Double diabetes is a nonscientific term used to describe the development of insulin resistance (a hallmark of type 2 diabetes) in people with type 1 diabetes. It is a recent phenomenon and has been linked to the growing epidemic of obesity, particularly childhood obesity.

Although type 1 and type 2 diabetes share many of the same characteristics, they are different diseases. Type 1 diabetes is an autoimmune disease in which the body mistakenly attacks the insulin-producing cells of the pancreas. Insulin is a hormone needed to process glucose (blood sugar) for energy. Poorly controlled glucose damages the blood vessels, nerves and other structures. Patients with type 1 diabetes must administer insulin regularly. This disease is usually diagnosed in childhood.

Type 2 diabetes is a metabolic disorder. Generally, the body keeps making insulin but has trouble using it. Obesity is typically a factor in this insulin resistance. In addition, the pancreas’ production of insulin may decline. Some patients control type 2 diabetes through exercise and diet, but many also require oral or injectable antidiabetic agents and/or insulin.

In some cases, people with type 1 diabetes go on to develop insulin resistance, typically as a result of obesity. Excess fat makes it harder for the body to use insulin. Because they are unable to process the insulin they take by injection or other means, people with double diabetes often require higher doses of insulin to control their glucose.

Heredity is another risk factor for double diabetes. Research has found an increased chance of developing double diabetes in type 1 patients who have inherited certain genes associated with insulin resistance and type 2 diabetes.

Because patients with double diabetes may have signs and symptoms of type 1 and type 2 diabetes, diagnosis and treatment may be difficult. Additionally, insulin resistance and a related condition called hyperinsulinemia (excess insulin in the bloodstream) are associated with numerous health problems, including:

• Atherosclerosis
• High blood pressure
• Unhealthy levels of cholesterol

• Heart conditions
• Obesity
• Metabolic syndrome
• Fatty liver
• Increased risk of certain cancers
• A skin condition called acanthosis nigricans
• Polycystic ovarian syndrome, a hormonal problem that can affect women

The many potential complications of diabetes include cardiovascular disease, kidney disease (diabetic nephropathy), nerve disease (diabetic neuropathy) and eye diseases such as diabetic retinopathy and glaucoma. Some research has suggested that people with double diabetes may be at greater risk for problems such as coronary artery disease. However, having double diabetes does not mean a person will develop more complications, according to the National Diabetes Education Program.

Some research has found that 10 percent or more of diabetic children may have double diabetes. Major research projects, including the multicenter SEARCH for Diabetes in Youth study, are trying to establish reliable statistics on the incidence of the various forms of diabetes.

Double diabetes is sometimes called hybrid diabetes, mixed diabetes, type 1.5 diabetes or type 3 diabetes. However, the term “type 3 diabetes” has been used also to describe other conditions, including gestational diabetes and Alzheimer’s disease, which according to one theory involves insulin resistance in the brain. The term “type 1.5 diabetes” has been used also to refer to two disorders that do not involve insulin resistance or obesity:

• Latent autoimmune diabetes in adults (LADA). A variation of type 1 diabetes that can develop in adulthood.
  
  
• Maturity-onset diabetes of the young (MODY). A rare genetic form of diabetes that is sometimes considered a form of type 2 diabetes and is sometimes misdiagnosed as type 1.