system
Given the following post, respond as if you're commenting on Reddit:
human
If a person has been overweight for along time is the fat old fat? Or does the body recycle the fat, tries to use old fat and stores new one?
gpt
With respect to fat cells themselves, the body does not "recycle" them in any sense.  What does happen is that the body is constantly synthesizing and breaking down the triglyceride contents of fat cells.

An overweight person does not necessarily have more fat cells than the skinny version of themselves.  Rather, it is merely that their fat cells are considerably larger due to containing more triglycerides.  

A skinny person becomes fat when the balance between fat synthesis and fat breakdown skews more towards fat synthesis.  As a result, over time, their fat cells will accumulate increasing amounts of triglycerides and expand accordingly, and eventually, the person is "overweight."

If that overweight person then undertook to lose weight, and managed to successfully do so, what one would observe would be that their adipose tissue ( the fat cells ) had shrunk, much like letting the air out of a balloon.  Overall, however, the number of balloons would still be the same, albeit occupying much less space in the jeans.

In general, the number and distribution of fat cells is fixed at birth, but some studies have suggested that under certain conditions, it may be possible for fat cells to divide and thereby grow in number.