In Chapter 6, Fung outlines the problems with current approaches to obesity and introduces an alternative theory. Researchers have focused on caloric intake, genetics, food types, behaviors, disorders, and socio-economic factors to explain obesity. According to Fung, existing theories are mutually exclusive and reductive, focusing on “only one true cause of obesity” (70). Drawing an analogy with heart disease, which can have many contributing causes, including family history, sex, age, smoking, and stress, Fung argues that there is no single explanation for obesity. He also stresses that obesity is time-dependent, meaning it develops over long periods. Fung defines obesity as a “hormonal dysregulation of fat mass” (71): The body maintains a strict set weight, or a “fat thermostat” (71). Obesity results when hormonal imbalances elevate this thermostat.

Fung draws a connection between insulin, obesity, and carbohydrate intake. Carbohydrates raise blood sugar more than other foods, which stimulates the release of insulin, a hormone that promotes fat accumulation and storage. While the body stores some excess glucose in the liver as glycogen, the process of de novo lipogenesis turns most of it into fat.