TW: If you're the type of person who would stop drinking water in order to lose weight/get thinner, don't read this.

Take everything I say with a grain of salt. Because salt is important for water retention haha. Also because I could be wrong about everything I just wrote.

Super basically: you lose more weight and appear thinner faster in the short term.

When you're dehydrated your body starts by using water reserves from fat cells and if you do it only one day, it will mainly be your fat cells shrinking instead of cells in muscle tissue and organs. So you lose a bit of water weight but you're also "losing fat" because your fat cells are shrinking thus making you thinner.

This is the more """"scientific"""" answer:

The idea is that your body stores water in a similar way it stores fat and in the same way your body taps into your fat reserves when you need energy, it will tap into your water reserves when you're dehydrated. All cells have water and they will all dehydrate and shrivel up as the concentration of water inside the cell exceeds the concentration outside (something something osmosis). But your body ain't a fucking idiot and it will prioritize which cells shrivel up. So the same way you break down fat instead of your organs when you need calories, your body will dehydrate fat cells instead of other more important cells.

The thing with what we call being "fat" is that it isn't dictated by the amount of fat cells, it's the size of fat cells. Loosing fat shrinks your fat cells. So if you dehydrate and shrink your fat cells, you will appear thinner the same way you would if you lost fat by burning it as calories.

So while you're dry fasting, the input of fat and water from the body to the fat cells has completely stopped but the output has to increase because no water or calories are coming from the outside world to the body.

During Ramadan plenty of people do dry fasts so it's not the end of the world but it has to be short periods of time. You don't have that much water in your fat cells and you need a lot of water to live. I can't see a dry fast longer than a day being healthy in any way because you also have a lot of water in your muscles and organs. So the same way your body starts breaking everything down when it needs calories, it will dehydrate everything cuz it needs water except 10x faster. Your health will nosedive like that*(imagine I just snapped). When you get severely, like, about to die dehydrated, your blood turns into the same consistency and color as sweet baby Ray's barbecue sauce, there's something fun to think about.

Again, I'm talking out if my ass using bits and pieces of info I've gathered on the internet, do some legitimate research if this interests you at all.