Dr. Jason Fung, MD

Fung came to insulin through kidney medicine, not weight loss. Working as a nephrologist with patients who had diabetes and obesity, he kept seeing insulin, not calorie intake, as the variable that predicted who got sicker and who improved. The Obesity Code turned that clinical observation into a full framework: obesity is a hormonal disorder driven by chronically elevated insulin, and, in his view, any approach that does not lower insulin is treating the wrong target.

Fasting is the lever Fung uses most, because it is the most direct way to give insulin an extended break. In The Complete Guide to Fasting, he lays out how eating windows of varying length can let insulin fall low enough for the body to use stored fat for fuel. Fasting is an option some people choose, not a requirement. His writing on therapeutic fasting is one of the sources RealFoods draws on for its fasting content. Fung has not reviewed or endorsed any RealFoods feature.

The Diabetes Code extends the argument to type 2 diabetes specifically, making the case that addressing insulin resistance matters more than chasing blood sugar numbers alone. That distinction, blood sugar readings versus the insulin resistance behind them, is one of the book's central themes. Anyone new to fasting can see how the stages work in practice below.

Fung's nephrology background also shapes how he talks about obesity and metabolic health more broadly. Kidneys are deeply involved in blood sugar and insulin regulation, and Fung has argued that treating obesity as simple math, what goes in against what goes out, ignores the hormonal signaling his own field of medicine takes for granted. That is part of why his fasting protocols are framed clinically, as a tool for lowering insulin on a schedule, rather than as a lifestyle trend. His wider point is that obesity and metabolic health sit downstream of a hormone, not a habit of overeating.