So for a pair of two-week stays, the volunteers lived in a metabolic ward where they ate the same thing every day for breakfast, lunch, dinner and snacks. Each person tried two different diets identical in calories: one diet cut 30% of their total calories, all coming from reductions in dietary fat while keeping carbohydrates and protein the same, while the other cut calories from carbohydrates, keeping fat and protein the same. “This is the first time a study has ever just selectively reduced these individual nutrients as opposed to changing multiple nutrients at once,” says Hall. Using special equipment, the researchers were able to see exactly how their bodies were burning both calories and body fat.