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We just need more water than it can provide. We have fairly dilute urine. There are other animals can rely on a higher percentage of water from food, some with more concentrated urine than us, we lose a lot of water.
Food not only contains water, but breaking down the sugars using cellular respiration also creates more water as a byproduct, adding another small percentage. It’s just a small percentage of our overall water, so we need to drink, but some animals do use it as an important water source. I can’t remember which ones, maybe some arid species.
Depends on what you eat. There are a few ways eating food can make you thirsty:
1. Salt. This is a big one. Salt needs to be filtered out of your blood by your kidneys, and your kidneys can’t filter blood without losing some water to flush out all the bad stuff. In other words, salt makes you pee and you need to replenish those fluids. So, eating salty food makes you feel thirsty. This is obvious for something like bacon or potato chips which are *very* salty. But the fact is we’re used to a diet that’s pretty universally high in salt. So many foods that taste neutral to you (not bland) are still likely to contain a substantial amount of salt.
2. Fiber. Dietary fiber is a bit similar to salt, in that it absorbs water, so eating a lot of fiber will also tend to make you feel thirsty.
3. Protein. Protein digestion involves *hydrolysis*, i.e. breaking apart the big protein molecules in a chemical reaction that consumes water. So, a high-protein meal can make you feel thirsty.
4. Sheer volume. The first stop in your digestive tract is the stomach. The stomach holds your chewed-up food so it can be passed to the intestines bit by bit, and also has a role in breaking it down into smaller pieces. This process requires the food in your stomach to be submerged in stomach acid. Stomach acid is mainly water. If your stomach is very full, it needs to produce more stomach acid to digest all that food. First, to simply cover all the food, and second, to replenish the acid that is lost every time the stomach opens up at the bottom to release food slurry into the intestines. So any big meal, regardless of its contents, can make you feel thirsty, because water is being used up for stomach acid.
5. Habits. Your body adapts to and anticipates your eating and drinking habits. If you eat at regular times, you will start to feel hungry around those times, (somewhat) independently of how much or how recently you ate beforehand, because it’s useful for the body to prepare itself for digestion, and generally regulate its metabolism around when food is expected. Similarly, if you always drink something alongside or after meals, then your body will expect fluids when you eat something. This alone may cause you to feel thirsty after eating.
It also depends on how much water is in the food you ate. Raw vegetables and fruits contain lots of water. Cooking reduces that. Meat contains less water to begin with, as do nuts and seeds, or flours created from nuts or seeds. So something like a cooked hamburger on a bun, with not so much water inside but plenty of salt, protein and bulk, is likely going to require quite a bit more water to digest than the little water it gives you.
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