You should read the Obesity Code by Dr. Jason Fung. He explains that obesity is a result of constant elevated insulin levels. These elevated levels are a result of people eating the standard American diet which is high in refined carbohydrates and sugar. Insulin spikes high from these types of food and elevated insulin causes the body to store fat, hence the rising rates of obesity in Americans. This eventually leads to type 2 Diabetes if the diet continues for long enough without changing eating habits.