A study that found that people's genes can determine whether they will lose weight on a low-fat or low-carb diet has made a splash. "Knowing your genotype for low-carb or low-fat diets could help you increase your weight-loss success," Stanford University's Christopher Gardner, a co-author of the study, told the Wall Street Journal. Interleukin, the sponsor of the study and the developer of the test, sells it for $149.