The diet centers on a "40:30:30" ratio of calories obtained daily from carbohydrates, proteins, and fats, respectively. The exact formula is always under debate, but studies over the past several years (including a non-scientific study by the PBS documentary show Scientific American Frontiers) have shown that it can produce weight loss at reasonable rates. The Scientific American Frontiers study compared the effectiveness of several popular 'diet' regimes including the Zone; somewhat to the surprise of the show's staff, the participants on the Zone experienced the greatest fat loss while simultaneously gaining muscle mass. Participants also reported the Zone as the easiest regime to adjust to, i.e. having the fewest adverse affects such as fatigue or hunger. Most people who report fatigue find that the fatigue diminishes by day 2 or 3.
"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 caloric balance is more efficient and 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. Using stored fat for energy causes weight loss.
Another key feature of the Zone diet, introduced in his later books, is an intake of the proper 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.