Much attention has lately been given in the media to high protein diets, such as The Zone™ diet plan. How valid is the rationale behind these diets? Are these diets appropriate for the athlete?
The Zone™ dietary plan bills itself as "not a diet, but a life long hormonal control strategy." According to Dr. Barry Sears, author of The Zone™ books, it is insulin which makes you fat and which keeps you fat. Insulin is a hormone which removes sugars and proteins from the blood, promoting energy utilization or storage, as well as protein synthesis. The Zone™ web site states that "the more insulin you produce the fatter you become," and contends that high carbohydrate intake leads to obesity. It goes so far as to say that the introduction of grain into the diet caused mankind to shrink from inadequate protein, and that heart disease, arthritis and obesity all result from carbohydrates and insulin.
The crux of this diet plan is the existence of compounds called eicosanoids. Eicosanoids are hormone-like compounds which include prostaglandins, thromboxanes, and leukotrienes. These compounds have diverse physiologic effects, both promoting and inhibiting platelet aggregation [blood clotting], dilating and constricting blood vessels, and causing and inhibiting inflammation. Dr. Sears suggests that there are "good" and "bad" eicosanoids, and that the ratio of good eicosanoids to bad can be changed by following the Zone™ diet. A connection is also made by Dr. Sears between eicosanoids, diet and exercise performance.
Let us examine the scientific literature available on eicosanoids, insulin, diet and exercise. At the heart of the Zone™ plan is the concept that ill health and obesity, results from carbohydrate intake and the insulin used to process these sugars. Insulin, in fact, is used to also process proteins and in the formation of lean tissue. It is true that obese people often have higher levels of insulin. Excess body fat tends to make the individual less sensitive to insulin's effects, and as a result more insulin must be manufactured to maintain a steady blood sugar. High insulin levels do not result in obesity; obesity leads to higher insulin. As the individual loses weight, the requirement for insulin decreases, thus lean individuals generally have lower insulin levels.
A central tenet of the Zone™ plan is that insulin and its counterpart, glucagon, regulate eicosanoid production. A review of literature, even the references cited by Dr. Sears, does not show that any diet can control eicosanoid production via insulin and glucagon. No scientific evidence is offered which would indicate that the Zone™ diet controls glucagon.
Carbohydrates make you fat? Catchy phrase, but is it so? There is scientific evidence quite to the contrary. It has been shown that carbohydrates are more likely to be used for energy than stored as fat, as compared to a diet high in fats [Sims, et al]. Carbohydrates will be converted to body fat only if excess calories are consumed. If one thinks of cultures that rely heavily upon carbohydrates the weakness of the Zone™ rationale will instantly come to mind. If one imagines the diets throughout southeast and central Asia, and the typical body size of those peoples, it is clear that carbohydrates are not responsible for obesity. In a roundtable discussion hosted by the Gatorade Sport Science Exchange , the typical Chinese diet was described by a leading nutritionist:
The 'typical' Chinese diet tends to be higher in carbohydrate and fiber, and lower in fat than the Western diet. In the morning people often take porridge or millet gruel for the main food. Soybean milk, salted vegetables, eggs [boiled or fried], deep fried dough sticks or cakes are the common choices for breakfast. For lunch and dinner, steamed rice and boiled noodles are major foods...eggs, chicken, fish, meat and vegetables are non-staple foods.
The diet described above is low in protein, and high in carbohydrate and sugars, yet obesity is uncommon among these people. Carbohydrates do not make you fat; excess calories make you fat.
How useful are high protein diets for athletes? Anecdotal reports are included in the Zone™ books of athletes competing successfully after following the Zone™ plan. Dr. Sears declares that humans have evolved to eat a very specific ratio of protein to carbohydrates, outlining a diet which provides 30% of its calories from protein, and 40% from carbohydrates. How much protein would this represent for an athlete? A hypothetical 150 pound triathlete who trains 6-8 hours per week, will require approximately 3000 calories per day. If 30% of these calories came from protein the athlete would eat 225 grams of protein per day.
Endurance athletes are estimated to require about 50% more protein per day than the Recommended Daily Allowance. The protein needs of our hypothetical athlete then would be about 80-100 grams per day. The Zone™ plan provides nearly 300% more protein than the athlete requires. What happens to this extra protein? It is converted to sugars, used for energy, or stored as fat just as carbohydrates would be. Protein is an expensive way to get calories. Although Dr. Sears' books cite scientific research to support the desirability of the Zone™ plan for athletes, the research was in fact done on high fat, not high protein, diets [Muoio, 94].
The Zone™ plan may be useful in encouraging the athlete to ingest more fluids and more servings of vegetables and fruits. It is first and foremost a weight loss plan. We followed the Zone™'s food plan for a 150 lb man. The recommended diet, as calculated by the Nutritionist IV software program, provided 118 grams of protein and only 1184 calories. Clearly, our hypothetical athlete on such a diet would lose weight, rapidly. In weight dependent sports such as running, weight loss may aid performance.
In summary, the claim that carbohydrates and insulin make you fat cannot be scientifically supported, nor can a connection be made at this time between carbohydrates and specific eicosanoid production. Athletes would be well served to eat a diet which is more well rounded than simply grain products: they should consume adequate protein sources, as well as dairy, fruits, and vegetables daily. Little research exists to corroborate the claim that endurance athletes should ingest very high protein diets. Given the minimal number of calories these diets provide they are an effective means of losing weight.