The Zone Diet. It sounds like something that Mulder and Scully from the X-Files should investigate. Instead, it is one of the popular diets that has "swept" the world via the United States, and includes athletes among its target group. We first heard of the diet from articles in American Swimming magazines in 1993 where it claimed credit for the 9 Gold medals won by members of the Stanford swim team at the 1992 Barcelona Olympics Games. Since then it has blossomed into 2 best selling books by its creator, Dr Barry Sears: "Enter the Zone" (1995) and "Mastering the Zone" (1997). Like most popular diets, it promises to solve all the nutritional and medical problems of the world - from AIDS, impotence and diabetes - and to promote permanent and painless weight loss. However there are special chapters on benefits to athletic performance, and additional credit taken for another 9 Gold medals at the Atlanta Olympics. Since a number of athletes claim to be "on the Zone", or at least interested in it, we have carefully conducted an investigation of the Zone and its promises:

Dr Sears' books and promotional materials make many emotive claims about the dramatic benefits of being on the Zone diet. It associates itself with winners and winning performances. How could an athlete resist the promise of such a winning edge? And it claims to make breakthroughs in scientific understanding, filling pages with complicated biochemistry explained in simple language. How could a coach resist the impressive sounding "science", backed up case histories or "research". The Zone diet follows the rules that make many books into bestsellers - intrigue, controversy, and repetition of brave new "facts".

According to Dr Sears, the Zone is reached by finding the correct balance between the "good" and "bad" hormones in our body - between insulin and glucagon and the "good" and "bad" eicosanoids. He pulls no punches in attacking current healthy eating guidelines and nutrition research. He claims that high-carbohydrate, moderate-fat diets actually cause an overproduction of insulin, and that this is the cause of weight gain, ill-health and poor performance.

The keys to entering the Zone are to eat a meal or snack at least every 5 hours that fits the following profile:

• 30% of energy from protein. In fact, this is the baseline of the whole diet since protein must be eaten in the exact amount to meet body requirements (set by Dr Sears at a daily intake of 1.8-2.2 g per kilogram of lean body mass). Once protein intake is established from Tables provide in the Zone books, this sets the levels of total energy intake and the intake of the other macronutrients (fat and carbohydrate)
• 40% of energy from carbohydrates - emphasising those with a low glycemic index
• 30% of energy from fat - emphasising monounsaturated fats.

In fact, the Zone diet is an energy-restricted diet. Calculations for the Zone needs of 2 different athletes (representing members of the AIS Department of Sports Nutrition) are presented:

How do you put the Zone diet onto your plate?

Although many athletes will find the principles of the Zone and supporting biochemistry hard to understand, it is the practice of the Zone which will really throw them. Without extensive knowledge of nutrition and food composition it is hard to visualise a dietary ratio of "40:30:30". To achieve these goals Dr Sears has organised food into "carbohydrate" blocks, "protein blocks" and "fat blocks" and has set up formulas of how many blocks should be eaten at each meal and snack. In his latest books "Zone friendly" recipes are also provided. Dr Sears and other companies have additional help for the nutritionally-challenged, in the form of personalised dietary plans and menu programmes, and special bars with a 40:30:30 composition. The "PR" company, which makes PR bars and PR powder, has sponsorship deals with the Ironman triathlon company and several big name triathletes and sports people. In the USA there is plentiful evidence that the Zone is more than just a brief fad. Not only do companies sell Zone dietary plans and bars via 1-800 numbers and internet websites, but there are a series of food products (e.g. pancake mix) all proposing to be "Zone favourable". Not to mention restaurants and fast food places offering menus and meals that also carry this tag. This suggests that the Zone diet has longevity and market clout. And the cause is helped when famous people are rumoured to be devotees of the Zone, or when the Zone diet is given the credit for the fabulous figures and well-publicised weight loss achievements of the Superstars.

In reality, however, very few foods are a pure "block" of just one macronutrient. And once food starts being combined into recipes and mixed dishes, it is almost impossible to control ratios. A common and valid criticism of the Zone diet is whether those who say they are following the diet actually achieve its important formula. In fact, a paper presented at a conference of the American College of Sports Medicine found some interesting results when it crunched numbers on the menu plans and food advice provided in the Zone diet books. The researchers found that when followed to the letter, these diets don't conform to the magic mantra of the book - in fact, the protein levels are higher and the carbohydrate levels are significantly lower (eg 30-35% of energy). Clearly, pre-packaged meals and food products make it easier, but just what are the rewards of getting it right?

Most scientists are sceptical about the claims made for the Zone diet. In the Department of Sports Nutrition at the AIS we are also confused by how the diet is supposed to be followed by athletes. Dr Sears has a theory to explain our views. We belong to the conservative guard of conventional nutritionists/medical experts and are upset that he is challenging our beliefs. However, in our defence we see little rigorous evidence of the miracles that are promised by the Zone diet. Instead we see clever marketing, confusing dietary information, and some unrealistic medical claims in relation to diseases like cancer and AIDS.

"Enter the Zone" is complicated to read, full of scientific discussions, emotive promises and repetitive nutrition messages. The reader is told that "everytime you open your mouth to eat, you are applying for a passport to the Zone", with the Zone promising optimal health, physical performance and mental alertness for the rest of their lives. Dr Sears repeatedly attacks current healthy eating guidelines and research, claiming that high-carbohydrate, moderate-fat diets actually are the cause of weight and health problems. However, there is no research to support the claims that the Zone promises optimal health, a cure for disease or winning performances. Testimonials from "satisfied customers" and success stories from people who have achieved benefits from following the Zone diet do not replace research. The couple of "studies" quoted in the book in which a group of athletes were tested before and after a period on the Zone diet do not constitute rigorous research. Because there was no control group in this "study" we don't know how much the Zone diet contributed to the changes seen in the athletes, and how much was a result of their period of hard training.

Like many popular diet books, the Zone diet has promises for everyone. However, there appear to be two target groups who are most likely to respond to the Zone marketing. The first are those who struggle with their weight and the associated problems of being overfat. For these people, the Zone offers an end to the battle, promising that the diet will allow people to tap into their body fat stores and lose weight forever. While being able to eat as much food as they like. Weight loss is indeed likely, but the "forever" and "unrestricted eating" parts are improbable. Although Dr Sears is coy about mentioning this, the Zone is essentially an energy-restricted diet (see the calculations done above for 2 people undertaking regular exercise).

Of course, weight loss on the Zone diet is cleverly disguised behind "scientific" explanations about more efficient fat-burning, and complicated instructions about mixing and matching foods together. Other diets such as the Beverly Hills Diet, and the Fit for Life diet have used the same strategies. Generally they "work" because people are surreptitiously organised into eating less food than they used to. While they follow the instructions and eat less food, they lose weight. When they go back to their normal eating patterns, they will regain weight. Simple! Nevertheless, studies are needed to see whether the Zone offers any advantages over other versions of low-Calorie eating.

An athlete who calculated their Zone intake, based strictly on the instructions provided in the books, would be in a state of energy deprivation. This strategy would certainly cause them to "access stored body fat". (i.e. lose body fat) There is nothing magic about this! Of course, this assumes that the athlete wants to lose body fat. And many athletes do. However, eating a very low energy intake is an unnecessarily extreme way to become leaner - not to mention, counterproductive to training. Without sufficient carbohydrate to replace muscle glycogen stores, it is impossible to undertake high-intensity exercise at optimum effort. An athlete might be able to get away with low-moderate intensity training on such a diet. However, quality sessions or interval sessions would suffer.

The sections on athletes and performance in the Zone diet are particularly confusing and contradictory. The back of the book promises: "athletes do better on a high fat diet". However, the Zone diet if followed to the letter of the book, is not a high fat diet. Rather it is a low energy, low fat diet - providing most people with 30-70 g of fat each day. If an athlete believed that adaptation to a high-fat diet is an advantage for performance - and that is the topic for another fact sheet - then the Zone diet will not look after these needs.

There is no direct explanation for the athlete who doesn't want to lose any more body fat. Clearly, lean athletes need to be in energy balance by eating far more Calories than the Zone book allows for. There are some suggestions in the Zone book that these athletes should make up extra fuel needs by eating more mono-unsaturated fat. However, an athlete who dies this will no longer eating a 40:30:30 dietary mix, and the book is light on practical advice on how to organise such an eating plan.

The "Zone" athletes to whom we have spoken interpret the diet in different ways and at different levels of adherence. Some simply treat it as a general principle and "do their own thing". They steer clear of pasta and bread (dietary enemies of the Zone). They eat larger serves of protein at meals. And they like the 40:30:30 sports bars. Let's face it - a high-carbohydrate low-fat bar is generally sweet and chewy, whatever flavour you make it in. But a bar that has a higher fat and protein content can be made with chocolate-coating, and fudge or peanut butter flavouring. What would your taste buds choose? Some of the athletes who sing the Zone's praises are those who have followed the energy restrictions closely for a period, and have rejoiced at the body fat loss. To be honest we have never seen an athlete achieve a true dietary mix of 40:30:30.

To date there is only one published study concerning the Zone diet and athletic performance. Dr Laurie Rauch and colleagues from the Sports Science Institute of South Africa tested ultra-endurance cyclists on two occasions, using a riding protocol that required 5.5 hours of submaximal work followed by a time trial estimated to take another half hour. On one ride they consumed carbohydrate via a sports drink during the ride, according to sports nutrition guidelines. On the other occasion, under identical conditions, they consumed 40:30:30 sports bars providing equal energy intake and an equal amount of fluid. The result: the 40:30:30 bars resulted in a fuel mixture with more fat and less carbohydrate during the exercise. However, the cyclists performed better with the carbohydrate drink - in fact, performance was directly related to the ability to burn carbohydrate. Two out of the 6 subjects were unable to finish the ride on the 40:30:30 bar treatment.

There are no published studies that support any benefits of the Zone diet on athletic performance. We are left with the present conclusion that the 40:30:30 diet is a well-marketed nutrition craze. Whether it will stand the test of scientific scrutiny, or the test of time, is a topic for the future. At the moment it is simply untested.

Further reading:

Cheuvront, S. N. The zone diet and athletic performance. Sports Medicine. 27: 213-228, 1999.

Rauch, H. G. L., J. A. Hawley, M. Woodey, T. D. Noakes, and S. C. Dennis. Effects of ingesting a sports bar versus glucose polymer on substrate utilisation and ultra-endurance performance. International Journal of Sports Medicine. 20: 252-257, 1999.