Critics point to the fact that there is little research to support Dr. Sears’ claim that his exact proportion of carb-to-protein intake yields the best insulin response. They believe that his classification of “good” and “bad” eicosanoids is oversimplified and misleading.

Most nutrition professionals favour a high-complex-carbohydrate, low-fat diet for general health and weight loss. They caution that high-protein, low-carbohydrate diets lack key nutrients for health, including dietary fibre, vitamin C, folic acid, and a few minerals. In addition, the high intake of meat products necessary to meet the protein requirements of the Zone Diet may place certain individuals at risk of heart disease due to increased intake of saturated fat and cholesterol. Persons with kidney failure are also warned to stay away from the diet, as high amounts of protein increase the amount of nitrogen-containing waste products the kidneys must process for elimination.

Critics of the Zone Diet concede that people who follow the diet carefully often experience significant weight loss. However, they point out that the Zone Diet is low in overall calories, with the average person taking in only 1,500 to 1,750 calories per day, and caution that the strict, and confusing, nature of the diet makes it difficult to follow. As a result, long-term compliance—and therefore lasting weight loss—with this diet is unlikely. Moreover, any diet low enough in calories will result in weight loss, regardless of its proportions of protein, carbohydrate, and fat.

Not all sports nutritionists and athletes believe that the Zone Diet is ideal for athletic training and performance. A significant amount of research in the field of exercise physiology has shown that fatigue during exercise is primarily caused by depletion of the body’s stored carbohydrates (called glycogen). Most athletes’ glycogen stores are depleted within 90 minutes of intense exercise, leaving muscles without any source of energy to fuel activity. As a result, many experts believe that a sufficient intake of carbohydrates before and during exercise is crucial for increasing endurance.