A calorie is a unit of energy used to measure food. Most weight loss strategies focus on reducing calories and increasing movement. By contrast, Fung argues that caloric reduction is ineffective because it slows the metabolism, raises hunger hormones, and lowers satiety hormones.

Carbohydrates are one of three macronutrients alongside proteins and fats. Carbs are composed of sugars and function as the body’s prime source of energy. Several popular diets, such as the Atkins diet and Paleo recommend avoiding carbs. Fung refines this approach, advising readers to reduce their consumption of processed carbs because they are quickly absorbed and cause insulin spikes.

Often called the stress hormone, cortisol is produced in the adrenal cortex and mediates the fight-or-flight response by enhancing the availability of glucose. High cortisol levels raise glucose and insulin, thereby driving weight gain while low cortisol levels result in weight loss. Stress management and adequate sleep are key to controlling cortisol.