“Under normal conditions, humans absorb only about 80% of the nutrients from the food they eat,” says A. Roberto Frisancho, Ph.D., a weight-loss researcher at the University of Michigan. But, he says, when the body is deprived of nourishment, it becomes a super-efficient machine, pulling what nutrients it can from whatever food is consumed. Start eating again normally and your body may not catch up; instead it will continue to store food as fat.
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."