system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does skin tighten up and not be saggy after losing weight, especially significant weight (30+lbs)?
gpt
The whole body is always in a renewing process. Cells are constantly dying and new ones are being formed. This is also the case with the skin. If you rub firmly after showering, you can recognize dead skin cells as white crumbs.

As the body slowly changes shape, the skin can adapt over time just by building more or less new cells. So the skin becomes smaller or larger without tearing. If the weight loss goes too fast, the skin can no longer adapt in time. 

In addition, it will be important that weight loss is associated with certain exercises to achieve firmness, because thin people can have saggy skin aswell.