The Zone diet proposed by Dr. Barry Sears continues to be one of the most popular and controversial new diet programs. This program revolves around a 40/30/30 ratio of carbohydrate/protein/fat. The amount of recommended food consumed is determined by calculating protein needs and then adding the appropriate balance of carbs and fat.
It is claimed that this dietary program is best for losing unwanted bodyfat and will promote improved overall health and well-being. One of the central features is that it is claimed that this diet will promote lower insulin and higher glucagon levels, thus promoting less bodyfat and more muscle, as well as higher levels of the "good" eicosinoids and lower levels of the "bad" eicosinoids. Eicosinoids are hormone-like substances in the body that have wide ranging effects, such as promoting increased or decreased inflammation.
Central to the theory of the zone diet's ability to promote leanness is it's claim that it results in lower insulin levels. Insulin, it is claimed, stimulates the accumulation of increased bodyfat, whereas glucagon tends to stimulate leanness and muscularity. The "correct" balance of carbs/protein/fat will result in lowered insulin/higher glucagon levels. 40/30/30 will make you both feel and look better.
40/30/30 food bars, meal replacement powders, zone diet centers, and dieticians promoting this type of dietary approach have sprung up all over the country.
The following is a critical review of Barry Sear's Zone diet
by respected nutritionist, Ellen Coleman. It was published in the
International Journal of Sports Nutrition, March 1996, pp. 19-21:
The BioZone Nutrition System -- a Dietary Panacea?
A currently popular sports nutrition fad revolves around a dietary regimen proposed by Barry Sears, Ph.D., the author of The Zone (ReganBooks, 1995) and creator of BioZone nutrition bars. The Zone is defined as the seemingly effortless, euphoric state where body and mind work at peak efficiency. While in the Zone, athletes will allegedly reach their maximum athletic performance. Sears' book attributes these benefits to altering the production of eicosanoids with the Zone diet. The Zone claims that eicosanoids are the most powerful of all hormone systems and have ultimate control over all physiological functions.
An understanding of the function and formation of eicosanoids is necessary to evaluate the performance and health claims made for the Zone diet. Eicosanoids are the biologically-active, hormone-like compounds known as prostaglandins, thromboxanes, and leukotrienes that are synthesized from 20-carbon (hence the name) unsaturated fatty acids. They have diverse and often antagonistic effects and are produced in response to stimulatory events such as infection, trauma, allergy, or toxin exposure. A balanced production of eicosanoids regulates the local tissue response to stimulatory events (10).
The production of eicosanoids may be manipulated by the type of dietary fat eaten, since different kinds of eicosanoids are synthesized from omega-6 compared to omega-3 fatty acids. Consumption of omega-3 fatty acids (found predominantly in fish oils) is associated with a greater depression in thromboxane relative to prostacyclin levels, which reduces platelet aggregation and increases bleeding time (5). The low incidence of heart disease in Greenland Eskimos has been attributed to their high intake of fish oils (5,10).
Optimal athletic performance and health according to The Zone means that the body makes more "good" eicosanoids than "bad" ones. In fact, The Zone's definition of disease is that the body makes more "bad" eicosanoids than "good" ones.
The Zone classifies prostaglandin E1 and prostoglandin I2 (prostacyclin) as "good" because they inhibit platelet aggregation, promote vasodilation, and are anti-inflammatory. Prostaglandin E2, thromboxanes and leukotrienes are classified as "bad" because they promote platelet aggregation, vasoconstriction, and are pro-inflammatory.
To control eicosanoids and so enter the Zone, The Zone advises a dietary regimen of 30% protein, 40% carbohydrate, and 30% fat at each meal and snack. Three small meals (not over 500 calories) and two snacks (not over 100 calories) are recommended. The protein to carbohydrate ratio of the Zone diet and BioZone nutrition bar allegedly maintain the proper balance between the hormones insulin and glucagon.
The correct insulin-glucagon balance in turn supposedly increases the production of "good" eicosanoids which bring about the Zone. According to Sears, high carbohydrate diets impair athletic performance and make athletes fat. The Zone recommends limiting carbohydrate to keep the body from producing too much insulin, because high insulin levels allegedly increase the production of "bad" eicosanoids. "Bad" eicosanoids purportedly impair athletic performance by reducing oxygen transfer to the cells, lowering blood glucose levels, and interfering with body fat utilization. The Zone asserts that "bad" eicosanoids ultimately even cause heart disease, cancer, and autoimmune diseases.
The protein content of the Zone diet supposedly increases glucagon levels, thereby maintaining the appropriate balance between insulin and glucagon. The Zone claims that glucagon helps to increase the production of "good" eicosanoids by opposing the effect of insulin. Sears' "glucagon favorable diet" supposedly maintains blood glucose, increases endurance by increasing fatty acid utilization, and reduces body fat by increasing the utilization of stored fat.
The lay reader may be impressed and intimidated by the scientific-sounding information presented in The Zone. However, the scientific basis for the Zone diet can be faulted on many fronts. I'll begin with the claim that high carbohydrate diets increase insulin levels, thereby causing low blood glucose and suppressing fat mobilization.
Carbohydrate feedings 30-60 minutes before exercise do raise insulin levels and lower blood glucose, but these responses are temporary and will not harm performance (3). This insulin response does not impair fat mobilization or cause accelerated glycogen depletion (3). In fact, consuming carbohydrate an hour before exercise has been shown to improve performance (7). Carbohydrate feedings three to four hours before exercise also enhance performance by "topping off" muscle and liver glycogen stores (8). Lastly, carbohydrate feedings during exercise improve performance (2) and improve glycogen repletion following exercise (4).
The Zone's claim that a high carbohydrate diet promotes greater body fat storage is also unfounded. Carbohydrates, not fatty acids, are used preferentially for energy during exercise at or above 70% of V02Max -- the intensity at which most athletes train and compete (2). Athletes don't usually work out long enough to burn significant amounts of fat during exercise. Rather, it is the caloric deficit resulting from the exercise session that promotes body fat utilization. Carbohydrates will be converted to fat only if they are eaten in excess of calorie requirements. However, when compared to dietary fat, dietary carbohydrate is more likely to be burned for energy than stored as fat (9).
The metabolic pathways presented in The Zone which supposedly connect diet, insulin-glucagon, and eicosanoids do not exist in standard nutrition or biochemistry texts (5,10). The idea that the Zone diet (or any diet) completely controls the secretion of insulin and glucagon is not supported by the relationship between nutrition and endocrinology (10). Next, the notion that the insulin-glucagon axis controls the production of eicosanoids is not supported by biochemistry (5). And finally, the belief that eicosanoids control all physiological functions (including athletic performance, health, and disease) is not only unfounded (5,10), it is an appalling over-simplification of complex physiological processes.
In science, no hypothesis is presumed to be true until it has been clearly demonstrated. Responsible health professionals should actively oppose the Zone diet because nutrition recommendations must be supported by scientific research. The Zone, however, abounds with unproven claims based on case histories, testimonials, and unpublished, poorly controlled studies. The Zone also ridicules double-blind, placebo-controlled, peer-reviewed published research and attacks current nutrition recommendations based on this research.
The Zone promotes the fundamental nutrition misbelief that "you are what you eat" -- that everything from health to physical and mental performance is determined by diet (6). Many people want to believe that there are simple cures for ailments and easy paths to perfection. Unfortunately, in spite of what The Zone claims, a dietary panacea does not exist.
At the very least, following the Zone diet may impair athletic
performance due to inadequate dietary carbohydrate (1), and
possibly, calorie intake. However, I feel that The Zone's
deceptive health claims are particularly insidious because they
create false hope for desperate people who seek a cure for
life-threatening illnesses such as heart disease and cancer.