"The Zone" is Sears' term for proper hormone balance. When insulin levels are neither too high nor too low and glucagon levels are not too high, then specific anti-inflammatory chemicals (types of eicosanoids) are released, which have similar effects to aspirin, but without downsides such as gastric bleeding. Sears claims that a 30:40 ratio of protein to carbohydrates triggers this effect, and this is called 'The Zone.' Sears claims that these natural anti-inflammatories are heart- and health-friendly.
Additionally, the human body in calorie balance does not have to store excess calories as fat. The human body cannot store fat and burn fat at the same time and Sears believes it takes time (significant time if insulin levels were high because of unbalanced eating) to switch from the former to the latter.
Another key feature of the Zone diet, introduced in his later books, is an intake of a particular ratio of Omega-3 to Omega-6 fatty acids. Dr. Sears is believed to have popularized the taking of pharmaceutical-grade Omega 3 fish oils.