the pregnancy ends, but women who have had gestational diabetes have a 20 percent to 50 percent chance of developing type 2 diabetes in the next 5 to 10 years. (26)