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 Ken Kornman, a dentist with a doctorate in microbiology and immunology who is founder and chief science officer of the Inherent brand's parent company, Interleukin Genetics. "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.
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.