Actually, only 6 days, as each subject was given first one of the two diets (low-carb or low-fat), then the other.
Nutrition is notorious with its flawed and sponsored studies!
Absolutely agree! In two weeks the body is barely flushing excess water and adjusting to use ketones as a fuel source when carbohydrates aren't available. I'd love to see a study 6+ months long and with varying levels of carb reduction.