You know the kind, says Jillian: "Everyone's like: 'Give up carbs!' 'Give up fat!' 'Wait, no, now I'm taking pills!' None of them are manageable long-term—and they wreak havoc with your metabolism! Because you're either starving yourself or you're cutting out a major food group. Then you go back into weight-gain mode, but it's even worse, because your body has adjusted to all that crazy fad crap."
The authors wanted to compare low-fat vs. low-carb diets, but they also wanted to study genetic and physical makeups that purportedly (their word) could influence how effective each type of diet will be for people. Previous studies had suggested that a difference in a particular genetic sequence could mean that certain people will do better with a low-fat diet. Other studies had suggested that insulin sensitivity may mean that certain people will do better with a low-carb diet.