Think about your group of friends: Among them is surely someone who waxes enthusiastic about the Atkins diet, another who tried it but then regained the 10 lost pounds, someone else who found success with a low-fat, higher-carb regimen, and another who couldn't stay on that for more than a week but swears by South Beach. The more science learns about the marvelous diversity among human beings—fueled by genetic, environmental, and other factors—the clearer it becomes that a one-size-fits-all approach to diet is bound to fail.
Indeed, the weight-loss industry has cottoned to the notion that one specific diet isn't likely to work for everyone and is churning out eating plans based on what authors say is scientific evidence that taking your individual differences into account makes success more likely. One (The Metabolism Miracle, by dietitian Diane Kress) holds that the estimated quarter of U.S. adults with the high blood pressure, abdominal obesity, insulin resistance, and other symptoms known collectively as metabolic syndrome need a period of low-carb dieting to rest their "overworked pancreas and liver while shrinking fat cells," then the "methodical" re-introduction of carbs. Another, the Women ' s Health Perfect Body Diet, offers two different eating plans keyed to your "carb tolerance." Meantime, The GenoType Diet, by naturopathic physician Peter D'Adamo, uses blood type, fingerprint analysis, and leg length measurements, among other factors, to prescribe one of six diets based on a "unique genetic type." And if you've got more extra cash than the $25 you'd spend on a book, various companies offer gene tests that they promise will identify the ideal mix of macronutrients (carbs, protein, and fat) and the exercise intensity that will most likely lead to weight loss.
Is there anything to the fancy formulas and rules? (T he GenoType Diet advises "Hunters," who have a "heightened sense of fair play," symmetrical bodies, and type O blood, to embrace beef tongue but spurn ham and kangaroo.) The bottom line remains that if you want to lose weight on a diet, you have to burn off more calories than you're taking in. As the authors of a study that compared diets with different compositions of macronutrients— fat, protein, and carbs—wrote in the New England Journal of Medicine earlier this year: "These findings together point to behavioral factors rather than macronutrient metabolism as the main influences on weight loss." They concluded that "any type of diet, when taught for the purpose of weight loss with enthusiasm and persistence, can be effective." This line of evidence stretches back years, says George Bray, chief of the division of clinical obesity and metabolism at the Pennington Biomedical Research Center of Louisiana State University and an author of the NEJM study. There's no evidence, he says, that substituting the same number of calories in fat for carbs is going to make you lose more weight or body fat.
But the hard part of dieting, as anyone who's tried it can attest, is not so much finding an eating plan with the appropriate number of calories but following it. That means adopting a balanced diet that's going to support your health, not just your weight loss, and that matches your preferences and lifestyle. Some of those preferences, and the rest of our eating habits, are surely influenced by genes.
For example, researchers have reported in the last year that genes predisposing people to obesity act in the brain to mediate behavior, rather than directly influencing metabolism. In other words, obese people are more likely to overeat. Satiety mechanisms are complex and involve multiple sensors, some of which detect one macronutrient better than the other two; there's probably some genetic variation in the way each sensor operates, says Dale Schoeller, a professor at the University of Wisconsin-Madison who researches biochemical and molecular nutrition. So yes, it's entirely possible that a carb-heavy breakfast can leave you hungry in two hours, while it keeps your neighbor full until noon. For someone else, it might be high-protein or high-fiber. (That doesn't mean that anyone should entirely eliminate a macronutrient or prohibit otherwise healthy foods entirely unless there's some medical reason for doing so.)
Clarified on 10/02/09: An earlier version of this story gave an imprecise title for Ken Kornman. He is founder and chief scientific officer of Interleukin Genetics, which makes Inherent Health brand tests.