Many people start to gain weight after running, but after putting on clothes, they obviously feel in good shape. They can wear underwear that they couldn't wear before, and their waistlines are thinner. This is actually the result of running to lose weight, because it means that almost all the fat has been reduced. Moreover, a week is too short, and the change of weight sometimes does not fully reflect the effect of reducing fat. Sometimes the same weight, but the ratio of fat to muscle has changed, with more muscle and less fat.