Since the publication of the original Enter the Zone diet book in 1995, the Zone diet has emerged as one of the most popular around. Its popularity is evident in the subsequent publication of at least 10 other Zone-based books, with the latest focusing on the benefits of ‘pharmaceutical fish oil.’ Biochemistry Ph.D. Barry Sears originated the Zone concept. Sales rhetoric aside, though, how scientific is the Zone diet?
Sears bristles when the Zone is referred to as a ‘low-carbohydrate diet.’ He notes that his plan allows a generous intake of various fruits and vegetables, all having low-glycemic-index numbers. The Zone philosophy rests on insulin control, and in that respect it’s similar to low-carb diets like the one advocated by the late Dr. Robert Atkins.
The Zone, however, differs from other diets that claim to control insulin’it demands adherence to a strict ratio of nutrients. Specifically, it mandates that each meal contain 40 percent carbohydrate, 30 percent protein and 30 percent fat. That ratio represents the ideal combination for limiting insulin release while promoting the release of glucagon, a hormone that counters the effects of insulin in the body.
Sears says that Zone-style eating fosters maximum health and fitness by manipulating the synthesis of eicosanoids, hormonelike substances produced from fat. He classifies eicosanoids as either ‘good’ or ‘bad’ and notes that when the good eicosanoids dominate’as they theoretically do in the Zone’your body taps into fat stores. The higher protein intake suggested for the diet helps you maintain lean mass during fat loss without promoting excess insulin release.
A recent independent review examined the Zone diet’s scientific claims.1 It notes that compared to more conventional diets on a calorie-by-calorie basis, the Zone generates no greater fat oxidation. Surprisingly, the level of insulin released after meals doesn’t vary that much either. The author of the review even suggests that the Zone may promote a greater release of insulin than conventional-diet meals. The weight loss that occurs from Zone dieting is more likely a result of its low-calorie aspects. A study published a few years ago calculated that eating the food blocks suggested by the Zone resulted in a daily total intake of only 1,400 calories for a man, which guarantees weight loss in an active person.
As for the eicosanoid concept, whether good or bad eicosanoids are produced depends on which enzyme system is stimulated. One reason that carbs are considered bad when you’re on the Zone diet is that they inhibit a certain enzyme system that favors good eicosanoids, but so does glucagon’which the Zone promotes. Sears never explains that notable contradiction.
On the other hand, the Zone does have some genuinely beneficial properties. The focus on maximum-nutrient-density foods and minimum calorie density is thought to be the key to longevity. Sears strongly suggests a generous intake of omega-3 fatty acids, found naturally in fatty fish, such as mackerel, sardines and herring. Omega-3 fat intake encourages decreased inflammation in the body, which fosters good health. Irrefutable research has confirmed as much.
But while Sears’ early books suggested getting your omega-3 fats from food sources, he’s lately begun selling what he calls ‘pharmaceutical-grade fish oil,’ and he even wrote a book extolling the merits of that particular oil supplement. He goes further, warning that other supplemental forms of fish oil on the market are basically garbage, possibly containing contaminants that will make you feel ill if you take them in ‘therapeutic ranges.’
Sears sells his pharmaceutical-grade fish oil for about $70 a bottle. While his product is probably as pure as he claims, what he fails to tell potential consumers is that none’and I mean not one’of the studies showing the value of omega-3 fats used his fish oil. All studies used either what Sears called ‘health-food-grade fish oil,’ or (gasp!) plain old fatty fish.
When claiming that most fish-oil supplements on the market are laden with impurities, Sears fails to disclose that any such contaminants would have to be measured in billionths of a gram, or at a level that wouldn’t cause problems in any form of life higher than an amoeba. Using Sears’ fish oil is a little like putting premium gas in a car that runs perfectly on regular’just a waste of money.
The purity issue is underscored by a recent survey of 16 brands of commercial fish-oil supplements by Consumer Reports magazine (July ’03). Two independent labs carefully analyzed the supplements for potency and impurities, and the products all matched their label-value contents of omega-3 fatty acids. None showed any indications of spoilage or rancidity, nor did any contain contaminants, such as mercury, PCBs or dioxin. Unlike fish, fish-oil supplements don’t contain mercury, which is water-soluble and stays behind when fish oil is extracted during processing. The same holds true for the heavy metals, such as cadmium and lead.
The Zone diet is better than many others, but the science behind many of its claims is science fiction, based more on speculation than fact. Even worse, few people are motivated enough to follow the Zone diet’s strict dictates about portion sizes that require weighing foods and so on. Of course, Sears offers a convenient alternative: prepackaged Zone-based meals and energy bars.
1 Cheuvront, S. (2003). The Zone diet phenomenon: A closer look at the science behind the claims. J Am Dietetic Assoc. 22:9-17.
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