The study reviewed here investigated the effects on insulin signaling in humans after consuming a fast-food, high calorie diet for 4 weeks that led to weight gain (Danielsson A., et al. 2009).
After we eat, insulin transports the glucose from the blood into the cells where it can be used for energy. People who develop type 2 diabetes first develop what is called insulin resistance, which means the insulin is no longer able to transfer the blood glucose into the cells as efficiently as it used to. This happens because receptors on the cells start to be less sensitive to the insulin signal.
The researchers conducting this study discovered that it did not take very long for the participants to develop insulin resistance. After only 4 weeks, they developed moderate systemic insulin resistance. The amount of insulin receptors was reduced similar to what happens in type 2 diabetes. These study participants gained 10% body weight, but they were still fairly lean with a body mass index of 24.3.
Even without being severely overweight these participants still developed insulin resistance.
Insulin resistance can not only lead to type 2 diabetes, but also many other serious diseases.