"All the health authorities have convincingly recommended to the nation at large that diets high in saturated fat are linked with [heart] disease and elevated cholesterol levels. So in that respect the fat is dangerous. It also lacks fiber, so it could be disconcerting to the [intestinal] tract. Also, the high protein content is taxing on the kidney, so if you have predisposition to kidney disease that could further escalate things," Zelman tells WebMD.
What troubles many experts is that some high-protein diets restrict fruits and vegetables as well as starches. "What you have in the grains and the fruits and vegetables ... are phytochemicals, and antioxidants and fiber," Zelman says. "You can't get phytochemicals in a pill. So what you are eliminating in your diet are these cancer-preventing nutrients that are found naturally in fruits and vegetables and grains."
"I think it is unfortunate because fruits and vegetables provide fiber and different nutrients than you'd get from high-protein foods," says Alice Lichtenstein, DSc, a professor of nutrition at Tufts University. "In the short term, I don't think I'd be concerned about it -- most people are not going to stay on this kind of diet as strictly as described for that long a period of time."
"First of all, I should point out that the Zone diet has no relation whatsoever to a high-protein diet; this is a very mistaken notion," Barry Sears, PhD, author of The Zone, tells WebMD. "A good definition of a 'high-protein' diet is any diet in which you are consuming a lot more protein than carbohydrates, and that is going to induce a state of ketosis. Nobody can say anything positive about ketosis." Sears is also the president of Sears Laboratories, a Chicago-based biotechnology company.
Sears says each meal should be 1/3 low-fat protein (the serving no bigger or thicker than the palm of your hand) and 2/3 fruits and vegetables, with a dash of monounsaturated fat. There is, however, little or no room for starches (like pasta and breads).
"The diet is based on two things: balance and moderation. You are balancing your plate in terms of protein and carbohydrates," says Sears. "Where I think the controversy of the Zone diet comes from is, it forces people to think of hormonal consequences, not simply calorie-counting. And that's an alien concept to nutritionists because they've been trained to think: calories in, calories out -- whereas the Zone says, even in [meals with similar caloric values] you can get these tremendous changes in hormonal responses simply by changing the balance of protein to carbohydrates within a meal." The object of the Zone diet is to keep certain measures of food metabolism, like insulin, within a target area or zone so that blood sugar levels stay constant and hunger levels stay low.