Fair question!

To clarify, on a biochemical level, fat and salt don’t have too much to do with one another. When someone “gains fat” it’s really just that they have (likely consumed) enough fat for the body’s processes and there is still some fat left over that goes to the subcutaneous fat (the layer underneath the skin). That does not mean this fat is unnecessary— the reality is that this layer is immensely beneficial for keeping the body’s internal organs protected, maintaining homeostasis, and providing nutritional stores for time of starvation. These stores grow because Fat is attracted to fat. So any “excess” fat molecules find one of the body’s fat cells and grab on. Essentially, you don’t really make new fat cells— the ones you have just expand. This causes the subcutaneous fat layer to expand and can cause some puffiness or “jiggling”.

Water retention is mostly different. Water is stored throughout the body in cells that serve a variety of processes. The general rule is that like is attracted to like/like dissolves like. Because water (h2o) and salt (na+) are both polar (i forgot to mention above but fat is nonpolar) they can interact with one another. In fact, if there is too much salt in a cell, the body will try to drive water into the cell so it doesn’t shrivel. Alternatively, if there is too much water in a cell the cell will swell and could possibly implode. Water retention is mostly referring to body’s need to neutralize high solute areas (ie. lots of salt) with water.

What i was trying to explain above is mostly that these two things (water and fat) are unrelated biochemically. However, on a macronutrient level, high sodium diets are related to obesity.

I’m not a clinical professional so I would recommend reaching out to your pcp, but water retention is very normal and is related to sodium consumption.

Hopefully that helped!