There are a number of factors that may cause double diabetes. The most significant cause of insulin resistance in a patient with type 1 diabetes is obesity because extra fat in the body impairs the effectiveness of insulin.

A healthy weight range can be determined by a person’s body mass index (BMI) or waist-hip ratio. A person with a BMI of 25 to 29.9 is considered overweight. A person with a BMI of 30 or more is considered obese. The distribution of weight is also a factor. People with excess weight above their hips, especially around their abdomen (central obesity), are at a higher risk than those with excess weight on their hips and thighs. For more information, see Obesity & Diabetes.

Excess weight also increases the risk of many other conditions including high blood pressure, unhealthy cholesterol levels, heart conditions, stroke, certain cancers, sleep apnea and sexual dysfunction.

The risk of developing insulin resistance generally increases with age, but many of the reported cases of double diabetes have occurred in adolescents and young adults. This is believed to be due to a combination of increased incidence of type 1 diabetes and the growing epidemic of childhood obesity.

Recent research shows that the proportion of patients who were overweight when diagnosed with type 1 diabetes tripled from the 1980s to the 1990s. Scientists also found that overweight children developed type 1 diabetes at an earlier age, possibly because insulin resistance hastened the loss of insulin-producing beta cells.

Other risk factors for double diabetes include a family history of type 2 diabetes and the inheritance of certain genes linked to insulin resistance. Race may also play a role. Some research has found double diabetes to be more common in black children than in white children, though whites are at greater risk for type 1 diabetes.