The Zone diet is a diet popularized in books by biochemist Barry Sears . It advocates balancing protein and carbohydrate in 3:4 ratios. It is not primarily a weight-loss "diet", though it can be used for that purpose.  Some nutritional experts, including some of Sears' former colleagues, are critical of his conclusions from the scientific evidence, contending that he has distorted or exaggerated the meaning of much of the basic research. They point out that no direct studies to verify his conclusions have been performed. Some experts consider The Zone a fad diet; others think it "not bad." 
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. 
Sears emphasizes a hormonal paradox contrary to the "low-fat" rationale, namely that low-fat diets increase the production of the hormone insulin, causing the body to store more fat. The example proposed by him is the cattle ranching practice of fattening livestock efficiently by feeding them lots of low-fat grain. He and others also point out the supposed irony that human diets in the West for the last twenty years have been full of low-fat carbohydrates, yet people are considered more obese now.
Additionally, Sears suggests fat consumption as essential for "burning" fat.
His rationale is: Monounsaturated fats in a meal contribute to a feeling of fullness and decreases the rate at which carbohydrates are absorbed into the bloodstream. Slower carbohydrate absorption means lower insulin levels which means less stored fat and a faster transition to fat burning. If the body needs energy and can't burn fat because of high insulin levels, a person feels tired as their brain starves and metabolism slows to compensate. This occurs because the brain runs on glucose and high insulin levels deplete blood glucose levels. Such condition, rebound hypoglycemia causes sweet cravings (which just starts the high-insulin cycle all over again).
Sears describes a Zone meal as follows: "Eat as much protein as the palm of your hand, as much nonstarchy raw vegetables as you can stand for the vitamins, enough carbohydrates to maintain mental clarity because the brain runs on glucose, and enough monounsaturated oils to keep feelings of hunger away."
The Zone is considered a low-carb diet.  It is not as restrictive in total carbohydrate intake as some of the other low-carbohydrate diets (e.g. the Atkins diet) that became extremely popular throughout the United States in 2003 and 2004. Sears claims these other diets miss the point. According to him, they ignore the importance of hormonal balance, as well as the influence of dietary balance on digestion and hormone production.
The introduction of the Zone in Italy began in 1997 by a physician, Aronne Romano M.D. who applied this nutritional style to patients and athletes. Since the 2nd edition of the book "Come Raggiungere la Zona" (The Zone), in 1999, the Chef Memo Romano and his brother Aronne modified the original recipes and menu to suit the local food and habits.
Famous obesity case
Possibly the most famous case of someone using the diet effectively has been Mexican Manuel Uribe. After weighing in at around 560 kg (1234 lbs or over 88 stone) but within a year had lost about 180 kg.  .
Several Hollywood stars, including Jennifer Aniston, Renee Zellweger, Cindy Crawford, Charlie Sheen and Tiger Wood are believed to have followed the Zone diet.
The American Heart Association does not recommend the Zone Diet due to high-protein, lack of essential nutrients, and little information on long-term effects.  However, characterization of the Zone diet as 'high-protein' may be inaccurate as the diet is not intended to increase protein intake beyond a typical American diet.
Most Vegetarian or vegan diets, according to Sears, are as far as you can get from The Zone because they generally utilize very little protein relative to carbohydrate consumption. This, says Sears, prohibits the body from operating truly efficiently. As critical as Sears is of vegetarian and vegan diets, individuals who promote a vegetarian diet are also very critical of aspects of the Zone and similar diets. In 2000 Dr. Sears published the Soy zone where he outlined a zone diet based around soy protein, making it more vegetarian friendly.
Other nutritional experts, including some of Sears' former colleagues, are critical of his conclusions from the scientific evidence, contending that he has distorted or exaggerated the meaning of much of the basic research. They point out that no direct studies to verify his conclusions have been performed. 
Official Zone books