Obesity, a primary risk factor for developing type 2 diabetes, is an abnormally high amount of body fat. U.S. health agencies define obesity as a body mass index (BMI) of 30 or more and overweight as a BMI of 25 to 29.9. BMI is a calculation using a person’s height and weight.

Excessive weight has reached epidemic levels in the United States. Thirty-four percent of American adults are obese, the U.S. Centers for Disease Control and Prevention (CDC) reported in late 2007. In addition, a similar percentage is overweight. The prevalence of morbid obesity (BMI of 40 or more) jumped fourfold between 1986 and 2000, according to the CDC. Obese people have a 50 to 100 percent increased risk of death compared to people of normal weight, the National Institutes of Health reports.

The exact cause of type 2 diabetes is unknown, but obesity is one of the strongest factors in its development. In fact, about 85 percent of people who have type 2 diabetes are overweight or obese, according to the CDC. The International Diabetes Federation estimates that the number of diabetes cases worldwide will grow from about 246 million in 2007 to 380 million in 2025, fueled largely by rising rates of overweight and obesity. Many large studies have concluded that there is a strong link between obesity and diabetes, as strong as the association between smoking and lung cancer.

One reason for the link is that when body fat increases, impaired fasting glucose and impaired glucose tolerance (prediabetes) can develop. Most people with insulin resistance or glucose intolerance are unaware of their condition as there may not be symptoms to warn them of the progression toward diabetes.

Obesity is also a risk factor for gestational diabetes and many diabetic complications. It may cause an earlier onset of type 1 diabetes and can trigger double diabetes, which is the development of insulin resistance in someone with type 1 diabetes.