Dr. Jason Fung opens The Obesity Code with his central, field-challenging thesis: obesity results primarily from hormonal dysfunction — specifically chronically elevated insulin levels — rather than simply reflecting a straightforward imbalance between calories consumed and calories expended, directly challenging the conventional "calories in, calories out" model dominating mainstream weight management advice.

Fung's foundational argument:

• Drawing on his clinical nephrology practice treating patients with diabetes and kidney disease, Fung observed that the conventional calorie-counting model frequently failed his patients despite genuine, sustained effort, leading him to examine underlying hormonal mechanisms as an alternative explanatory framework
• He argues that insulin, functioning as the body's primary fat-storage hormone, directly determines whether consumed calories get stored as body fat or made available for immediate energy use, meaning chronically elevated insulin levels — regardless of total calorie intake — can drive persistent fat accumulation and resistance to weight loss
• This foundational reframing directly challenges the conventional model's implicit assumption that all calories affect body weight identically regardless of their hormonal effects, arguing instead that specific foods' effects on insulin secretion matter as much as, or more than, their pure caloric content

This foundational hormonal framework sets up the book's overall project: examining specific dietary and lifestyle factors that influence chronic insulin elevation, and presenting fasting-based interventions specifically targeting this hormonal mechanism as more effective than conventional calorie-restriction approaches.