With the rats in the same study, the body weight difference was only 6%, while the different amount of white fat tissue in the belly was 22%. So it is pretty cl

With the rats in the same study, the body weight difference was only 6%, while the different amount of white fat tissue in the belly was 22%. So it is pretty clear the body is changing composition (more muscular, less fatty). BMI is worthless in such a case because it penalizes muscle, which is heavier than fat. I know countless very fit friends who have bad BMIs just because they have a lot of heavy muscle. BMI only allows you to be a certain weight at a certain height, it doesn't allow you to carry muscle instead of fat.