Insulin resistance increases as weight increases and physical activity decreases. Some people with type 2 diabetes must inject insulin, but most can control the disease through a combination of weight loss, exercise, and prescription oral diabetes medication. Scientists are not certain, but like type 1 diabetes, type 2 disease also runs in families, which suggests some genetic component. In fact, a genetic link seems even stronger in type 2 diabetes than in type 1. Scientists have not yet put their finger on a single gene that causes the disease, but they are finding errors in several that may contribute to the disease.
Researchers also suspect a genetic susceptibility to obesity.
Obesity is the single most important cause of type 2 diabetes. Definitions of obesity differ, but in general, you're obese if you weigh at least 20 percent more than what's recommended for your height and build. Three-quarters of type 2 diabetics are overweight. For reasons that remain unclear, carrying excess body fat somehow causes insulin resistance. That's why type 2 diabetes is usually treated with diet and exercise.