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? (The 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.)

It's not yet at all clear what determines the differences between dieters--although Jose Ordovas, director of the Nutrition and Genomics Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts University, is optimistic that the day will come. One aim of nutritional genomics is to discover how genes and food interact in order to prescribe diets that will better address individual predisposition to things like high cholesterol, osteoporosis, cancer, and diabetes as well as weight loss. "In the future, we'll predict the risk of the individual, and from the beginning we'll be able to put them on the kind of diet that will work for them," he says. "So if we're successful, the individual will know from day one that it's going to work for him."

But, he says, the gene tests on the market are not likely to help much in their current iterations. There's a long road from identifying an association between a variation in a certain gene and the ability to thrive on a certain type of diet to translating the knowledge into anything useful. Associations are often found in small populations--say, among 69 obese Spaniards, those with a particular gene variation are less likely to lose body fat on a low-calorie diet--but don't necessarily hold true for large populations. There's plenty of promise in this area, says Ordovas, but much more comprehensive data is needed. "We have to be very careful," he says. "We need to create a new science that's solid and can't be confused with snake oil."

Proponents of using existing knowledge to prescribe personalized diets say that some information, albeit imperfect, is better than nothing. "We don't have a good idea of how these individual gene variations interact, but we can integrate this information with what we know about health and family history to identify the best path to take in a very educated way," says Colleen Fogarty Draper, a registered dietitian, nutritional genomics consultant, and founder of Nugenso Nutrition, a private nutrition consultation practice in the Boston area. She uses Inherent Health's weight management test, based on the DNA taken from a cheek swab, to identify five key variations in genes regulating fat absorption and fat-cell formation, among others. The test is supposed to identify which diet and exercise pattern clients are most likely to respond to, although she says she's careful to tell them that it's not a definitive prediction. Even so, she says, there's a psychological effect, too. "Once they see it on paper and see a test result, they're more likely to comply," she says. "It offers a little more inspiration." (The investment of $149 undoubtedly helps, too.)

"We can't define the perfect personalized diet," says Inherent founder and chief science officer Ken Kornman, a dentist with a doctorate in microbiology and immunology. "What we feel confident that we can do is to substantially improve upon the current random results that someone gets when they go on a diet." The company recently announced the results of a small (100-person) unpublished study that suggests people who followed diets matched to their genotype using the company's test lost more weight than those who weren't correctly matched.

If you don't want to pay for what might at this point turn out to be just an expensive bit of inspiration, how can you individualize your diet by yourself? First, focus on working on your total health, not just weight loss. Dietitians don't advise simply eating 1,300 calories of whatever you want, whether it's carrots or Coke, to lose extra pounds; while two foods may be equal in calories, they don't necessarily offer the same nutritional value. Whatever mix of carbs, fat, and protein you end up with, it should consist primarily of what experts generally agree are the best-bang-for-the-calorie-buck foods: a balanced combination of lots of whole fruits and vegetables, whole grains rather than processed carbs, legumes, lean protein like fish, and "good" fats like omega-3 fatty acids and olive oil rather than saturated or trans-fats. That kind of diet is more likely to meet your nutritional needs and keep you satisfied. Fruits and vegetables are good at filling you up. On the other end of the caloric scale, nuts, for example, seem to promote weight loss, possibly because they are slowly digested and control hunger, says Schoeller.

If you're diabetic or have another chronic disease, or are at high risk for one, you should talk with a nutritionist about special tweaks you may want to make to your diet -- being sure you avoid saturated fat or excess sodium, for example, or, in the case of diabetes, picking foods that keep your blood sugar steady. (That's what the Nutrisystem D plan does; it's a low-calorie diet, but its foods have a low glycemic index, meaning they more slowly affect blood sugar levels, so dieters can manage their disease better.)

People with the metabolic syndrome are generally advised to lose weight by limiting calories and following a healthful diet rich in fruits and vegetables and low in unhealthful fats, rather than with any special carb-restriction regimen, says Maria Collazo-Clavell, an endocrinologist at the Mayo Clinic. (Dietitian Kress says she has unpublished data on nearly 4,000 patients showing that her diet for those with "Metabolism B," as she calls metabolic syndrome, works better than the standard low-fat, calorie-controlled diet advice.)

If you're generally healthy but just need to drop some weight, you might do some experimenting. Cassandra Forsythe, author of the Women's Health Perfect Body Diet, recommends eating a high-carb breakfast of a bagel and sugar-free jam one day and monitoring your mood, energy levels, and hunger throughout the morning. Then try a low-carb breakfast of scrambled eggs with feta, plus a small apple, the next. (Repeat to confirm the results.) She puts forth two different meal plans depending on which breakfast makes you feel better: one for the carb-energized, which includes about two more servings of carbs per day than the other, and one for the carb-sluggish, which substitutes about two servings of fat. Even if you don't want to go to the trouble of a formal experiment, just pay attention to how you feel after different foods. Do you feel satisfied? Can you go about your day without obsessing over your next meal? "Think about what you do best with. Maybe you feel fuller after lentils and rice rather than chicken or fish," says Forsythe.

Notice she didn't mention choosing between four different types of Lunchables. Remember, whatever ratio of carbs, proteins, and fat you find works best for you, your diet should primarily be comprised of healthful whole foods. Sounds boring. Won't sell many books. But at this point, it's still the simplest and best route to success.










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