system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do our bodies store excess fat, sugar, cholesterol etc but not water or vitamins?

It's pretty annoying to have to drink water all day, throughout the day in proper amounts. Not enough? Headaches, dry mouth, etc. Too much? "Oh we don't need this right now. Pee it out!!!" Same with vitamins. Extras just get excreted out so you have to gradually consume them to stay healthy.

But yet you eat too much fat, sugar, or cholesterol and your body is like "hey let's hang onto this, we might need it in 27 YEARS!!!"

Why is it so inefficient about what it does/doesnt store?

Edit: Wow thanks to everybody who answered! Apparently we do store vitamins and minerals. I thought we didn't because i heard taking supplemental vitamins was a waste because our bodies just pissed out the extras.

I'm still salty about needing to drink water constantly though. I work in a microbiology lab and have a lot of PPE to deal with every time i leave and reenter the lab to drink. I can't even chug a gallon of water and be good for the day; it has to be gradual.

Edit 2: Oh goodness too many replies to keep track of but you guys rock

Edit 3: Gold! Thanks strangers! And it turns out our bodies store water as well. Just not how i thought. But my eye does twitch when i drink a boatload of water one day, pee most of it out, then next day my body is like "heyo i want more water even though I got rid of most of it yesterday".
gpt
Because water is actually pretty hard to store. We need a *lot* of it, far more than we need fat or sugar. Water is also pretty easy to obtain, so we don't *need* to store it. Things that do only do so because they can't get it regularly: cacti, camels... 

Water is also not only something we discard, it's how we discard *other* things, things that we have to discard due to their toxicity like urea. A high water intake and output is important for our overall health because it keeps us efficiently removing toxic waste products from our systems. 

We actually do store certain vitamins - fat soluble vitamins. Some vitamins are water soluble though, so we have no way of storing them. To store any chemical our body has to sequester it from the water inside our cells. Fat-soluble molecules naturally remove themselves from water anyway, but water soluble ones have to be made water *insoluble* in order to store them, which means modifying them so that they're not vitamins anymore. 

The problem with storage of fats and sugars is that we evolved to be able to survive droughts and other unpredictable conditions. We store excess energy (in the form of fats and sugars) so that we can't be caught off-guard if we ever enter a period of starvation. Fat storage is also a useful insulating tool remaining from when we hadn't invented central heating. So, we have all these mechanisms for storing fats and sugars, but we now live in an environment where starvation pretty much never happens unpredictably and we don't need to build large energy stores. The body always assumes the worst because it's evolved to do so, and if you're over-eating regularly its assuming that a starvation period is coming. So if you're constantly over-eating your body is constantly trying to stockpile energy in fear of an oncoming winter or something, but that never comes and you just keep on eating. 

The body is actually *incredibly* efficient about what it does and doesn't store. It stores everything it needs to be able to store (and can store at all). That's how evolution works after all - it tends towards making the most efficient versions of things possible, as long as more efficiency means a higher survival rate. If it doesn't need to store more energy, then it sends us messages telling us we don't need to eat. If you're storing more fat and sugar than you need, it's because you're *eating* too much fat and sugar, ignoring those messages, or because of some genetic defect that is meaning that messaging system isn't working properly. Or because of gut bacteria. That's the new focus of dietary science.

Also, the reason we can't store water soluble things is because it's actually toxic to do so. A lot of our body works on the relative concentration of water soluble molecules, and if we have too much of a water soluble molecule, we actually can't absorb any more of it without converting the water soluble molecule to an insoluble form. We actually have to use a very advanced mechanism to absorb glucose at all, that relies on the fact there's more sodium in our intestine than inside our cells.