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The Zone diet is a diet popularized in books by biochemist Barry Sears. It advocates consuming calories from carbohydrates, protein, and fat in a balanced ratio.
The diet centers on a "40:30:30" ratio of calories obtained daily from carbohydrates, proteins, and fats, respectively. The ideal formula has been under debate, although 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' regimens 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. There is no evidence that eating in this way affects hormone levels.
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. Using stored fat for energy causes weight loss.
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 popularised the taking of pharmaceutical-grade Omega 3 fish oils.
Sears believes in a hormonal paradox contrary to the "low-fat/high carbohydrate" rationale of most diets (including the USDA "Food Pyramid"). He claims that the relatively high proportion of carbohydrate in these diets—by comparison to protein— increases the production of the hormone insulin, causing the body to store more fat. The example proposed by Sears is the cattle ranching practice of fattening livestock efficiently by feeding them high amounts of high-carbohydrate grain. However, excess fat gains can be attributable to the amount of the grain and thus the total amount of calories consumed. Sears also points out the supposed irony:
"data analysis ... shows that in spite of the fact that the American public has dramatically cut back on the amount of fat consumed, the country has experienced an epidemic rise in obesity." 
Additionally, Sears suggests fat consumption is essential for "burning" fat. His rationale is: Monounsaturated fats in a meal contribute to a feeling of fullness and decrease 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 a 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 non-starchy 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."
Comparison to low-carb diets
Whether the Zone diet is a low-carb diet is a matter of opinion and definition. It is much less restrictive in total carbohydrate intake than the Atkins diet that became extremely popular throughout the United States in 2003 and 2004. Sears claims that diets specifically designed as "low carb" miss the point. According to him, they ignore the importance of moderation and balance: hormonal balance, as well as the influence of dietary balance on digestion and hormone production. A reasonable argument could be made that the typical American follows a "high carb" diet, and that the Zone diet is simply a moderate one.
Famous obesity case
Possibly the most famous case of someone using the diet effectively has been Manuel Uribe. After weighing in at around 560 kg (1234 lbs or over 88 stone), he had lost about 184 kg within 2 years (he now weighs 376 kg). After decades of failed weight-loss methods, including a botched liposuction that nearly killed him, Uribe believes devotion to the Zone Diet has been the only weight-loss program to have such a positive impact on him. Dr. Barry Sears, along with two of Mexico's most prestigious physicians, Dr. Silvia Orozco Avina and Dr. Gustavo Orozco Avina, are at the helm of the interdisciplinary team of doctors, nutritionists and exercise physiologists who continue to work diligently to help Manuel reach his goal. Aside from Dr. Sears, the other physicians on the team are more skeptical of Uribe's use of the Zone diet, but feel that if it helps Manuel stick to a diet and lose weight, they will tolerate his use of it. Manuel's current diet consists of about 2,000 calories per day broken up into six meals. His specially-designed menu includes a wide variety of meals including egg-white omelets, fresh salads, chicken fajitas and fish filet in a bed of spring greens. In fact, Manuel's biggest problem is not lack of appetite control, but eating all the food he is supposed to eat.
The recommended protein intake, taken as meat, is a significant source of saturated fat. However, Sears believes the characterization of the Zone diet as 'high-protein' is inaccurate, as more calories come from carbohydrates than protein. In his book, he advocates a formula based on lean body mass and activity level to determine the appropriate daily intake of protein. For example, a female of average height and average build who has a moderately active lifestyle is encouraged to eat around 60g of protein daily.
The most common vegetarian or vegan diets, according to Sears, are highly dissimilar from The Zone because they generally utilize very little protein relative to carbohydrate consumption. Low-protein vegetarian diets, says Sears, prohibits the body from operating truly efficiently. As critical as Sears has been of what he calls typical vegetarian and vegan diets, vegetarians who promote low-protein diets have also been very critical of aspects of the Zone and similar diets. In 2000, Sears published the Soy Zone where he outlined a zone diet based around soy protein and soy foods, for vegetarians who wish to follow a Zone diet.
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.
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