I have read accounts of people who have lost lots of weight e.g. half their body weight saying that the loose skin does go back to the way it was, but it can take a few years. One woman who lost as I recall, 110 pounds, said that the skin all went back after 7 years. Another guy who lost a huge amount of weight and has sagging skin has noticed that there has been an improvement after one year. The skin is a living organ hence it is not inactive, but it has to be given time.
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